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Interrupt Current Codes and Surge Protective Devices (SPDs)

Posted By Bryan Cole, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

Transient Voltage Surge Suppressors (TVSS), also more commonly referred to as Surge Protective Devices (SPDs), have been applied as part of an overall power quality strategy for almost a decade. Moving into the future of utility deregulation and the increased reliance on electronic devices to control, process, and communicate information, SPDs will continue to be recognized as an essential component of power quality.

However, because SPDs have only been utilized for the past ten years as part of a power quality strategy, there are many issues that cloud the safety, performance, and the correct application of the device. In the United States, many documents have been produced to evaluate both the performance and safety attributes of SPDs. Many of the documents have been produced by the telecommunication companies who are in the process of building the internet and enhancing the telecommunications infrastructure.

Additional documentation has been provided by independent organizations which address the performance and safety aspects of SPDs: the National Electric Code (NEC), the Institute of Electrical and Electronic Engineers (IEEE), and Underwriters Laboratories, Incorporated (UL).


The National Electric Code (NEC), a top-level document detailing the guidelines for application of electrical and/or electronic equipment connected to low-voltage power distribution, is primarily concerned with the safety aspects of all electrical/electronic devices, including SPDs. An essential component of ensuring the safe application of a SPD, the NEC requires that virtually every component of the building wiring or device connected to the building wiring be provided with a means of overcurrent protection [1]. In particular, Article 110-10 of the NEC states: "The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more circuit conductors. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section.”

Figure 1.

Furthermore, Article 240-3 of the NEC states that "Conductors, other than flexible cords and fixture wires, shall be protected against overcurrent in accordance with the ampacities as specified in Section 310-15, unless otherwise permitted or required in [sub-clauses] (a) through (g).”


The IEEE has been very active in providing recommendations and guidelines that help engineers correctly describe transient environments, and help to identify the many attributes of a correctly applied SPD. The most notable IEEE document on surge environments is titled IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, standard C62.41-1991 [2]. The IEEE has also authorized a working group to explore and provide guidelines that will help engineers in the application and specification of SPDs.

To ensure the coordination of a SPD to the distribution network of application, the IEEE Guide for the Application of Surge Protective Devices in Low-Voltage AC Power Circuits, Draft 5 recommends that the interrupt current ratings of all devices within a particular panel be equally rated [3]. If the available interrupt current (AIC) rating of the SPD is less than the panel, then the SPD has been incorrectly applied.

UL Standards

Testing of the SPD to determine the available interrupt current (AIC) rating is accomplished by an evaluation to Underwriters Laboratories, Incorporated (UL), standard 1449 [4] and is defined in Section 37.3. The conformity to this requirement was required by the implementation of the second edition of the standard dated 1996 August.

Figure 2.

Testing consists of subjecting representative samples of a particular SPD model or product line to an over-voltage condition with a specified short circuit current. Testing is only performed on single-, split, or three-phase wye configurations. Three-phase delta configurations are not required to be tested. SPDs are subjected to the maximum system voltage present with the available fault current limited only by the effective source impedance.

Three representative samples are tested in a single mode configuration, i.e. line-to-neutral, line-to-ground, or neutral-to-ground. At the completion of the test, the SPD can not have any holes in the enclosure that could result in the accessibility of live circuits, produce a flame that damages the cheese-cloth that surrounds the SPD during the test, or ignite the enclosure.

The amplitude of the short circuit fault current is determined in advance of the test procedure by the manufacturer of the SPD. Table 1 shows the amplitudes of the fault currents possible. It is important to note, the method of disconnect may be internal or external to the SPD. Either way, once the representative samples of the parallel SPD products have been evaluated by UL, all production models are required to have documentation available to the end-user indicating the maximum fault current of the SPD. This documentation can be provided in the form of a label on the product or within the installation instructions.

An Application Example

The best way to illustrate the incorrect and correct application of a SPD in accordance with the codes, guidelines, and conditions of acceptability by the NEC, the IEEE, and UL respectively is by example.

Table 1. The amplitude of the short circuit fault current is determined in advance of the test procedure by the manufacturer of the SPD. Table 1 shows the amplitudes of the fault currents possible.

For example, assume that a SPD has obtained an available interrupt current (AIC) rating of 25,000 amperes from UL. Assume that this particular SPD was placed behind a circuit breaker with an AIC rating of 65,000 amperes, as shown in Figure 1. Additionally, assume that the maximum AIC present to this particular location on the facility’s power distribution system is 42,000 amperes. In this particular example, the SPD is incorrectly applied. Because the SPD has obtained an AIC rating of only 25,000 amperes, it may not be capable of withstanding an interrupt current of 42,000 amperes.

