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Wire Temperature Ratings and Terminations

Posted By Jim Pauley, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

Many electrical inspectors can tell you that confusion about wire temperature ratings and equipment termination temperature requirements results in their rejecting installations. Information about this topic can be found in the National Electrical Code (NEC), testing agency directories, product testing standards, and manufacturers’ literature, but many people do not consult these sources until it is too late.

Why are temperature ratings important?

Conductors carry a specific temperature rating based on the type of insulation used on the conductor. Common insulation types can be found in Table 310-13 of the NEC, and corresponding ampacities can be found in Table 310-16. Table 1 shows the ampacity of a 1/0 copper conductor based on different conductor insulation types.


Figure 1. Based on the table, a 1/0 copper conductor is acceptable. The installation would be as shown in Figure 1, with proper heat dissipation at the termination as well as along the conductor length.

The ampacity of the 1/0 Cu conductor depends on the temperature rating of the insulation. At the same ampacity, a smaller conductor with higher-rated insulation can be used instead of a larger conductor with lower-rated insulation. As a result, the amount of copper and even the number of conduit runs needed for the job may be reduced.

One of the most common misapplications of conductor temperature ratings occurs when the established temperature rating of the equipment termination is ignored. This is particularly true for equipment rated for 600 V and less since the equipment is tested as a complete system using conductors sized by the NEC rules. Reduced conductor sizes result in the system having less ability to dissipate heat and therefore increase the operating temperature of the equipment terminations. Conductors must be sized by considering where they will terminate and how that termination is rated. If a termination is rated for 75°C, the maximum temperature at that termination is 75°C when the equipment is loaded to its ampacity. If 60°C insulated conductors are used in this example, the additional heat at the connection above 60°C could result in conductor insulation failure.


Figure 2. This may have led to overheating at the termination or premature opening of the overcurrent device due to the smaller conductor size

When a conductor is selected to carry a specific load, the user/installer or designer must know the termination ratings for the equipment in the circuit. For example, consider a circuit breaker with 75°C terminations and a 150A load. If a THHN (90°C) conductor is chosen for the job, review Table 310-16 in the NEC and look for a conductor that will carry the 150A. Although a 90°C conductor is being used, ampacity must be chosen from the 75°C column because the circuit breaker termination is rated at 75°C. Based on the table, a 1/0 copper conductor is acceptable. The installation would be as shown in Figure 1, with proper heat dissipation at the termination as well as along the conductor length. Had the temperature rating of the termination not been a consideration, a No.1 AWG conductor might have been chosen, based on the 90°C ampacity. This may have led to overheating at the termination or premature opening of the overcurrent device due to the smaller conductor size (see Figure 2).

In this same example, a conductor with a 75°C insulation type (THW, RHW, USE, etc.) also would be acceptable since the termination is rated at 75°C. A 60°C insulation type (TW) is not acceptable since the temperature at the termination could rise to a value greater than the insulation rating.


Table 1

The NEC Rules


Figure 3. Conductors rated 60°C

Equipment Rated for 100A or Less— NEC 110-14(c)(1)(a) through (d)

NOTE: The equipment sizes and ampacities shown in the figures are arbitrary. The rules apply to any equipment rated 100A or less.

For equipment with termination provisions for circuits rated 100A or less or marked for No. 14 AWG through No. 1 AWG conductors, the NEC allows conductors to be used based on the following four conditions:

a. Conductors rated 60°C (see Figure 3).

b. Conductors with higher temperature ratings, provided the ampacity is determined based on the 60°C ampacity of the conductor (see Figure 4).

c. Conductors with higher temperature ratings, provided the equipment is listed and identified for use with such conductors (see Figure 5).

d. Conductors for specific motor applications (see Figure 6). This permission is specific to Design B, C, D, or E motors because those motors are temperature evaluated with conductors based on 75°C ampacity according to NEMA MG-2 (Safety Standard for Construction and Guide for Selection, Installation, and Use of Electric Motors and Generators).

Equipment Rated Above 100A—NEC 110-14(c)(2)(a) and (b)


Figure 4. Conductors with higher temperature ratings, provided the ampacity is determined based on the 60°C ampacity of the conductor

For equipment with termination provisions for circuits rated above 100A or marked for conductors larger than No. 1 AWG, the NEC 110-14(c)(2)(a) and (b) allows conductors to be used based on the following conditions:

a. Conductors rated 75°C (see Figure 7).

b. Conductors with higher than 75°C ratings provided the conductor ampacity does not exceed the 75°C ampacity of the conductor size used (see Figure 8). This condition also permits the conductors to be used at ampacities higher than 75°C if the equipment is listed and identified for the higher rating. However, for equipment rated 600V and less, there is no listed equipment with termination ratings above 75°C.

The equipment termination ratings versus conductor insulation ratings are summarized in Table 2.


Table 2. The equipment termination ratings versus conductor insulation ratings are summarized in Table 2.

Caution on using lug ratings


Figure 5. Conductors with higher temperature ratings, provided the equipment is listed and identified for use with such conductors

When terminations are inside equipment such as panelboards, motor control centers, switchboards, enclosed circuit breakers, safety switches, etc., follow the temperature rating identified on the equipment labeling instead of the rating of the lug itself. Manufacturers commonly use 90°C-rated lugs (i.e., marked AL9CU) on equipment rated only 60°C or 75°C. The use of the 90°C-rated lug in this type of equipment does not allow the installer to use 90°C wire at the 90°C ampacity. The Underwriters Laboratories® General Information on Electrical Equipment Directory states the following about terminations: "A 75°C or 90°C temperature marking on a terminal (e.g., AL7, CU7AL, AL7CU or AL9, CU9AL, AL9CU) does not in itself indicate that a 75°C or 90°C insulated wire can be used unless the equipment in which the terminals are installed is marked for 75°C or 90°C.”

Review the labeling of all devices and equipment for installation guidelines and possible restrictions.


Figure 6. Conductors for specific motor applications

Available Equipment Terminations

Remember that a conductor has two ends, and that the termination on each end must be considered when applying the sizing rules. For example, consider a conductor wired to a 75°C termination on a circuit breaker at one end, and a 60°C termination on a receptacle at the other end. This circuit must be wired with a conductor that has an insulation rating of at least 75°C (due to the circuit breaker) and sized based on the ampacity of 60°C (due to the receptacle).

For electrical equipment rated for 600V and less, terminations are typically rated to 60°C, 75°C, or 60/75°C. No distributions or utilization equipment is listed and identified for the use of 90°C wire at its 90°C ampacity. This includes distribution equipment, wiring devices, and even utilization equipment such as HVAC, motors, and light fixtures. Installers and designers who have not realized this fact have equipment that does not comply with the National Electrical Code and that has been turned down by the electrical inspector.


Figure 7. Conductors rated 75°C

In equipment rated over 600V, the effect of the conductor as a heat sink is minimized, and ratings higher than 75°C are available. NEC 110-40 recognizes that conductors with sizes based on the 90°C ampacity can be used in installations over 600V.

An example of how 90°C wire might be used at its 90°C ampacity is shown in Figure 9. Note that the conductor does not terminate directly in the distribution equipment, but in a terminal or tap box using 90°C-rated terminations.

Frequently, manufacturers are asked when distribution equipment will be available with terminations that will permit 90°C conductors at the 90°C ampacity. This would require not only significant equipment redesign (to handle the additional heat), but also coordination of the downstream equipment where the other end of the conductor terminates. Significant changes in the product testing/listing standards also would have to occur.


Figure 8. Conductors with higher than 75°C ratings provided the conductor ampacity does not exceed the 75°C ampacity of the conductor size used

A final note about equipment—some equipment requires the conductors that are terminated in the equipment to have an insulation rating of 90°C, but an ampacity based on 75°C or 60°C. This type of equipment might include 100 percent rated circuit breakers, fluorescent lighting fixtures, etc., and is marked to indicate such a requirement. Check with the manufacturer of the equipment to see if you need to take into account any special considerations.

What about higher-rated conductors and derating factors?

One advantage to conductors with higher insulation ratings is noted when derating factors are applied. This advantage is noted in the last sentence of NEC 110-14(c): "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.” Derating factors may be required because of the number of conductors in a conduit, higher ambient temperatures, or internal design requirements for a facility. By beginning the derating process at the ampacity of the conductor based on the higher insulation value, you may not be required to upsize the conductor to compensate for the derating.

Remember these points while studying the derating process example:

  • The ampacity value determined after applying the derating factors must be equal to or less than the ampacity of the conductor based on the temperature limitations at its terminations.
  • The derated ampacity becomes the allowable ampacity of the conductor, and the conductor must be protected against overcurrent in accordance with this allowable ampacity.

Example of the derating process

Assume that you have a 480Y/277 Vac, 3-phase, 4-wire feeder circuit to a panelboard supplying 200A of noncontinuous fluorescent lighting load. Assume that the conductors will be in a 40°C ambient temperature and the conductors originate and terminate in equipment with 75°C terminations.

Additional information to consider from the NEC:

  • Since the phase and neutral conductors all will be in the same conduit, consider the issue of conduit fill. NEC 310-15(b)(4)(c) states that the neutral must be considered to be a current-carrying conductor since a major portion of the load is a nonlinear load (electric discharge lighting).
  • Based on this, four current-carrying conductors will be in the raceway. NEC 310-15(b)(2)(a) requires a 20% reduction in the conductor ampacity based on having four to six current-carrying conductors in the raceway.
  • According to the ambient correction factors at the bottom of Table 310-16, an adjustment must be made of 0.88 for 75°C and 0.91 for 90°C.

Calculate, using a 75°C conductor such as THWN:

300 kcmil copper has a 75°C ampacity of 285A.

Using the factors noted earlier: 285A x 0.80 x 0.88 = 201A


Figure 9. An example of how 90°C wire might be used at its 90°C ampacity

201A is now the allowable ampacity of the 300 kcmil copper conductor for this circuit. Had the derating factors for conduit fill and ambient not been required, a 3/0 copper conductor would have met the needs for this application.

Calculate, using a 90°C conductor such as THHN:

250 kcmil copper has a 90°C ampacity of 290A.

Using the factors noted earlier: 290A x 0.80 x 0.91 = 211A

211A is less than the 75°C ampacity of a 250 kcmil copper conductor (255A), so the 211A would now be the allowable ampacity of the 250 kcmil conductor. Had the calculation resulted in a number larger than the 75°C ampacity, the actual 75°C ampacity would have been used as the allowable ampacity of the conductor. This is critical since the terminations are rated at 75°C. Note that the conductor size was reduced by one size (300 kcmil to 250 kcmil) and still accommodated all of the required derating factors for the circuit. This is the primary advantage of using 90°C conductors.

Therefore, when using 90°C wire for derating purposes, you can begin derating at the 90°C ampacity. You must compare the result of the calculation to the ampacity of the conductor based on the termination rating (60°C or 75°C). The smaller of the two numbers then becomes the allowable ampacity of the conductor.

Summary

There are a number of factors that affect how the allowable ampacity of a conductor is determined. The key is not to treat the wire as a system in itself, but as a component of the total electrical system. The terminations, the equipment ratings, and the environment all affect the ampacity that can be assigned to the conductor. If the designer and installer keep each of the rules in mind, the installation will go more smoothly.


Read more by Jim Pauley

Tags:  Featured  March-April 2000 

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Installing Transient Voltage Surge Suppressors (TVSS)

Posted By Alan Manche, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

A Historic Perspective on Surge Protection


Photo 1. Direct bus connections between the TVSS and panel reduces; the conductor length, unnecessary conductor bends, and impedance with bolted connections.

Surge protection was introduced into the first National Electrical Code (NEC) published in 1897. The primary focus at that time was lightning arresters. In 1981, NEC Article 280 was revised and re-titled "Surge Arresters” in order to align with industry terminology. The title change in the NEC also recognized that surge arresters were being installed where the surge source was other than lightning, such as utility switching, or equipment switching within industrial and commercial facilities.

The vast introduction of electronic equipment such as computers, answering machines, microwaves, HVAC electronic controls, security systems, etc., during the last 20 years has also presented a challenge to the electrical industry to protect this sensitive equipment from less severe surges than lightning. The Transient Voltage Surge Suppressor (TVSS) is the newest product becoming commonplace in residential, commercial, and industrial facilities in order to protect sensitive electronic equipment. In response to the development of TVSS products, Underwriters Laboratories, Inc. (UL) published the first TVSS safety standard, UL 1449, in August 1985. [See photos 1 and 2]

Lightning Arrester, Secondary Surge Arrester, Transient Voltage Surge Suppressor.Where do these fit in the electrical system?
The lightning arrester is generally installed by the utility at the serving transformer and basically acts as a spark plug. When lightning hits the distribution line, the increase in voltage causes an arc to form across the spark gap and the lightning current is diverted to ground, protecting the utility transformer.

Surge arresters and secondary surge arresters are generally installed at the service equipment either on the line or the load side of the service disconnect. They may also be found connected to non-service panelboards where branch circuits extend outside the building. For example, branch circuits feeding parking lot lights are a possible source for a surge to enter the building by backfeeding the electrical system from external sources.


Photo 2. Integral TVSS units within Busway Plug-in Units and MCC buckets reduce the conductor length and enhance performance of the surge protection of the system.

Transient Voltage Surge Suppressors may be installed in any part of the electrical system starting from the load side of the service disconnect in the service equipment to the electronic product being protected. The TVSS may integrate protection of AC, coaxial cable, and telephone to establish a common ground reference for the different services. TVSS products may also be found connected to non-service panelboards where branch circuits extend outside the building.

Installation in Accordance with the NEC

TVSS products may be installed:

1. as an integral part of a listed panelboard.

2. as a field installed product within a panelboard listed and marked for a TVSS kit.

3. external to a panelboard or switch and connected on the load side of a circuit breaker or fuse inside the panel.

4. as an integral component of a Listed wiring device to protect specific electronic equipment connected to that branch circuit.

Article 250 – Grounding and Bonding

Establishing a solid foundation for a safe TVSS installation starts with the grounding and bonding system. Section 250-2(a) clearly states the grounding system be connected to earth in order to limit the voltage levels imposed by surge events.

250-2. General Requirements for Grounding and Bonding

(a) Grounding of Electrical Systems. Electrical systems that are required to be grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines and that will stabilize the voltage to earth during normal operation.

The connections of the grounding electrode, grounding electrode conductor, and the bonding jumper are important to facilitate a safe, low resistance path to ground for any surge current being diverted by a TVSS. When adding a TVSS to an existing electrical system, it is important to reinspect the grounding system to ensure a safe and effective path for the surge current.

Article 280 – Surge Arresters

TVSS products are not specifically recognized in the NEC. Since TVSS products are similar in function to a surge arrester, we must turn to Article 280 for installation requirements. The voltage rating of the TVSS must be equal to or greater than the continuous phase-to-ground voltage. Section 280-4 also requires all surge arresters less than 1000V to be listed. UL 1449 only covers products rated 600V and less.

280-4. Surge Arrester Selection

(a) Circuits of Less than 1000 Volts. The rating of the surge arrester shall be equal to or greater than the maximum continuous phase-to-ground power frequency voltage available at the point of application.

Surge arresters installed on circuits of less than 1000 volts shall be listed for the purpose.

The TVSS must be connected on the load side of the service disconnect unlike surge arresters that can be connected to the line or load side of the service disconnect as permitted in Section 230-82. The surge arrester is often evaluated to higher surge current levels than TVSS products. A TVSS evaluated to UL 1449 assumes connection on the load side of the "”main disconnect.”" The listing of a TVSS product may also require connection to the load side of a specified overcurrent device. UL 1449 permits this marking to be placed either on the TVSS or in an instruction bulletin.

A service may contain six disconnects, as permitted in Section 230-71. The main disconnect supplying the TVSS at this service counts as one of the six disconnects. A TVSS is not power monitoring equipment.

230-71. Maximum Number of Disconnects

(a) General. The service disconnecting means for each service permitted by Section 230-2, …, shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be no more than six disconnects per service grouped in any one location. For the purpose of this section, disconnecting means used solely for power monitoring equipment or the control circuit of the ground-fault protection system, installed as part of the listed equipment, shall not be considered a service disconnecting means.

The routing and length of conductors connecting the TVSS is an important concern, addressed in Section 280-12. The objective of the TVSS is to get any overvoltage conditions to ground as quickly as possible without causing a strikeover or arc-to-ground before the transient is dissipated to earth, ultimately protecting sensitive electronic equipment. Unnecessary bends and conductor length increases impedance in the surge path, driving the surge voltage higher. As the voltage grows, the potential for a strikeover grows since the voltage is not suppressed. The unsuppressed voltage passes on to the electronic equipment that the TVSS was intended to protect. Depending on the size and material (Cu or Al) of the conductor, each foot of wire may add as much as 165V to the suppression rating of the device.

Other Safety Issues


Photo 3. The integral TVSS is becoming common place in panelboards that serve sensitive electronic loads such as school computer facilities, office buildings, and industrial facilities

TVSS products function similar to surge arresters; however, in order to provide protection for electronic equipment, TVSS devices begin to operate (i.e., conduct electricity during a surge), much closer to the system operating voltage than a surge arrester. The NEC does not require all electrical systems to be grounded nor does it prohibit the installation of TVSS products on an ungrounded system; however, there are concerns with ungrounded systems. The voltage in an ungrounded system is not stable and the system voltage-to-ground can rise as high as eight times over the normal operating voltage on the system if a ground fault occurs on the circuit. A voltage rise of this magnitude can be destructive to a TVSS; therefore, a TVSS should not be permitted on ungrounded systems. [See photo 3]

It is important to look for TVSS products that have a short circuit current rating (SCCR), even the more common single-port or parallel devices. TVSS technology commonly uses metal oxide varistors (MOVs) to make direct phase-to-phase and phase-to-ground connections unlike a spark (air) gap used in lightning arresters. When an MOV fails, a short circuit is established. The available fault current will attempt to flow along the shorted path. The circuit breaker or fuse, to which the TVSS is connected, affords short circuit protection for the wire, but the TVSS may or may not be protected unless it has been tested and marked. An example of markings on TVSS products that have been tested for short circuit current ratings would be similar to:

This TVSS is suitable for use on circuits capable of delivering not more than:

25,000A rms, 240V when protected by a 30A max circuit breaker, or

10,000A rms, 240V when protected by a 30A max non-current limiting fuse.

TVSS products can see an extremely high amount of energy before the circuit breaker or fuse limits and disconnects the short circuit current. This energy can be destructive to the TVSS if the internal components are not electrically coordinated with the overcurrent protection. Containment of the TVSS failure within the TVSS enclosure is an important safety concern. The rupture of the TVSS enclosure is a safety concern whether external or internal to other electrical equipment. The short circuit current rating for single-port devices is a safety gap that presently exists within industry standards and can only be evaluated by product testing. UL 1449 currently has a short circuit current test for two-port devices but a similar test is only required at minimal levels for the more common single-port or parallel device. Short circuit current rating markings on products is a good indication that the manufacturer has considered the available fault current to which the TVSS is going to be connected.

Photo 4

The markings on a TVSS mounted externally to a panel should include the SCCR directly on the TVSS. A panel with an integrated TVSS will have SCCR markings on the TVSS, the panel’s wiring diagram, or panel markings that indicate acceptance of a TVSS within the panel. The short circuit current ratings on the panel will apply for an integrally mounted TVSS as part of the panel’s listing. Do not confuse the short circuit current rating with the surge current ratings on the TVSS, they are not the same.

The integration of TVSS products into panelboard enclosures is becoming common. Listed equipment is evaluated to ensure the integrity of the panel with the TVSS installed. The panelboard with an integral TVSS is evaluated in accordance with the UL 67 safety standard for panelboards for heat rise, conductor (bus bar) spacing, wire bending space, wire fill, short-circuit current, etc. A TVSS will reduce the volume of the enclosure, which impacts the heating, wire gutter, and wire bending space. The reduction in volume of the enclosure can also impact the short-circuit performance of the panelboard due to venting characteristics of different circuit breakers. Extending the enclosure using a panelboard extension and adding the TVSS above or below the panelboard will not resolve these safety issues without proper evaluation and testing. Paragraph 30.12 in UL 67 clearly states that components for use in panelboards must be included in the panelboard markings.

30.12 Field-installed equipment

30.12.1 A panelboard to which a unit, such as a circuit breaker, switch, or the like, may be added in the field shall be marked with the name or trademark of the manufacturer and the catalog number or equivalent of those units for which it is intended.

Look for a panel marking that indicates the panel has been listed for use with an integral TVSS. A marking must be provided as part of the panelboard listing that the TVSS has been evaluated for use within the panel.

TVSS Performance Considerations


Photo 4. The "Whole House" surge protection unit integrates surge protection of AC, Telephone/Data, and Coax/cable and Satellite TV by establishing a common reference where the utilities enter the house.

TVSS conductor length is a consideration in product performance. If we consider the conductor lengths by using 165V per foot to calculate new suppression ratings after connection to the electrical system, we start looking for methods to reduce the conductor length in order to increase our protection from surges and transient voltages. Efforts to reduce conductor length to maximize protection from surges prompted the introduction of TVSS products being manufactured integral to panelboards in order to minimize the impact of conductor impedance. [See photo 4]

The entrance of utilities such as electric, cable, and telephone at the same point on the structure is important for effective protection using a TVSS. Electronic equipment (computers, telephones,…) has multiple paths for surges to enter. A surge will destroy electronic equipment due to a voltage differential (between AC, phone, and coax) on the electronic boards within the equipment. These multiple paths can effectively be protected in a residence using a TVSS that integrates protection of AC, coaxial cable, and telephone. By referencing all utilities to a common ground at the service entrance, we have an effective low impedance ground path and establish a common reference for all external surge sources. The utilities will electrically rise and fall together when a surge enters the building, assisting in the removal of any voltage differential between the communication system and electrical system within the computer, phone, or answering machine.

Large residences may have multiple panelboards, such as two 200A panelboards. You enhance your protection of electronic equipment that is connected to multiple utilities, such as telephones and computers, by connecting the TVSS to the panelboard that supplies the branch circuits for those loads. "Point-of-use” protection may also be recommended by an appliance manufacturer or the panelboard TVSS manufacturer to ensure proper protection of sensitive electronic equipment for warranty purposes. Point-of-use TVSS products are also listed to UL 1449 and may be found in the form of a receptacle, direct plug-in connected, or cord-connected devices.

Summary

The NEC does not currently address the unique safety concerns of TVSS installation as compared to surge arresters. However, fundamental elements have been discussed to assist in a safe and NEC compliant installation of transient voltage surge suppressors. Please note those items covered in this article that are safety related but not specifically addressed in the NEC at the present. The following list provides a summary of the items discussed and should be reviewed during the installation of a TVSS:

1. Review the grounding system and connections to ensure an effective ground path for surge current. NEC 250–2(a). The TVSS should not be installed on an ungrounded electrical system.

2. The TVSS must be listed by a recognized certification body. You may find surge arresters installed that may be listed as secondary surge arresters. NEC 280-4(a).

3. The TVSS must be installed on the load side of the service disconnect. UL 1449 (Not in the NEC). Review the TVSS markings for the required overcurrent protection.

4. When the TVSS is installed at the service with six disconnects, it must be connected to one of the six disconnects. Seven is a violation of NEC 230-71.

5. Review the panelboard marking that indicates the panel is listed for use with an internal or integral TVSS. NEC 110-3(b) and UL 67.

6. Look for a short-circuit current rating on permanently connected TVSS products. NEC 110-10.

 


Read more by Alan Manche

Tags:  Featured  March-April 2000 

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1999 Code Requirements for Low-Voltage Systems

Posted By Brooke Stauffer, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

The National Electrical Code (NFPA standard 70-1999) contains rules for all electrical products and systems. This includes low-voltage and limited-energy systems of all types ranging from telecommunications/LAN to fire alarm to closed-circuit TV systems used for security purposes. There are important reasons for making sure that low-voltage and limited-energy systems are installed in accordance with all applicable NEC requirements:

  • Telephone—ringing voltages can be as high as 90 volts AC
  • Audio—system voltages can be as high as 70 volts AC
  • Shock hazard—Incorrectly-installed low-voltage wiring may accidentally become energized at line voltages, thus endangering both installer and users.
  • Grounding—Proper grounding of communications circuits, CATV cables, TV and satellite dish masts, computer room raised floor systems, etc., is vital for both and performance reasons.

Proper installation of low-voltage and limited-energy systems to 1999 National Electrical Code safety requirements is so important that the National Electrical Contractors Association (NECA) has just published a new technical guide on the subject. In the meantime, here’s a quick rundown of 1999 Code changes affecting low-voltage and limited-energy installations:

Sections 230-82(4) and 230-94, Exception No. 4—Emergency Power. Circuits for emergency systems can no longer be connected to the supply side of the service disconnecting means. This will require a separate power service (a second feeder from the utility transformer out in the street, a generator, batteries, etc.) for emergency systems such as fire alarms, exit lights, etc.