To rectify this particular example so that the SPD has been installed in accordance the NEC, the IEEE recommendations, and UL conditions of acceptability, a few approaches can be considered. The first approach would be to apply a SPD with an AIC rating of 42,000 amperes or greater. Even though the circuit breaker proceeding the SPD is greater than the SPD, the maximum short-circuit current available at the panel is 42,000 amperes. Therefore, the SPD has an AIC rating equal to the power distribution system at that point.

The second approach is to utilize a current limiting circuit breaker that will limit the AIC to the SPD to a value of 25,000 amperes or less [5]. This option presents some complications. In accordance with the NEC, the device must be applied in accordance with its listing requirements. Testing of the SPD must be with a specified circuit breaker at a specified current rating. Using a circuit breaker produced by a different manufacturer, at a different AIC rating, or even a different part number is not acceptable and is outside the scope of its safety agency listing. If the SPD is installed utilizing an untested or improperly tested current limiting circuit breaker, then it is the same as not utilizing a product evaluated by an independent safety agency, such as UL.

The third approach would be to install a finite amount of impedance in the line prior to the panel. The amount of impedance would have to be high enough to limit the amount of AIC to 25,000 amperes or less. One particular way to implement this action item is by installing a transformer in the power distribution system to lower the overall AIC rating at that particular point. This option can be very expensive and may require additional space to incorporate.

Figure 2 represents a SPD that includes the over-current protection within the device itself. In this case, the SPD does not require that the device be preceeded with a circuit breaker. However, the same rules apply. The SPD has a specified AIC rating which has been evaluated by UL, it is marked either on the product or in the accompanying installation instructions, and must be connected appropriately.


As SPDs have become a common device in the overall picture of a complete power quality strategy, their performance and safety attributes have continued to be examined and tested. Among one of the top attributes has been the safe and correct application of a SPD to a facility’s power distribution system. Analysis of the AIC ratings of the power distribution system, any over-current protective devices, and the SPD itself is required.

The NEC long ago established rules determining the relationship between the available interrupt current of a device and the over-current protective device required. An added clarification was implemented by the second edition of UL 1449 by testing SPDs to a particular set of conditions and providing SPDs with an AIC rating. Additionally, contributions made by the members of the IEEE on guidelines and recommendations of a correctly installed SPD will help ensure the correct application of a SPD. Responsibility for the coordination of the SPD to the application rests on the specifying engineer, the contractor or end-user installing the SPD and the inspecting official.

In conclusion, the only set rule for determining the AIC rating of a SPD is to examine the paperwork and installation instructions of that particular SPD. Once the AIC rating of the SPD has been determined, and an analysis has been performed on the power distribution system, the appropriate SPD can be installed. Failure to ensure the proper application of the SPD with the development of specifications, guidelines, and standards can result in a potentially dangerous condition. The inappropriate application of an SPD can no longer be tolerated or excused.


1. National Fire Protection Association, National Electrical Code Handbook, 1999, Eighth Edition, Qunicy, MA.

2. The Institute of Electrical and Electronic Engineers, IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits, 1991 February 21, document C62.41-1991, NY, NY.

3. The Institute of Electrical and Electronic Engineers, IEEE Guide for the Application of Surge Protective Devices in Low-Voltage AC Power Circuits, Draft 5, 1999 document PC62.72, NY, NY.

4. Underwriters Laboratories, Incorporated, Standard for Safety, Transient Voltage Surge Suppressors, Second Edition, 1996 August 15, Melville, NY.

5. The Institute of Electrical and Electronic Engineers, IEEE Recommended Practice for Electric Power Distribution for Industrial Plants, IEEE Red Book, IEEE Standard 141-1993, page 217, NY, NY.

Read more by Bryan Cole

Tags:  Featured  March-April 2000 

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America Needs to Power Up Its Rank of Skilled Electricians

Posted By Square D, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

A century ago, building and construction trades experienced their first boom period as the U.S. rapidly shifted from an agricultural economy into an industrial colossus.

Today, with an economic boom pointing America toward the new millennium, the nation is again facing a major shortage of skilled career employees in its largest industry—construction.

With an economic boom pointing America toward the new millennium, the nation is facing a major shortage of skilled career employees in the construction industry.

The tight labor market is interfering with young job-seekers’ decisions to pursue long-term career opportunities in skilled building and construction trades, Square D has learned from its electrical contracting customers.

In some parts of the U.S., entry-level electrical apprentices might earn nearly as much today in fast-food restaurants or other less-skilled service jobs. Unskilled service jobs, however, cannot offer as much career achievement or earnings potential as electrical construction, an industry that sees no end to its growing technical sophistication and complexity.