Article 370, Part D—Manholes. With utility deregulation, responsibility for building and maintaining underground vaults and manholes, for both power distribution and communications systems, is increasingly falling on electrical contractors. New Part D blends existing NEC manhole requirements with those of the National Electrical Safety Code (the utility wiring code).

Section 410-15(b)—Lighting Poles. Metal poles for lighting fixtures can’t contain low-voltage or limited-energy circuits. Specifically, they have been defined as "”raceways”" which is intended to prevent conductors for security cameras, loudspeakers, etc, from being installed inside lighting poles with power wiring. The NEC usually tries to keep power and non-power wiring separate.

Article 640—Sound-Recording and Similar Equipment. Article 640 was completely re-written to update it technically and provide a clear, unambiguous article for the use the Authority Having Jurisdiction (i.e., electrical inspectors) when inspecting audio installations.

Section 770 – 770-52(a)—Optical Fiber Cable. A new rule states that raceways shall be of a type permitted by Chapter 3 (general wiring rules) unless specifically listed for use with fiber optic cables [770-6]. Rules on mixing optical fiber cables with power conductors were revised and expanded [770-52(a)].

Article 800—Telecommunications. Section 800-4 was revised to require that listed telecommunications equipment also comply with Section 110-3, general rules for equipment installed under the Code. A new rule states that raceways shall be of a type permitted by Chapter 3 (general wiring rules) unless specifically listed for use with communications circuits [800-48].

Article 810—Radio and Television Equipment. The scope of Article 810 was expanded to include satellite dish antennas.

Article 830—Network-Powered Broadband Communications Systems. This article was added to cover a new technology intended for interactive (two-way) internet access, movies on demand, multimedia, etc., and was pushed by the telecommunications industry led by Lucent Technologies. An example of a Network-Powered Broadband Communications System would be hybrid fiber coax (HFC) delivering video and telecommunications signals simultaneously, though this new article is not limited to that wiring method. Article 830 defines two different classifications of wiring. Low-Power Circuits operate at up to 100 volts, while Medium-Power Circuits can be rated as high as 150 volts.


Read more by Brooke Stauffer

Tags:  Featured  March-April 2000 

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Secondary Ground-Fault Protection in Neon Signs and Outline Lighting

Posted By Michael Faser, Saturday, January 01, 2000
Updated: Monday, February 11, 2013

All UL Listed signs and outline lighting which incorporate electric discharge tubing are required to use neon transformers and power supplies that comply with UL 2161, the Standard for Neon Transformers and Power Supplies. UL 2161 includes requirements for secondary ground fault protection (SGFP).

Previous Situation

Prior to September 3, 1999, gas tube sign transformers and power supplies were acceptable for use in UL Listed Signs and Outline Lighting. Gas tube sign transformers are evaluated for compliance with the Standard for Specialty Transformers, UL 506. Gas tube sign power supplies are evaluated for compliance with UL 506 and the Standard for Power Units Other Than Class 2, UL 1012. It should be noted that gas tube sign transformers and power supplies continue to be listed to the requirements of UL 506 and UL 1012 for applications other than signs. The requirements in UL 1012 were developed to evaluate constant voltage power supplies (power supplies used in computers, stereo equipment and the like) rather than constant current type gas tube sign power supplies. As the gas tube sign power supply industry developed and the use of these products in signs became more prominent, it became clear that a single UL safety standard covering both neon transformers and power supplies was needed.

Standard UL 2161

In 1993 UL initiated development of a new standard, UL 2161, to evaluate transformers and power supplies specifically intended for use with neon type circuits. During the three-year development cycle of UL 2161, SGFP in neon signs was proposed for the National Electrical Code (NEC) and was subsequently adopted.

The purpose of the SGFP requirements is to reduce the fire hazards associated with neon installations. In many cases, fires could be traced to improper installation methods. However, attempts to improve installation methods by education and other means were, for the most part, not successful. Therefore, SGFP was required in the NEC as a means to reduce the number of fires without relying on improved installation practices.

It is important to note that SGFP addresses a risk of fire and not a risk of electric shock. Injurious electric shock from neon circuits has not been reported as a field problem. The SGFP requirements are different when compared with ground-fault circuit interrupter (GFCI) requirements for line voltage circuits that are intended to address a risk of electric shock.

The development of UL 2161 has been a complex technical issue that included combining the requirements for transformers and power supplies into a single standard, significant upgrades in both the transformer and power supply requirements and the development of requirements for SGFP. Many of the revisions to the Standard did not involve SGFP but were related to including requirements to address new technology and adding new tests, test conditions and construction requirements to address the field reports.

SGFP also added a complex task to the development of the UL 2161 requirements. See Fig. 1. Even though SGFP was proposed and published in the NEC, manufacturers did not have a circuit that would perform even the most rudimentary function of secondary ground fault protection. Protective circuits were available, but they only protected the gas tube sign transformer or power supply and would not sense a ground fault under an assortment of conditions and shut off the output voltage. Therefore, minimum requirements had to be developed, tested by manufacturers in field conditions, and then adjustments made to the requirements until the SGFP requirements would accomplish the intended function while considering nuisance tripping. The result was UL 2161, an 86 page standard covering the construction and test requirements used to evaluate neon transformers and power supplies. It should be noted that in addition to the SGFP requirements, UL 2161 includes many significant upgrades in requirements over the requirements used to evaluate gas tube sign transformers and power supplies.

When requirements for SGFP were included in the 1996 NEC, technical details of the requirements such as trip current, trip time, testing to determine reliability, fault conditions under which the circuits should continue to function and many other technical details were not included in the NEC. These technical details had to be developed by the standards organization. To illustrate with an example, within ½ second of ground fault current reaching 15 mA, the SGFP circuitry is required to trip. The timing is very quick. However, some people believe the allowable fault current is high, which is not the case. Where a fault to ground involves an arc to ground (heat to increase the risk of fire), the arc will draw all of the available energy in the output circuit because the resistance of an arc is a conductance (the opposite of resistance). Also, the trip current cannot be too low because there are capacitive coupling currents that could be as high as 5 to 10 mA and nuisance tripping could result.


Figure 1. SGFP also added a complex task to the development of the UL 2161 requirements.

The SGFP requirements were developed with an emphasis on the SGFP circuit being reliable, not easily defeatable, and sensitive enough to react to reduce the risk of fire. The reliability of the circuits involves subjecting the circuitry to a number of possible field conditions such as failure of the SGFP components to function due to shorting or opening, thermal aging, over and under voltage conditions, power interruption, transient surges and humidity conditioning.

One of the most significant aspects of the requirements in UL 2161 is to require that the SGFP circuitry not be defeatable without a tremendous effort on the part of a user, manufacturer or installer. Test considerations include improper installation such as not providing an equipment ground, not providing a return conductor from the load when one is required and grounding of the load instead of providing a return conductor. Additionally, the SGFP circuit must be physically integral to the transformer or power supply, within the enclosure or wiring compartment and electrically connected such that no wiring connections except normal input and output connections of the transformer or power supply are accessible to the installer. These requirements make it very difficult to defeat the SGFP circuit. See Fig. 2.


Figure 2.

All these technical requirements are included in UL 2161. It should be noted that significant revisions and additions to the standard, which required re-testing of most products, were recently added to the Standard and became effective in September 1999.

Exceptions to SGFP in the NEC

During the development of UL 2161, it was determined during the early meetings that two types of transformers were much less likely to experience a secondary ground fault condition. A lack of field reports related to specific installation scenarios provided the basis to propose two exceptions for consideration in the NEC. These exceptions were included in the 1996 edition of the NEC.

One exception is for isolated output transformers with a maximum output voltage of 7500 volts. This exception is included because without a ground reference in the secondary circuit, the high voltage does not arc to ground. However, in UL 2161 this exception does not apply to neon power supplies with high output frequencies since high frequency couples readily to ground without a grounding connection within high frequency output neon power supplies.

The second exception is for neon transformers and power supplies with integral glass or porcelain electrode receptacles that require no field wiring of the secondary circuit. This exception was included because with a single continuous glass circuit, a break in the glass would result in the high voltage being available only at the transformer or power supply terminals. Therefore, a ground fault could occur only at the transformer or power supply terminals and the probability of a ground fault was significantly reduced. In a circuit with multiple glass segments, UL 2161 specifies that the interconnection of the tube segments must be accomplished by a Listed, More Than 1000-Volt Lampholder, without the use of GTO cable. These lampholders are constructed of glass or porcelain and, therefore, the probability of these insulating materials degrading to the point of permitting a ground fault at that location is significantly reduced. See Fig. 3, page 30.


Figure 3.

Components evaluated under these two exceptions to the NEC are covered under UL’s Component Recognition Service and are not UL Listed. They are required to bear the Recognized Component Mark.

They are covered under UL’s Component Recognition Service for use in UL Listed Signs and are not intended for use as field installed components. Component Recognition Service provides UL with the ability to oversee the use of these products to establish assurance that they will be used as intended and within their limitations. Since isolated output transformers were covered by UL under UL 2161 as Listed products prior to September 1999, Listed products of this type may still be available in the distribution channels.

Additional Exceptions to SGFP in UL 2161

When the SGFP requirements were first published in the NEC, UL 2161 was under development. The objective to achieve with the SGFP requirements was the reduction in the incidence of fires. The field report data suggested that most fires were occurring in installations in which the voltage to ground was 5 kV or greater in channel letters and border neon where 12 and 15 kV open circuit voltages were driving the neon. The typical currents for such applications were between 30 and 60 mA. In these applications the amount of energy available to a potential ground fault was significant. Additionally, at voltages below 5 kV to ground, the electrostatic field around high voltage conductors is small enough to not significantly degrade insulating materials, reducing the potential for ground faults to occur.

Subsequent to the deadline for proposals for the1996 edition of the NEC, it became apparent there were other types of products that were less likely to cause a risk of fire. The necessity of requiring dedicated SGFP circuitry in all transformers and power supplies came into question based on the field report data, limited output and use considerations. It was indicated that some products could be considered as inherently protected, and UL proposed requirements that would permit some transformers and power supplies without dedicated SGFP circuitry. Therefore, the following two additional exceptions to the requirement for specific SGFP circuitry were added to UL 2161 based on the determination that they were inherently protected.

1. Neon transformers and power supplies with a maximum output current of 15 mA between all output terminals or leads and from any output terminal or lead to ground.

2. Neon transformers with a maximum output current of 33 mA (30 mA + 10 %) between all output terminals or leads and from any output terminal or lead to ground and an output voltage to ground of less than 3001 volts at a frequency of 100 Hz or less. Neon power supplies with a maximum output current of 33 mA (30 mA + 10 %) between all output terminals or leads and from any output terminal or lead to ground and an output voltage to ground of less than 2001 volts at a frequency greater than 100 Hz.

Note to item 2. Although the field report data indicated that most problems were occurring in installations in which the voltage to ground was 5 kV or above, maximum voltage of 3 kV to ground was established as an additional margin of safety.

Since the deadline for proposals for the 1999 NEC coincided with the implementation of UL 2161, it appears that proposals to add these two exceptions were not submitted.

Transformers and power supplies intended for use with cold-cathode lighting systems and marked, "This product is for use in cold cathode lighting systems intended for installation in accordance with Article 410 of the NEC” also appear as an exception to the requirements for dedicated SGFP circuitry in UL 2161. This is because these lighting systems are only installed using lampholders over 1000 volts and Article 410 of the NEC covers cold-cathode lighting systems and does not require SGFP. These components are covered under UL’s Component Recognition Service and are not UL Listed. They are covered under UL’s Component Recognition Service for use in UL Listed Cold Cathode Lighting Systems. It is important to note that these transformers and power supplies are not intended for use in or with signs or outline lighting systems installed in accordance with Article 600 of the NEC. A neon transformer or power supply using cold cathode installed as outline lighting or as a sign is required to have SGFP protection.

Implementation of UL 2161

Implementing the requirements of UL 2161 was difficult and time consuming. Many transformer and power supply manufacturers produce hundreds of different models so the volume of work was large. The evaluation of these products is time and equipment intensive and stretched UL’s resources. Additionally, some of the requirements in UL 2161 were enhanced after manufacturers had completed Listing of their products. These products had to be revised to meet the enhanced requirements of UL 2161 and submitted again for evaluation. In addition, many of the early SGFP designs had significant nuisance tripping problems and had to be redesigned. The result of all of the implementation conditions has caused delays in making products available that comply with UL 2161.

Installation Considerations

If installers do not follow the proper spacing and wire routing requirements the installation may give the appearance of a secondary ground fault. Since such a condition will result in no perceived output from a neon transformer or power supply, the installers will be motivated to learn the proper installation practices. Additionally, those who do not use components and insulating materials in the manner intended will likely find the neon transformer or power supply will not provide an output because there is a ground fault.

Identification

Transformers and power supplies that comply with UL 2161 can be identified by the category name that is included as part of the Listing Mark. The category name will be either "Neon Transformer” or "Neon Power Supply.” Additionally, transformers and power supplies that were manufactured after September 3, 1999, will be marked, "Complies with Secondary Ground-Fault Protection Requirements of UL 2161.”"

Transformers and power supplies evaluated by UL to the requirements of UL 506 or UL 1012 are not evaluated for compliance with SGFP requirements and are no longer acceptable for use in UL Listed Signs and Outline Lighting. However, these transformers and power supplies have applications other than signs, such as in copiers and amusement machines. Therefore, the requirements for gas tube sign transformers and power supplies in UL 506 and UL 1012 will not be withdrawn until 2002. This will provide time to develop requirements specifically for these products other applications. They can be identified by the category name "Gas Tube Sign Transformer” or "Gas Tube Sign Power Supply” that is included as part of the Listing Mark.

Summary

Although the requirements for SGFP have been in the NEC since 1996, their widespread usage is just now occurring with their required use in UL Listed Signs and Outline Lighting. With the implementation of these new transformers and power supplies, and recent upgrades in the requirements for GTO cable, the risk of fire in neon signs should be mitigated. It should also be noted that the 1996 NEC in Section 680-23(b) required transformers and electronic power supplies to have secondary ground-fault protection. The UL Standard 2161 still addresses this type of protection as secondary ground-fault protection, but adding the word "circuit”" was a change inserted into the 1999 NEC. In the 1999 NEC, in Section 600-23(b) the protection is now referred to as "secondary-circuit ground-fault protection (SCGFP), both in the title and the text.


Read more by Michael Faser

Tags:  Featured  January-February 2000 

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Electrode Receptacles and Enclosures

Posted By Paul R. Davis, Saturday, January 01, 2000
Updated: Monday, February 11, 2013

There are other factors that complicate the situation. Some components are UL Recognized Components while others are UL Listed. UL recognized components are suitable for use only in an overall listed end use product. These recognized components are not suitable for use as field-installed units. Other products may carry a CSA Certification which may differ in some cases from a UL Listing. CSA and UL are both standards writing bodies. Verify the components that are allowable in your jurisdiction to be sure they are suitable for the intended use and acceptable to the AHJ. The rules here are not clearly black and white. There are some gray areas that require investigation.

Electrode Receptacle Terminology

There are housings, P-K housings, housings sold under the P-K name, which are standard #100, #200 and #300 borosilicate receptacle housings. There are electrode enclosures G Cups, GG Cups, G-2s, D-2s, 2UPs (a D-2 like housing) and myriad of boots, cups, and caps, which are referred to by the Code as electrode enclosures.

A commonly used term for the non-glass and ceramic electrode enclosures referred to as boots, cups and caps is "rubber-like.” Some prefer "polymeric” instead of rubber-like. Webster defines "polymeric” as "of or relating to a polymer.” The definition of "polymer” is "a naturally occurring or synthetic substance consisting of giant molecules formed from polymerization.” Rubber-like tells a better story.

Most rubber used today is synthetic. Most everyone understands rubber-like to indicate a flexible and pliable material. That’s the type material (a flexible and pliable material) referred to in this story.

Article 600—Electric Signs and Outline Lighting of the National Electrical Code underwent major changes in 1996. The most significant involved field installed outline lighting (commonly called border tubing). The requirements for border tubing fall under Part B of Article 600 (Field-Installed Skeleton Tubing). Misapplication of the requirements of Part B has caused numerous problems with border tubing, including fires caused by poorly installed neon, and the use of combustible electrode enclosures. Many of these rubber-like boots, cups, and caps were not UL Listed or Recognized.

The earliest edition of the Underwriters Laboratories Standard for Electric Signs that can be referenced for the story is March 1930.

The following was part of the UL Requirement before 1930, the exact date is uncertain:

Electrode Receptacles—129.It is strongly recommended that electrode receptacles be used.

130. Electrode receptacles for gas tubes shall, if used, be of a noncombustible, nonabsorptive insulating material.

131. Fiber, rubber, hot moulded shellac, or phenolic compositions are not considered suitable materials for electrode receptacles for ounting of high potential parts.

132. A rubber gasket may be used in connection with the mounting of an electrode receptacle.

133. A rubber gasket may be used around a gas tube in an electrode holder to prevent the entrance of dust, moisture, etc., provided the gasket is not depended upon for the insulation of the tubing and is not in contact with grounded metal such as the sign face.

134. An electrode receptacle shall not permit water to accumulate therein and thereby form a conducting path to grounded metal such as the sign face.

135. Each electrode receptacle shall be of substantial construction and shall provide means for reliability securing it to the sign face. Electrode receptacles shall not be secured to sign faces by means of sheet metal screws.

136. Each electrode receptacle shall be constructed so that, when installed, the spacing mentioned elsewhere in these requirements will be maintained.

This may seem like ancient history to some, but if every sign and neon installer followed these eight basic rules for neon installations, the amount of fires would be minimized and the reliability of the average neon system would be greatly improved. The removal from the 1999 NEC of "Electrode receptacles for the tubing shall be of noncombustible, nonabsorbent insulating material” for wet location situations, created a whole new set of problems.

Now back to the main subject at hand. There are a dozen varieties of glass electrode receptacles and enclosures available. Most of these are available from more than one manufacturer, and the others are available through at least three master distributors, and dozens of sign supply distributors.

The glass electrode receptacles (#100, #200 and #300 housings) are generic and are available from at least three manufacturers. The rubber-like electrode enclosures, polymeric boots, cups, and caps are not so generic. They are all different in configuration to some extent. They have different conditions of acceptability, and the conditions are not readily available in the exact form stated in the listing and/or recognition.

There are presently no polymeric or plastic boots, cups or caps to cover an electrode, straight or double-back that are UL Listed for wet locations. Be sure to note, this is UL Listed and, as indicated earlier, some of these products have CSA Component Acceptance for certain uses in wet locations. The conditions of acceptability with the CSA Component Acceptance are not absolute, so each product should be verified for specific uses with CSA. Contact CSA directly. As always, acceptability to the AHJ should also be verified. If in doubt, contact the local inspector or authority having jurisdiction and communicate.

CSA Component Acceptance Information

The old CSA Component Acceptance Mark is a CA connected like script. From Appendix E—CSA Marks, E.3: "The ‘old’ Component Acceptance Mark which indicates that a component has been accepted by CSA International on the condition that the use in an end-product is subject to further investigation.

"The mark is being phased out and replaced by the Component Marking, item E.9.15.”

The new mark is the standard CSA circular mark with an equilateral triangle at the lower right. From Appendix E—CSA Marks, E.15: "Component Acceptance Marking, intended primarily for factory-installed components, indicates that a component has been evaluated and accepted for specific applications and/or requirements, as stated in the accompanying literature.

 
Photo 1. In the early days, many housings were made of porcelain glazed ceramic.

In the early days, many housings were made of porcelain glazed ceramic. (See Photo 1.)

Photo 2 shows the manufacturer (KOLUX), Model No. 112, manufactured in U.S.A. and the voltage rating of 7500V. TRANSCO now has a porcelain glazed ceramic insert for the "P-K Type” Metal Clad Housing.

Photo 3 is another unusual housing made from borosilicate glass. The brass honeycomb component at the bottom of the housing is the contact for the electrode. The honeycomb replaces the spring contact in the #100, 200, and 300 styles. The electrodes used with this system had the equivalent of a nail about 1/2-inch long sticking out the end of the electrode where the wire is on current electrodes. This nail established the contact with the honeycomb brass device. The high-voltage power was connected to the wire projecting from the right side of the housing. This was covered by a separate piece of heat resistant glass (not shown) to insulate the connection from objects near the terminal point. The housing was held in place with a friction fit metal ring in most cases.


Photo 2. The manufacturer (KOLUX), Model No. 112, manufactured in U.S.A. and the voltage rating of 7500V.

Photo 4 is looking directly into the housing from the top. The honeycomb brass contact, and the nail in the electrode, provided a foolproof method of connection. You can see the electrode at the left of Photo 5. This housing and electrode are from the mid to late 1920s.

You have all heard of Pyrex glass. All Pyrex is borosilicate, but not all borosilicate is Pyrex. One blend of Pyrex, No. 7740 has a softening temperature of 820º C, while common borosilicate has a softening temperature of only 593ºC.

This of course is compared to most of the rubber-like materials, which are rated at a maximum of 105º C. The borosilicate will not burn, and the polymeric materials do melt and/or burn.


Photo 3. Another unusual housing made from borosilicate glass.

There is another major factor in the rubber-like electrode enclosures vs. glass electrode receptacle debate. All glass housings, including all the G Cup series must be securely attached to the background (sign or building) to meet UL Requirements. None of the rubber-like boots and cups presently has to meet this same requirement. This attachment to the background is critical to both safety and reliability.

All of the glass electrode enclosures are UL Listed for dry, damp and wet locations, except for the casino bushing, and it is UL Recognized. D-2 housings are not UL Listed for wet locations either. The 2UP is the only "D-2 Type” which is UL Listed for wet locations.


Photo 4. Looking directly into the housing from the top. The honeycomb brass contact, and the nail in the electrode, provided a foolproof method of connection.

The G Cup is the only UL Listed housing for double backs which has a wet location listing when installed with proper orientation. See Drawing – 19R and Drawing – 23a to see how they must be installed to meet the conditions of the listing. The GG Cup shown in Drawing – 50 is also the only receptacle available for this type installation which is UL Listed for wet location. The 45º angular positioning on both of these devices is part of the listing requirement.

The G-2 Cup in Drawing – 47 is to be used in a channel letter with a housing type transformer. This is required in 600-23(b)(2). Field wiring as required for boots and caps is not allowed with housing type transformers. A D-2 [see Drawing – 52] or the 2UP [Drawing – 52(b)] can also be used for this.

The glass housings are numbered as follows, and you will find drawings of each in this story.


Photo 5. You can see the electrode at the left of Photo 5. This housing and electrode are from the mid to late 1920s.

The most common housings, prior to the advent of the rubber-like products, were the #100, #200 and #300 borosilicate housings. In the early days, they were all Pyrex.

The #100 is the largest diameter [Drawing – 55 (d)] and requires a panel opening of 1-3/4 inches. The #100 has a spring contact to connect the electrode to the secondary wiring system. This housing, allows water to wash the bugs and dirt out of the housing, the best of all the glass panel mounted housings except for the #300.


Drawing 19R. The G Cup is the only UL Listed housing for double backs which has a wet location listing when installed with proper orientation. See Drawing 19R and Drawing 23a to see how they must be installed to meet the conditions of the listing.

The #200 housing comes in two lengths. It is identical in basic configuration to the #100, but is smaller in diameter, and requires only a 1 5/16-inch panel opening. The #200P is 3.125-inches in overall, compared to 4.563-inches for the #200. The length is the only difference between the #200 and #200P.

The casino bushing (Drawing – 54) is the smallest glass housing available and, as pointed out earlier, is UL Recognized, not UL Listed. The panel opening is only 1-inch in diameter. The electrode connection to the secondary is made with a twisted wire connection, or better yet the connection can be made with a open end crimped type splice cap. This type of crimped connection is much more reliable than a twisted wire connection.


Drawing 23a. The G Cup is the only UL Listed housing for double backs which has a wet location listing when installed with proper orientation. See Drawing 19R and Drawing 23a to see how they must be installed to meet the conditions of the listing

The #300 and #300P probably cause the least problems of all the housings from #100 to #300 series of glass housings. They are open back as you can see in Drawing – 55(b). The #300 makes the secondary connection with a spring contact, and the #300P uses a spring clip. The positive connection of the spring clip to the electrode wire is by far the best way to go. With this method there is no room for error. It’s connected securely and misalignment doesn’t matter. With spring connections, misalignment is always possible, and can cause problems.


Drawing 50. The GG Cup shown in Drawing 50 is also the only receptacle available for this type installation which is UL Listed for wet location. The 45º angular positioning on both of these devices is part of the listing requirement.

Using electrode caps is a good approach. This provides a much larger surface area for contact between the spring and the electrode. The electrode button can easily slip by the spring, and cause an arc. The arc is contained in the glass housing, and rarely, if ever, will cause a major problem like a fire but, simply put, the electrode button is just not as reliable as the electrode cap. (Arrow 2 points to the electrode cap on the neon tube in photos 1 and 5.)

Last, but not least, is the P-K Type metal clad housing [Drawing – 55(c)]. This is probably the most maligned electrode receptacle in the neon industry. It has been wrongly blamed for more problems associated with neon installations. The improper installation and connection of the device is, and has always been, one of the major problems.


Drawing 47. The G-2 Cup in Drawing 47 is to be used in a channel letter with a housing type transformer. This is required in 600-23(b)(2). Field wiring as required for boots and caps is not allowed with housing type transformers.

Here’s a little bit of history about the P-K. The original patent application was filed March 12, 1947. Patent 2,488,065 was issued to C. M. Peterson on November 15, 1949.

From the Patent: "The invention relates to high tension terminal housings and has particular reference to insulator thereof.

"The principal object of this invention is to produce a housing and an insulator for high tension use, which will prevent the corona effect between the high tension cable and its conduit at a point adjacent the terminal.

"A further object is to produce a device which may be used with ordinary equipment now in use without materially altering its construction.

"A still further object is to produce a device which is economical to manufacture, easy to use and one which complies with all standard insulating requirements.”

 


Drawing 52. This is required in 600-23(b)(2). Field wiring as required for boots and caps is not allowed with housing type transformers. A D-2 [see Drawing 52

One improvement on the original patent was the introduction of a metal clad housing with a one-piece flange to fasten the shell to the channel building, letter body, or sign cabinet. This provides a positive bonding connection between the shell and the letter or cabinet. The flange will also hold the housing tightly to the building. Other metal clad housings have a two-piece flange device which clamps around the basic metal shell of the housing, but are somewhat more difficult to install.

 

There are many opinions about what’s best when it comes to neon installations, and especially neon receptacles. The facts outlined here are based on field experience with the various components. Safety and reliability should always be priority one even though, today, too many times it is not a priority.

 


Drawing 52(b). This is required in 600-23(b)(2). Field wiring as required for boots and caps is not allowed with housing type transformers. A D-2 [see Drawing 52

The lack of proper bonding and grounding of the high-voltage part of a neon system, is most often the primary cause of poor reliability and fires. If all metal conduit is used, the bond is hard to lose. When PVC and other nonmetallic materials are used in the conduit system, it’s necessary to run a separate bonding conductor. This part of the equation becomes very difficult, especially with the changes to the Code in 1999.

 

The 1999 NEC added the following requirement for nonmetallic conduit.

"600-7. Grounding. Signs and metal equipment of outline lighting systems shall be grounded. Listed flexible metal conduit or listed liquidtight flexible metal conduit that encloses the secondary wiring of a transformer or power supply for use with electric discharge tubing shall be permitted as a bonding means in lengths not exceeding 100 ft (30.5 m). Small metal parts not exceeding 2 in. (50.8 mm) in any dimension, not likely to be energized, and spaced at least 3/4 in. (19 mm) from neon tubing shall not require bonding. Where listed nonmetallic conduit is used to enclose the secondary wiring of a transformer or power supply and a bonding conductor is required, the bonding conductor shall be installed separate and remote from the nonmetallic conduit and be spaced at least 1 1/2 in. (38 mm) from the conduit when the circuit is operated at 100 Hz or 1 3/4 in. (44.45 mm) when the circuit is operated at over 100 Hz. Bonding conductors shall be copper and not smaller than No. 14. Metal parts of a building shall not be permitted as a grounded or equipment grounding conductor.


Drawing 54. The casino bushing (Drawing 54) is the smallest glass housing available and, as pointed out earlier, is UL Recognized, not UL Listed. The panel opening is only 1-inch in diameter.

"FPN: Refer to Section 600-32(j) for additional restrictions on length of high-voltage secondary conductors.”

Accomplishing the feat of stringing a bonding wire "1–1/2 in. (38 mm) from the conduit when the circuit is operated at 100 Hz or 1 3/4 in. (44.45 mm) when the circuit is operated at over 100 Hz” is not a simple operation. The bonding wire is exposed and susceptible to physical damage.

One other critical part of this situation here is the fact that the most important part of the high-voltage system (the bonding wire, the path to a safe ground.) is 1 1/2 to 1 3/4 -inches away from the components that can start the fires.

Neon systems are not all that difficult to install. Good planning and following the rules of the game are the two obvious keys to a safe and reliable installation.

Well made and correctly installed neon will last for 30,000 plus hours. It will be virtually free from fire hazards. If getting someone’s attention is important, brightly illuminated neon at night is one of the best answers.


Read more by Paul R. Davis

Tags:  Featured  January-February 2000 

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Determining Proper Loading for Neon Sign Transformers

Posted By Telford Dorr, Saturday, January 01, 2000
Updated: Monday, February 11, 2013

Neon sign transformers differ from most other types of transformers one is likely to encounter. Unlike a more conventional transformer, for normal operation a neon sign transformer is specified to operate a minimum, as well as a maximum load. Why is this? To understand this requirement, we must look at what makes a neon sign transformer different from other types.

Basics


Figure 1. This regulation is accomplished by inserting a ferrous magnetic shunt (fig. 1) into the transformer core, such that the magnetic flux from the primary winding has an alternate (although high impedance) path around the secondary winding.

Neon tubes require a high voltage at a low current to operate. This power is supplied by a specialized transformer. Secondary voltages typically range from 1,000 to 15,000 volts, and secondary currents range from 20 to 60 milliamps (and higher, for large diameter "cold cathode” tubing). The current passing through a neon tube needs to be limited by some means, otherwise once the tube lights, the current will rise to an excessively high value. This regulation is accomplished by inserting a ferrous magnetic shunt (fig. 1) into the transformer core, such that the magnetic flux from the primary winding has an alternate (although high impedance) path around the secondary winding. As the current draw on the transformer secondary winding increases, more primary magnetic flux diverts1 through the magnetic shunt. While this gives the transformer a poor voltage regulation characteristic, it also tends to keep the neon tube operating current reasonably constant. We may electrically model this type of transformer as a conventional transformer which has an inductor in series with each of its high voltage secondary leads2 (fig. 2).


Figure 2. We may electrically model this type of transformer as a conventional transformer which has an inductor in series with each of its high voltage secondary leads

Note that some transformers may have more than one secondary winding and more than one secondary shunt. If a transformer has two secondary windings, the midpoint connection between the secondaries may be grounded to the transformer case. Depending on the exact configuration of shunts and secondary windings, a transformer may be referred to as having either a "balanced” (fig. 3) or an "unbalanced” (fig. 4) design, and this in turn determines what types of secondary wiring methods may be used. Refer to the transformer manufacturer’s literature for more details on this.

This constant current nature of neon sign transformers allows one to greatly vary the tube loading on a transformer. Unfortunately, one can radically misload a transformer and it will still appear to work, in the short term. Long term, transformer failure will usually result. For any given transformer, the tube load should fall within specified limits. It must not be too high or too low3. The question is, how does one determine the proper loading?

Methods of Determining Proper Loading


Figure 3. Depending on the exact configuration of shunts and secondary windings, a transformer may be referred to as having either a "balanced" or an "unbalanced" design, and this in turn determines what types of secondary wiring methods may be used.

There are several answers to this question. The most commonly used method is by reference to manufacturer supplied loading charts. These charts indicate the minimum and maximum total length of neon tubing that may be used on a given transformer, as a function of the tube diameter, gas fill pressure and type of gas used (typically either straight neon or an argon/neon/mercury vapor mix). When using a chart, one deducts some amount of tube length for each pair of electrodes used (which occurs when multiple tubes are wired in series.) Other rules-of-thumb allow compensation for connecting tubes of varying diameters and gas fills in series. While this loading method seems straightforward and simple enough, and is certainly a good place to start in determining proper transformer loading, it does not always produce the desired results. This is because neon tubes may not exhibit the standard characteristics the loading charts are based on, due to processing variations and other factors. Therefore, we need to understand a little more about transformer characteristics, and look at some alternate loading techniques.

Checking Transformers

In the USA, neon sign transformers are rated primarily in terms of their open circuit secondary voltage and their short-circuit secondary current. While these ratings are specifically intended for use in calculating proper loading, they are also useful in determining if a neon sign transformer is functioning properly. The open circuit voltage may be measured with a good voltmeter, equipped with a high voltage probe. Typically, the higher voltage transformers (6000 volts and above) have their secondary winding midpoints grounded to the transformer case. The voltmeter common lead is connected to the case and a voltage measurement is made at either secondary terminal. The sum of these readings should equal the secondary rating. Be sure to measure the primary voltage, as the open circuit secondary voltage will vary proportionally with the primary voltage.

Secondary current may be measured by connecting an AC milliamp meter directly across the secondary terminals. The transformer will withstand this short circuit for a reasonable amount of time, because the magnetic shunts previously described allow what would otherwise be seen as abuse to be tolerated by the transformer primary winding. Again, the current reading should be reasonably close to the secondary short circuit value on the transformer rating plate.


Figure 4. Depending on the exact configuration of shunts and secondary windings, a transformer may be referred to as having either a "balanced" or an "unbalanced" design, and this in turn determines what types of secondary wiring methods may be used.

Be sure to check the manufacturer’s literature for their specific recommendations on testing SGFP type transformers, as the above general test methods may not work properly.

This brings us to a second method of determining proper transformer loading. For US made transformers, the typical transformer secondary operating current is approximately 80 percent of the rated short circuit current. For a transformer rated at 30 mA, this would be approximately 24 mA. We can connect a milliamp meter in series with a transformer’s tube load and measure this current. Various manufacturers make high voltage milliamp meters specifically for this task.

European Methods

I am told that in some areas of Europe, transformers are specified somewhat differently than in the USA. Typically, a transformer is rated in terms of its open circuit secondary voltage, its operating secondary current, and its "G” factor. The "G” factor is the desired ratio of loaded to open circuit secondary voltage. Typically, the "G” factor is around 0.5 (although this may vary), meaning for example that a transformer rated at 9000 volt open circuit should operate with a tube load connected at around 4500 volts. This may be measured conveniently with a voltmeter equipped with a high voltage probe. This is the preferred method of checking the loading on a European transformer, and in practice is somewhat easier and more sensitive than measuring the tube operating current. This method is starting to catch on in the USA as well. Unfortunately, US manufacturers don’t specify the "G” factor, but typically a value of 0.5 may be assumed.

One quick note here: when replacing a European transformer, the "G” factor must be taken into account. A replacement transformer with a different "G” factor will not operate a given tube load properly, even though its rated open circuit secondary voltage and operating current may be the same as the that of the original transformer.

Choosing a Loading Check Method

One would think that by using a loading chart in combination with measuring the secondary operating current and voltage that correct tube loading may always be determined. Unfortunately this is not always the case. There is one more variable in the mix: stray capacitance. This capacitance results from the close proximity of high voltage secondary wiring to its enclosing conduit (or other grounded metal objects), and between the neon tubing and the sign sheet metal. It is desirable to minimize this capacitance as much as possible. This means that any secondary wiring operating at high voltage relative to ground should be kept as short as practical, as capacitance varies directly with wire length. Various standardized wiring techniques, such as "mid-point return” and "virtual mid-point” have been devised to accomplish this goal. Both of these techniques involve keeping the wiring between the transformer and the electrodes on the first tube as short and direct as possible. Needless to say, the wiring techniques used must comply both with those methods allowed by the transformer manufacturer and with methods allowed by the NEC (for US installations), or with the appropriate European regulations for installations done there.

So what happens when capacitance intrudes in a neon installation? Two things, both bad. First, capacitance tends to counteract the current regulation of the neon sign transformer. Specifically, it tends to cancel the inductance in the secondary circuit of our previously described transformer model. This can cause excessive secondary, and thus, tube current. Unfortunately, installers tend to counteract this effect by adding additional tubing load to the transformer, or by selecting a smaller transformer. While this tends to restore the operating current, it is at the expense of operating voltage, which tends to rise excessively, leading to transformer failure. This is why both the operating voltage and current should be checked.

With some types of NEC 600-23(b) SGFP type transformers, the stray capacitance seen by either transformer secondary terminal should be similar. An imbalance may be interpreted by the protection circuitry inside the transformer as a fault, causing "nuisance tripping.” One of the best ways to avoid this situation is to insure that the GTO wires connected to either transformer secondary terminal be reasonably equal in length, as well as keeping them as short as possible.

Second, stray capacitance in combination with higher voltage transformers (typically units over 9000 volts) operating neon-filled tubes may invite transformer secondary circuit oscillations. These oscillations sometimes manifest themselves as flickering tubes and "buzzing” transformers, and are extremely destructive to both the transformer and to the high voltage wiring. Neither a voltmeter or a milliamp meter will conclusively detect this condition. Using an oscilloscope4 equipped with a high voltage probe, these oscillations may be easily seen. Other than reducing the capacitance as much as possible by mechanical methods, the best solution to this problem is to re-layout the sign wiring to use lower voltage transformers. This will, of course, require using more transformers to do the job.

Solid State Transformers

We have neglected to include so-called "solid state” transformers in our discussions of proper loading. This is because the operating frequencies used by these transformers preclude the use of regular voltmeters and milliamp meters, as they do not function well at these frequencies. The oscilloscope still works well, but few sign shops have them. On the other hand, most of these transformers are designed to operate with widely varying loads. When using these transformers, it is best to carefully follow the recommendations in the manufacturers’ data sheets.

Acknowledgments

Before publishing, this article has been passed for review to the members of the INA "Codes and Installation Forum.” I would like to thank everyone who reviewed this article for content and accuracy. Your assistance is greatly appreciated.

1 This is a bit of a simplification, but for purposes of illustration, it’s close enough.

2 Specifically, this is a model of a "balanced” midpoint grounded type of transformer. See figure 3 for a physical illustration.

3 A note on terminology: sign installers refer to a transformer as being "overloaded” or "underloaded” with tubing. Unfortunately, this refers to the length of tubing connected, not the electrical condition, which tends to be the opposite of that of the tubing. This is to say, when a transformer is "underloaded” with tubing, the transformer tends to draw excessive primary winding current.

4 Unfortunately, this type of equipment has been, in the past, both expensive and awkward to use in the field. Newer compact / portable equipment coming into the marketplace may make this measurement technique more practical in the future.

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Neon Tubing Secondary Wiring Methods

Posted By George Doll, Saturday, January 01, 2000
Updated: Monday, February 11, 2013
This article is intended to address a few of the common neon installation challenges associated with the secondary wiring from the transformer or power supply to the electrodes, as observed by a member of both the life safety and neon communities.

Secondary wiring methods as required by the National Electrical Code, Section 600-32

A common misunderstanding is that standard 600-volt twist-on wire connectors can be used to make high-voltage connections. This is not true; these types of connectors may not be used for these connections, according to Code. There are listed high voltage splice termination enclosures suitable for this purpose. High voltage GTO cable should not be installed in 3/8″ flexible metal conduit due to the effects of capacitance. High-voltage GTO cable cannot be paired in the same conduit or be run more than 20 feet off the output terminals of the transformer in metal conduit.

Besides these issues, one should understand the impact of Section 600-23(b), Secondary-Circuit Ground-Fault Protection, or its exceptions regarding transformers. This information is paramount for the well being and safety of persons and property. These transformers provide a level of ground-fault protection for the secondary circuit wiring. Included in the secondary wiring circuit is the point of transition from code wiring methods to connections at the electrodes and tubing. One should also understand the aspects of Section 600-42(f)&(g), which addresses wet locations and electrode enclosures, which will be the focus of this article.


Figure 1. The dotted white line represents the route of the border neon, but where do we mount our trannies? How do we design our glasswork?

Regarding electric-discharge outline lighting, which is used most frequently in the neon industry, one should ask whether one is "…familiar with the construction and operation of the equipment and the hazards involved,” or with the listed acceptable components as presented by the United States standards laboratories and the United States-based independent third party electrical safety testing laboratories.

Does familiarity with these things matter? If staples are becoming a "now and again method” of installing high-voltage neon cable all over a pine or cedar wet-location commercial site, these violations of the NEC and the unsafe conditions they present should be addressed.

Electrode enclosures in wet locations


Photo 1. When dealing with exposed high voltage wiring in a "wet" location, it is imperative to adhere to space and listed "wet" location components. Exp. 3-1/2" glass "standoffs" with glass "wet orientated" electrode enclosures.

In many parts of the country, exposed wet-location neon and argon/mercury tubing are often the outline lighting system of choice. However, in the far north, even with cold-weather gases, long runs of glass are not practical in a -40°F outdoor environment. With high average snowfall, the resulting snow pack on a building’s roof can pull down glass as easily as it can shingles. Therefore, most neon tubing installations tend to be channeled with clear faces and listed per a third party independent electrical safety testing agency prior to acceptance by the authority having jurisdiction.

Climatic differences can have an impact on electrical installations in areas where the winter temperature tends to be only as cold as an early fall day in Vermont. Needs and parameters may change with the conditions. The interpretation of the Code for exposed border neon rests on NEC Article 600-3(a), Listing, applied in conjunction with Chapter 6 Part B, 600-30, Field Installed Skeleton Tubing.


Photo 2. In comparison, this border and lettering display exhibits virtually every code violation possible. Total absence of grounding or bonding; inadequate spacing; illegal "wet" location electrode enclosures, etc.

These sections of the Code were recently revisited during a prototype installation for a restaurant chain in the southeast part of the country. This site, besides having listed neon displays, called for 324 feet of 15-mm white, wet-location border tubing. After reviewing the specs and corporate signage standards that had been written in 1997 (and revised in October 1998), it was realized that some of the issues described no longer met the letter of the ’99 NEC or that current listing standards had become a concern.

Another problem was that various distributors in the locale did not carry many of the items needed to meet the minimum NEC requirements. The best approach at attaining compliance and safety the first time appeared to be teaming up with transformer and component manufacturers for help. Through the efforts of the authority having jurisdiction (electrical inspector), manufacturers, and other valuable industry resources, many issues were discussed, evaluated, and resolved. Communication certainly is a key component for success. [See photos 1 through 4]

Focusing on the entire secondary circuit

Electrode enclosures [Section 600-42(g)], spacing, and secondary-circuit ground-fault protected or isolated transformers [Section 600-23(b)] were the main areas of concern with this project, as they are with most neon installations of this type.

First, there are currently no listed "wet location” rubber or polymeric electrode enclosures (boots) per our United States based Nationally Recognized Testing Laboratories (NRTLs). The only listed organic products allowable in a wet location, and required for this project, were the polymeric electrode receptacle covers for our two metal electrode housings. Also, it should be understood that polymeric or plastic sleeving could not be used as a stand-alone wiring method for these high voltage secondary circuits. Therefore, in the consensus of all parties involved, the majority of the design had to be based on listed wet-location glass electrode enclosures and listed wet-location glass conduit plug assemblies.


Photo 3. Code violation alert. This is no way to wire in a secondary high voltage system. One shall not take high voltage cable and use "soda" hose as a wiring method. Only listed "wet" location conduit with proper conduit connectors is allowable.

The next area of concern was the fact that 324 feet of this border tubing was to be mounted on dry, stained and painted cedar, with roof flashing and metal capping appearing at various points along the route. In any electrical installation, space is usually an issue. However, in the world of high-voltage discharge lighting (neon/argon) where capacitance is an ever-present concern, routing and length of the secondary conductors needs to be addressed carefully.

The neon industry produces a 3½” long glass tube support, which, in the case of border tubing, is a minimum acceptable length for this type of installation. This minimum exists because the required clearances cannot be maintained when utilizing the standard 1–3/4-inch tube supports at a neon double-back. (Note: A neon double-back is a glass/electrode design in which the electrode is bent and brought up behind the visual length of the glass.)

The last issue under this scope has to be the wireway layout and the design of the transformer secondary circuit conductors. There is an aspect of long lengths of outline/electric neon discharge lighting that must be understood for achieving a safe installation. At the restaurant neon installation site, the average length of border glass that one of the many transformers supplied was around 60 feet. By conventional (series) wiring, we would have 30 feet of GTO wire running inside of 30 feet of metallic conduit out to each end of that section, which would be in violation of the Code. However, more importantly, even the compliant 20 feet used in this manner could be very destructive to both higher voltage 30 mA transformers and to the HV system being supplied by such transformers.


Photo 4. Code violation alert. Without the proper spacing and listed "wet" location electrode enclosures some installations are sure to cause problems--as indicated by the charring behind the rubber boots.

The neon industry and its transformer manufacturers have advocated alternative methods of wiring these runs of glass tubing. The methods applied are called "midpoint” and "virtual mid-point” wiring. This practice minimizes the length of GTO in conduit between the neon transformer HV outputs and the closest pair of electrodes to be energized. In reality it could take a 17- to 20-foot run of energized high-voltage cable in metallic conduit and reduce those runs to 6 inches. This topic, however, is an article by itself.

A prudent avenue to follow in regard to border/electric-discharge outline lighting may be to have this border tubing fabricated in listed (channelized) fixtures and then wired in accordance with Chapter 3 of the NEC.

Need for training

Perhaps the most common challenge is the need for training in Article 600, Electric Signs and Outline Lighting. To say that the neon industry needs training in the rudimentary aspects of commercial electrical wiring practices would not be an understatement. This training should not just be about box fill calculations or ampacity tables; it should also be about grounding and bonding, twisting off of conductors, bending pipe and other rudimentary jobs. Many contractors and installers in the neon industry have received little electrical trade training and, consequently, do not fully understand Article 600 of the NEC and many, unfortunately, are not aware of the existence of the Code. On the other hand, many electrical inspectors have received little neon trade training. Too often, this lack of training in the proper procedures of electrical wiring when it comes to the installation and inspection of neon, that special light source that adorns most commercial structures, could inadvertently lead to fires, ranging from the minor to the serious.

Final thoughts on education

Communication and education are the two main ingredients for safe and code-compliant neon installations. If one feels he is not qualified or is in need of improving knowledge and understanding in Article 600—one of the most misunderstood sections of the Code—the following symposiums offer a chance to turn a weakness into the strength it should be.

Those involved in the industry should be encouraged to attend one of the four symposiums scheduled for this year: January (Fort Worth); mid-February (Normal, IL); June (Columbus, OH); August (Anaheim, CA). These symposiums are either free or charge a nominal fee to the life safety community.

Then there is testing and education on new products that come to market. Since moving the corporation from Vermont to the foothills of western North Carolina, we have encountered opportunities and situations that shaped the foundation of this article. In conjunction with the issues discussed above, four of the major ferro magnetic manufacturers afforded us the opportunity to install a variety of new required 600-23 ’96 & ’99 code-compliant (UL 2161) power supplies. These transformers, which in most cases were just coming off the production lines, did not demonstrate any of the problems that were prophesied, such as nuisance (false) tripping or unreliability.

If we focus on education instead of our differences and band together the neon industry, electrical inspectors, and manufacturers for the common purpose of life safety, we will all have made a giant stride forward.


This article has been passed for review before publishing to various state and provincial electrical inspectors, manufacturers, electrical safety associations, corporate/industrial safety officers and the INA "”Codes and Installation Forum.”" Special thanks to: Les Beros, Sask Power, Regina, Saskatchewan; Regan Dickinson: Sign Business Magazine, Broomfield, CO; Telford Dorr, Microtron, Encinitas, CA; Jeff Hinkle, City of Hickory Code Enforcement, Hickory, NC; Kathryn Ingley: IAEI, Richardson, TX; Mike Johnston: IAEI, Richardson, TX; Al Smith, France Transformers, Fairview, TN

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Back to Basics: Grounding and Bonding as it Relates to Signs and Neon Installations

Posted By Michael Johnston, Saturday, January 01, 2000
Updated: Monday, February 11, 2013

Some features of electrical circuits and electrical systems are so fundamental they have appeared in some form in every edition of the National Electrical Code. These include insulation for wire type conductors, conductor (wire) sizing, and overcurrent protection for circuits (fuses or circuit breakers). Another long-time electrical safety requirement is grounding of electrical systems and equipment for safety. The grounding of metal electrical equipment and metal enclosures has been practiced in some quarters since the use of electricity began. This article will focus on grounding and bonding requirements as they relate to metal parts and metal equipment of electric signs and neon installations.

Figure 1. The term "grounded" is defined in the NEC in Article 100

Fundamentals

The term "grounded” is defined in the NEC in Article 100 as, "Connected to earth or to some conducting body that serves in place of the earth.” The earth as a conductor is assumed to have a voltage potential of zero. Conducting bodies that serve in place of the earth can include, but are not limited to, conduit, metal enclosures, transformer cases, raceways, etc. Basically when metal equipment is grounded it is connected to the earth. (See Figure 1).


Figure 2. Grounding or earthing metal electrical enclosures puts both the earth and the metal enclosure at the same potential (voltage).

This can be accomplished in a few different ways. A metal object such as a box or other equipment enclosure that is grounded by connecting (bonding) it to the earth by means of an equipment grounding conductor to a grounding electrode conductor and, finally, to the grounding electrode (the conducting element connection to earth) of the system is thereby forced theoretically to take on the same zero potential as the earth. Grounding or earthing metal electrical enclosures puts both the earth and the metal enclosure at the same potential (voltage). (See Figure 2).


Figure 3. When a grounding conductor (could be a wire, conduit, or raceway) is broken, is inadequate in size, is not connected or has a poor connection, a hazardous, above-ground potential on the metal object may be present, creating a shock and fire haza

Any attempt to raise or lower the potential (voltage) of the grounded objects results in the passing of current (amps) over the grounding path until the potential (voltage) of the objects and the potential (voltage) of the earth (zero) are equalized. Usually, this above-ground potential is caused by a line- (hot conductor) to- ground fault. With both the metal enclosures and earth at the same potential (electrically), shock hazards are reduced and an electrically conductive path for any fault current to flow is established. When a grounding conductor (could be a wire, conduit, or raceway) is broken, is inadequate in size, is not connected or has a poor connection, a hazardous, above-ground potential on the metal object may be present, creating a shock and fire hazard. (See Figures 3 ,4 and 4a).

Grounding Electrical Systems and Equipment


Figure 4. When a grounding conductor (could be a wire, conduit, or raceway) is broken, is inadequate in size, is not connected or has a poor connection, a hazardous, above-ground potential on the metal object may be present, creating a shock and fire haza

The grounding of an electrical system and equipment is usually accomplished at the electrical service equipment of a building or structure. The grounded conductor (usually the neutral or white conductor) and metal enclosure of the electrical service are connected to the earth by using a grounding electrode conductor which connects to a grounding electrode system. [See figure 5]

Once this system and metal enclosures are grounded, the power is then distributed to the electrical panels with feeder conductors (the larger conductors supplying power to the electrical panel) that include an equipment grounding conductor, and finally to the branch circuit conductors (the conductors between the final fuse or circuit breaker in the electrical panel and the electric sign or neon transformer enclosure), which include an equipment grounding conductor for grounding the non-current-carrying metal parts of electrical equipment. These equipment grounding conductors are the extension of the grounding circuit, and as a result, are the conducting body that serves in place of the earth. (See Figure 6)


Figure 4a. When a grounding conductor (could be a wire, conduit, or raceway) is broken, is inadequate in size, is not connected or has a poor connection, a hazardous, above-ground potential on the metal object may be present, creating a shock and fire haz

Bonding of Electrical Enclosures and Metal Parts

Bonding of electrical equipment and enclosures simply means that the enclosures will be connected together in an appropriate manner to ensure electrical continuity and to ensure the capacity to conduct safely any fault current likely to be imposed on those enclosures. When a metal conduit is connected to a metal electrical junction box with a proper conduit connector or proper fittings, the two parts become one electrically because they are bonded together. (See Figure 7)

It is important that all connections and metal continuity be installed and maintained wrench tight. Wrench tight is a workmanship issue. The pride of workmanship must be held in high regard to comply with the rules and safety aspects of electrical installations contemplated by the NEC. It is important that care be taken to tighten locknuts and setscrews of all fittings as they enter electrical enclosures (i.e. junction boxes, timeclocks, electrical panels, transformer boxes, etc.). Loose connections can lead to arcing conditions when conduit or equipment grounding circuits are called upon to carry fault current. Loose connections can also lead to isolated metal equipment and enclosures, which become a silent and sometimes lethal shock hazard when energized.


Figure 5. Purpose of equipment Grounding Conductor

The Scope and Purpose of Grounding

The scope of grounding and bonding and the general requirements of grounding and bonding are contained in Section 250-2 of the National Electrical Code. These requirements include:

• Grounding of electrical systems,

• Grounding of electrical equipment,

• Bonding of electrically conductive materials and other materials, and

• Performance of the fault-current path.

Using the National Electrical Code


Figure 6. These equipment grounding conductors are the extension of the grounding circuit, and as a result, are the conducting body that serves in place of the earth.

The Code requirements for electric signs and neon installations are found in Chapter 6, Special Equipment, and specifically in Article 600 of the NEC. Grounding and bonding requirements for electric signs and neon installations are outlined in Sections 600-7 and 600-32. It should be pointed out that all of the Code rules in Article 250 are applicable to signs and neon installations unless the rules in Article 600 modify or amend those general requirements.

Section 90-3 of the NEC explains the basic arrangement of the Code. Chapters 1 through 4 apply generally, except as amended by Chapters 5, 6, and 7 for the particular conditions. The NEC contains the minimum requirements for electrical installations that are essentially safe, thus one must do at least that much. The main purpose of the NEC is the protection of persons and property from the hazards that arise from the use of electricity.


Figure 7. When a metal conduit is connected to a metal electrical junction box with a proper conduit connector or proper fittings, the two parts become one electrically because they are bonded together.

Back to Basics—The Path for Normal Current and Ground Fault Current

It is important to have a basic understanding of the paths for electrical current. For electrical current to flow properly it must have an adequate path. In the normal electrical circuit, current will seek out the source, taking any and all paths to try to return to that source.

In order for an electrical circuit to work properly, the circuit must be complete.


Figure 8.

In other words, for electrical current to flow in the circuit to an electric sign, the common 120-volt circuit usually will contain an ungrounded conductor (hot) and a grounded conductor (neutral). When properly connected, normal current will flow in this circuit. (See Figure 8)

The other type of current one must be familiar with is fault current, which will also follow all paths available to it to try to return to the source. Fault current in most cases is an abnormal or accidental situation. It is important that a proper path for fault current, in the form of a conductor, be provided with the circuit for safety. This conductor is referred to as the equipment grounding conductor of the circuit. With all metal parts and enclosure associated with the sign or neon installation effectively bonded together and connected to an equipment grounding conductor, two basic but important things are accomplished. First, the metal enclosures and parts are essentially put at the same electrical potential (voltage). Second, if a ground fault should occur in the circuit, an effective path is provided back to the source and to ground which ensures overcurrent device operation.


Figure 9.

The equipment grounding conductor and proper bonding are essential elements for safety in electrical signs and neon installations. This safety component of the circuit acts as the silent servant waiting to perform its ever-important function.

The high voltage secondary circuits (GTO in a wiring method that extends from the transformer to the discharge tubing) for neon installations also introduce another electrical component into the electrical circuit. This component is called capacitance. Capacitance coupling can actually raise the potential (voltage) on ungrounded metal equipment and metal parts. Proper grounding and bonding of metal enclosures and associated metal parts ensure that these parts remain at earth potential. Other electrical resource material on electrical theory is available that expands on this term.


Figure 10.

Electrical installations for sign circuits and neon installations are not exempt from these basic safety requirements. Transformers installed and wired using the balanced mid-point reference wiring method require the secondary output conductors to be as short as possible, and the secondary return leads must terminate on a mid-point grounding connection terminal provided for that purpose by the transformer manufacturer.

Proper grounding and bonding connections of the entire branch circuit wiring methods and secondary circuit wiring methods are critical for proper operation and safety of these secondary circuits wired by this mid-point reference method. Mid-point reference wiring methods are just mentioned in this writing and will be expanded upon in further writings.

The Sign and Neon Branch Circuit Wiring Methods


Figure 11.

All conductors of the branch circuit supplying power to a sign or primary (line side, usually 120-volt input) of a neon transformer are required to be installed in the same raceway, cable, trench, wiring gutter, unless permitted otherwise by the NEC. This includes all conductors (wires) of the circuit, including the equipment grounding conductor, which can be in many forms. It can be in the form of a conductor, conduit, tubing, cable armor, or combination of cable armor and conductor. (See Section 250-118 of the NEC).


Figure 11a.

In Article 600 of the NEC, there are requirements for branch circuits supplying signs and outline lighting systems. The wiring method (cable, conduit or raceway) used to supply signs and outline lighting systems must terminate within the sign, outline lighting system enclosure, junction box, or a conduit body. This circuit that terminates at the sign or neon transformer or power supply enclosure contains the equipment grounding conductor for the circuit. This conductor should be terminated to the metal enclosure. (See Figure 9)

This termination should be made by use of approved means. Sheet metal screws (tek screws) are not acceptable for attachment of equipment grounding conductors per the NEC. (See Section 250-8). Proper grounding clips, screws, or lug type terminations are available for this purpose.

This termination of the equipment grounding conductor establishes the connection to ground for those enclosures and puts them at the same zero voltage potential as the earth. Section 600-7 of the NEC requires signs and metal equipment of outline lighting systems to be grounded. The Code also allows for listed flexible metal conduit or listed liquidtight flexible metal conduit (per Article 351 of the NEC) to be used as a bonding means in lengths not exceeding 100 feet. (See Figures 10 and 11) One should keep in mind that there is a length limitation on secondary GTO conductors of 20 feet when installed in metallic wiring methods and 50 feet when installed in nonmetallic wiring methods. See Section 600-32(j).


Figure 12.

Small associated metal parts not exceeding 2 inches in any dimension, and not likely to become energized (such as the metal mounting means for tubing supports), and spaced at least ¾ inch from the neon tubing are not required to be bonded. Where listed liquidtight nonmetallic conduit is used for installing the secondary high voltage GTO conductors from the transformer or power supply to the neon tubing and where there are associated metal parts that require bonding, a bonding conductor is required to be installed. (See Figure 11a) This bonding conductor is required to be installed separate and remotely spaced from the nonmetallic conduit. It should be pointed out here that this wiring method is not electrical nonmetallic tubing, which was deleted from the 1999 NEC as an acceptable wiring method for GTO secondary conductors. See Section 600-32 of the NEC. The wiring method referred to here is rigid nonmetallic conduit.


Figure 13

A spacing of 1½ inches is required to be maintained when the secondary circuit operates at 100 Hz or less. When the secondary circuit operates at over 100 Hz, the spacing requirement increases to 1¾ inches. (See Figures 12 and 13). This conductor is required to be not smaller than No. 14. Metal parts of a building or structure are not permitted to be used as a grounded or equipment grounding conductor.

Summary

Proper grounding and bonding is a basic requirement in the NEC and is found in Chapter 2, which is appropriately titled, "Wiring and Protection.” The minimum Code requirements are set forth to protect persons and property from the hazards that arise from the ever-expanding use of electricity. Following these basic minimum requirements for grounding and bonding of signs and neon lighting installations contributes to the safe use of electricity.

The Code requirements in this writing are based on the 1999 edition of the National Electrical Code. Always consult the local authority having jurisdiction if in doubt as to the NEC requirements or any local amendments or requirements.


This article is being published in the January 2000 issue of Sign Business Magazine.

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Check and Be Sure of GFCI Safety Protection

Posted By NEMA, Saturday, January 01, 2000
Updated: Monday, February 11, 2013

"A safety revolution is underway in the electrical wiring of buildings,” writes Earl Roberts in his book Overcurrents and Undercurrents.1 He is writing about the use of electronics in circuit protection and specifically in the ground-fault circuit interrupter (GFCI). Just as electronics have enhanced the world of consumer appliances and communication, the use of electronics in the GFCI has resulted in a significant improvement in the safety of electrical systems.

Proven GFCI Safety Record

Notice Roberts’ use of the word "safety” as being under revolution. The GFCI indeed revolutionized the safety of electrical systems. Its purpose is to protect people from electrical shock or electrocution. It first appeared in the 1968 edition of the National Electrical Code (NEC) in Section 680-4 to protect underwater lighting of swimming pools. The successful use of GFCIs in providing electrical shock protection has resulted in continued expansion of GFCI requirements in successive editions of the NEC. Its application under the NEC has grown such that in the 1999 edition it is required in specified locations in kitchens, bathrooms, garages, unfinished basements, pools, fountains, rooftop receptacles, construction sites and elsewhere. Confirmation of the GFCI’s contribution to safety is seen in the reduction in electrocutions since the GFCI’s introduction. Whereas in 1975 there were 650 reported deaths related to consumer products, in 1996, there were 190—less than one-third of those in 1975. This information comes from the Consumer Product Safety Commission. Many of the lives saved can be attributed to GFCI protection.

Useful Service Life

As with any electronic device, the GFCI will eventually reach the end of useful service. Think about a TV set that has finally reached the end of its useful performance. The picture or sound that had once been clear and enjoyable is no longer so. In fact, the unit may simply be "dead.” We have to replace it if we wish to continue receiving our programs. The end of life for other electronic devices, however, may not be immediately evident to the consumer. For safety related devices, it is therefore essential that, if test features are integral in the design, the consumer has the discipline to use those features.

Although GFCIs have proven to be dependable products with a long service life, they must be replaced if their useful service life has expired. They are providing valuable protection that should be maintained. Replacement is also required to maintain compliance with the NEC. To help know when to replace them, manufacturers provide a test button feature available to every user. The instructions provided with or marked on every GFCI unit tell the consumer to test the unit monthly. The test-button feature and standardized instructions are an industry wide requirement of UL 943, Underwriters Laboratories Standard for Safety for Ground-Fault Circuit Interrupters.

The push-button test is simple for anyone to perform: push a button and confirm that the unit trips. Each GFCI has a test button that can be pressed to give a visual indication that the protective electronics in the GFCI are operating. The button is marked "TEST”on both circuit breaker and receptacle type GFCI’s. When this test button is pressed either the circuit breaker handle will move to the trip position or the "RESET” button on the receptacle type GFCI will pop out. There will also be an accompanying audible click. If the GFCI is working correctly, all power to downstream outlets will be disconnected when the test button is pushed. Power will be restored when the "RESET” button is pressed on the receptacle GFCI or the circuit breaker handle is reset.

If there is no tripping indication, the unit is no longer functioning as intended and must be replaced. The GFCI will still permit electrical current to flow if it is kept in service, but the electronic circuitry will no longer provide protection. The person doing the testing must respond to the test. A GFCI is not like the TV that is no longer useful when it stops functioning. The GFCI is designed to continue to permit current to flow, as if it were a standard receptacle or circuit breaker, to supply power until the device can be replaced.

Field Status Questioned

The November/December 1999 IAEI News carried an article titled, "Are All Those GFCIs Out There Working?” The survey cited in the article draws attention to the possibility that a percentage of GFCIs are no longer operational and then concludes that a feature developed by the company employing the author be adopted for all GFCIs as a solution. The issue of potentially non-operational units deserves careful investigation.

The survey information presented in the article was first communicated to manufacturers of GFCIs in August 1999. NEMA member companies have begun an evaluation of the facts. NEMA manufacturers have no information that supports the level of non-functionality indicated by the information presented in the article. Several manufacturers have reviewed their records and conducted informal surveys that seem to contradict the information contained in the article.

As we review the information in the article, we find that the survey failed to take into consideration the method used to test the GFCI, the age of the home, the age of the GFCI and the cause and mode of GFCI non-functionality. A number of items from this early information need to be better understood before the information can be used constructively by GFCI manufacturers or by standards developers for the purpose of revising product standards.

In order to establish the facts scientifically, NEMA members are initiating a study of the state of GFCI units in the field to be completed in 2000. To accomplish the study, training is being provided to those performing the checks, a specific test protocol has been established, and non-operational units will be collected and examined for cause. Age and condition of any non-operational units will be recorded. UL has been supportive of the NEMA study and UL will coordinate documentation of information collected. The intent is to learn if units are non-operational and, if so, to determine the cause and percentage of non-operational units.

Product and standard improvements over the years have already dealt with many potential causes of GFCI failure such as lightning, environmental conditions and incorrect installation. Products produced before these improvements were made may expire earlier than newer products. One value of a study such as the one NEMA is initiating is that the cause of non-operation can be learned and addressed. Date of manufacture will also be learned for any products that have expired. Until the causes of non-operation are known and the level of non-operation has been verified, improvements cannot be identified, if any are needed.

It should be clarified that GFCI test functions are performing as intended, to the best knowledge of manufacturers. The test button feature provided correctly indicates products that have expired and must be replaced. It may be found that residents are not checking units and replacing those that are non-operational. Although manufacturers have taken steps to improve reliability and surely will continue to do so, residents must do their part in checking their units and replacing any that have expired. Some units have been in use for over 25 years.

Conclusion

The solid protection brought to the use of electricity by the GFCI is a matter of record. It is a record that endorses the early work of Professor Charles Dalziel who helped define what protection is and introduced the first GFCIs. It also endorses the advent of electronics that permits the protection to be widely available. The industry is committed to knowing the status of GFCI products in the field. To retain the solid protection that is already available, residents must include periodic checks of their GFCIs just as they do for fire alarms to be sure that safety equipment is operating properly.


1Earl W. Roberts, Overcurrents and Undercurrents, Mystic Publications, Mystic, CT, 1996.

About NEMA: NEMA is the trade association of choice for the electrical manufacturing industry. Founded in 1926 and headquartered near Washington, D.C., its approximately 450 member companies manufacture products used in the generation, transmission and distribution, control, and end-use of electricity.

Tags:  Featured  January-February 2000 

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Ground-fault Protection of Equipment and Ground-fault Circuit Interrupters

Posted By Michael Johnston, Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
There are two important areas regarding protection of property and persons. Appropriately, these requirements are located in Chapter two of theNEC,labeled, Wiring and Protection. This article will focus on ground-fault circuit-interrupter (GFCI) protection for persons and ground-fault protection of electrical equipment (GFP). One must understand that there are two main differences between these two forms of protection.
Photo 1

Photo 1

Ground-fault protection of equipment and ground-fault circuit interrupters for personnel are essential to safety. Often people in the electrical industry learn about the requirements for ground-fault protection in electrical service equipment by the "school of hard knocks” when a ground-fault in a service not equipped with a ground-fault protection device endures the forces of a destructive ground-fault leading to a total burndown of their electrical service, or equipment, to their facilities. These situations lead to questions: How are we protected? How is the electrical equipment protected?” Why is this protection required? Too late, it seems, some in the industry learn that injuries, death, property loss, and downtime for facilities are all situations that can result when the required ground-fault protection for equipment or ground-fault circuit interrupters are not installed as required by theNational Electrical Code.

Figure 1

Figure 1

"The purpose of thisCodeis the practical safeguarding of persons and property from hazards arising from the use of electricity. ThisCodecontains provisions necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazards.” These principal purposes are so critical to the users of theCodethat they are set forth in the initial section of theCode.NFPA Standard 70 is a minimum electrical requirements safety standard, which simply means that one must, at the very least, comply with those minimum requirements. See Section 90-1(a) and (b) of theNEC.

Photo 2

Photo 2

For the protection of personnel

Article 100 of theNECdefines ground-fault circuit interrupter as, "A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.”

Figure 2

Figure 2

GFCI protection is required to provide protection against the hazards of electric shock and electrocution. The Underwriters Laboratories (UL) requirements for Class A ground-fault circuit interrupters is that the device will open (trip) when the continuous 60 cycle differential current exceeds 6 mA, but shall not trip at less than 4 mA. One can see by these values of current at which GFCIs operate that these types of protective devices are sensitive to low-level current leakage (see figure 1).

The GFCI is a current sensing device that, in basic terms, monitors the current balance in the ungrounded (hot) conductor and the neutral (grounded) conductor (see figure 2). If the current in either conductor changes by more than 6 mA, the GFCI will open. In other words, the GFCI monitors the current coming and going through the sensors. In a ground-fault condition, the current is seeking all paths back to the source. The device trips. So when one is troubled by the alleged nuisance tripping of a GFCI, chances are the device is just doing its job. One should troubleshoot the circuit, not defeat the GFCI to eliminate the alleged nuisance tripping.

Photo 3

Photo 3

The requirement for ground-fault circuit interrupters first appeared in Section 210-22(d) of the 1971National Electrical Code,which read, "For residential occupancies all 120-volt, single-phase, 15- and 20-ampere receptacles installed outdoors shall have approved ground-fault circuit-interrupter protection for personnel. The effective date of this requirement shall be January 1, 1973.” The 1971 edition of theNECalso required GFCI protection for all construction site receptacles rated at 15- and 20-amperes, effective January 1, 1974. The requirements set forth in Section 555-3 for marinas and boatyards stated that ground-fault protection for shore power receptacles "may be provided with GFCI protection,” which was permissive at that time. Those rules have since become mandatory requirements. See Section 555-3 of the 1999NEC.

Figure 3

Figure 3

The requirements in Section 680-6 for receptacle location and GFCI protection was, and still is, a mandatory requirement. As theCodedeveloped over the years, more and more additional requirements for GFCI protection were made mandatory. In the 1999 edition of theNEC,the requirements for GFCI protected circuits and receptacles are widespread and numerous. These devices have certainly contributed to the personnel safety and protection as required by theNEC.

Photo 4

Photo 4

For the protection of equipment

Now compare ground-fault protection (GFP) for equipment. Once again one must look to the definitions in Article 100, where ground-fault protection of equipment is defined as, "A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.”

First, consider the service equipment. Section 230-95 requires ground-fault protection for all solidly-grounded wye electrical services of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase for each service disconnect rated 1000 amperes or more (see figure 3). These provisions do not apply to fire pumps or continuous industrial processes where a non-orderly shutdown would result in increased or additional hazards. The maximum operational current setting for these devices is 1200 amperes. The maximum time delay is one second for ground-fault currents equal to or greater than 3,000 amperes. These equipment protection requirements are a result of a history of destructive burndowns of electrical equipment operating at these voltage levels.

Photo 5

Photo 5

An electric arc generating a tremendous amount of heat is readily sustained at these higher voltage levels. Conductive ionized gases produced by these arcs contribute to the electrical explosion and can go from a phase-to-ground fault to a phase-to-phase short circuit and result in destructive magnetic and thermal forces that cause the equipment to literally melt down (see photos 1 and 2). TheNECdoes not require ground-fault protection for services at voltage levels less than 150 volts phase-to-ground; however, it is permitted to be installed on those services.

There are basically two types of ground-fault protection (GFP). The zero sequence type, which may have more than one form, and the residual type, sometimes referred to as a neutral ground strap type. Both are designed to protect downstream equipment from destructive arcing burndowns (see figures 7, 8, and 9). It should be pointed out here that ground-fault protection equipment will not protect equipment from line-to-ground faults on the line side of this ground-fault protective device.

Figure 8

Figure 8

Ground-fault protection of equipment first became a requirement in theNational Electrical Codein the 1971 edition of Section 230-95. The requirement for ground-fault protection in health care facility electrical feeders downstream of a GFP service disconnecting means was introduced into the 1975 edition of theNEC.The main reason for this second level of protection downstream is to localize a destructive ground-fault condition to the device in trouble and keep the continuity of service to the facility and prevent a total blackout condition at the facility. This feeder protection is required to be 100 percent selective so that where a ground-fault occurs downstream from the feeder overcurrent device, only the feeder overcurrent device will open and the service and feeder main will remain closed. The coordination is achieved by theCoderequiring a six cycle, or greater, separation between the tripping times of both levels. These ground-fault protective devices are required to be tested, in accordance with the instructions provided with the equipment for the particular type of ground-fault protection used, when first installed. This is a performance test that includes injecting a current and measuring current and time, and is not just the pressing of a test button on the equipment. Testing is also required for service and feeder ground-fault protection devices in other than health care facilities. A written record of these test results shall be made available to the authority having jurisdiction. See Sections 230-95(c) and 110-3(a) and (b).

Observance of minimum standard required

Figure 9

Figure 9

TheNational Electrical Codeis a minimum standard, so we need to do at least that much. With that in mind, one can conclude that GFP is required for all service disconnecting means at the voltage rating of over 150 volts phase-to-ground but not exceeding 600 volts phase-to-phase. Looking a bit further, if we had an electrical service rated at 1600 amperes and 480/277 volts and the "six disconnect” rule were being applied as allowed by Section 230-71, meaning six disconnects rated at 400 amperes each, there would be no requirements for GFP to be installed for the service; installation would be optional. Obviously the system could benefit if a ground-fault protective device were installed as a main disconnect for the service, but this would not be required by theNEC.Ground-fault protection for equipment is required by Section 215-10 for feeder disconnects, 230-95 for service disconnects and in health care facilities, 517-17 for second level protection and coordination where a service disconnect is a ground-fault protective device.

Making the comparison between ground-fault circuit interrupters and ground-fault protection of equipment by carefully using the definitions and rules set forth in theNational Electrical Code,one can easily see the differences between the two types of protection. Ground-fault protection of equipment protects equipment, and ground-fault circuit interrupters protect people. Both types are equally important in maintaining the spirit and content of theCode.We hope this helps clear up some of the confusion between the requirements and reasons for both types of ground-fault protection. These two topics are covered more extensively in theIAEI Soares Book on Grounding,7thEdition.


Read more by Michael Johnston

Tags:  Featured  September-October 1999 

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