An estimated 240, 000 skilled contracting workers each year are retiring or leaving contracting for different occupations. Among electricians, the shortage is having a ripple effect on Square D and other manufacturers of electrical products, say executives at the Palatine, Ill.-based North American electrical industry leader. It’s been obvious for several years that the short supply of electrical contractors could hamper the long term growth potential of the electrical industry—including, of course, Square D products, systems and services, said Chris Richardson, Square D president. We’ve been a leader in this industry for 95 years and we needed to step up and get involved.

Square D is leading the effort among electrical contractors and manufacturers from around the country by investing in electrical industry career development, awareness and professionalism. To date, Square D has provided $1.25 million for partnership programs with two leading not-for-profit national educational foundations, the National Center for Construction Education and Research (NCCER) and the Electrical Contracting Foundation (ECF), an arm of the National Electrical Contractors Association (NECA).

NCCER Targets New Contracting Workforce Entrants

NCCER—affiliated with the University of Florida and headquartered in Gainesville—focuses on attracting high school students and young adults into more than 20 different construction specialties. The educational group concentrates on standardizing quality craft training, management education and safety programs.

Within electrical construction alone there are more opportunities than ever, and it is a very rewarding career for those willing to invest the time required to master today’s advanced technology, said Dan Bennet, NCCER president. There is an especially high demand for electricians with skills in such areas as computer networking and fiber optics.

The curriculum developed and published by NCCER currently is being adapted to CD-ROMs for students to use on multimedia computers. NCCER course materials are taught nationwide in thousands of secondary and post-secondary schools. The group reaches out not only to students, but also parents, teachers and potential employers. Bennet says that NCCER urges the schools, local contractors and trade associations to develop partnerships that encourage youths to consider careers in the construction trades.

Contractors Emphasize Technical Change

Today’s electricians and their contractor employers must adapt to rapid technical change as well, according to the Electrical Contracting Foundation, the National Electrical Contractors Association’s career research and educational arm. NECA and ECF share offices in Bethesda, Md.

Established in 1990, ECF has developed a spectrum of programs and strategies to educate the current and future electrical contractor workforce. Financing and overseeing the foundation’s work is its Electri’21 Council, whose members including chief executives of electrical contracting firms and top management from manufacturers like Square D. Among its many projects, ECF offers important quality and field management training to help established contractors improve their business performance

We want to promote the electrical contracting career, said Russ Alessi, ECF’s executive director. We need to make sure that current managers can meet the challenges of our changing electrical industry environment, to gain the technical and business skills that are required today.

Like NCCER, the ECF’s career awareness programs include summer internships and apprentice programs for high school and college students. Their high school awareness activities also reach students and school guidance counselors. It also has developed a model electrical construction management curriculum now being offered by Purdue University and Iowa State. Alessi expects more colleges and universities to come aboard.

We realize that the workforce shortage is a serious impediment not only to our business but the nation’s economy, says Square D’s Richardson. There’s a whole lot riding on our collective ability to bring qualified people into electrical contracting.

Tags:  Featured  March-April 2000 

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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.


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.


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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The new Section 210-12 of the 1999 National Electrical Code (NEC) requires arc-fault circuit-interrupter protection for some branch circuits. Does UL List such devices?

Posted By Underwriters Laboratories, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

Question: AFCI for branch circuits

The new Section 210-12 of the 1999 National Electrical Code (NEC) requires arc-fault circuit-interrupter protection for some branch circuits. Does UL List such devices?


Arc-Fault Circuit-Interrupters (AFCIs) are currently covered under the category Circuit Breakers, Molded Case, Classified for Mitigating the Effects of Arcing Faults (DIWL). The Guide information can be found on page 11 of the 1999 General Information for Electrical Equipment Directory (white book). This category covers Listed molded case circuit breakers, which are also Classified after being evaluated for their ability to mitigate the effects of arcing faults that may pose risk of fire ignition under certain arcing conditions.

On Feb. 26, 1999, UL published UL 1699, Standard for Arc-Fault Circuit-Interrupters. Manufacturers have already begun to seek UL Listing of their AFCI products to UL1699, and interest in these products is expected to increase significantly as contractors, electricians and consumers become more aware of their availability and intended application. However, only the branch/feeder type can be used to satisfy the current requirements of Section 210-12, since it requires AFCI protection of the branch circuit. Section 210-12 does not come into effect until Jan. 1, 2002, but is included in the 1999 NEC.

UL tests these devices by using methods that create or simulate arcing conditions to determine their ability to recognize and react to arcing faults. AFCIs are also evaluated to determine their resistance to nuisance tripping caused by arcing that occurs in control
and utilization equipment under normal operating conditions that may closely mimic an arc fault.

Question: Open neutral protection and GFCI

I am very concerned about not having open neutral protection provided on permanently installed ground-fault circuit-interrupters (GFCIs). Who can I contact to try get this requirement changed? Presently, UL 943, Standard for Ground-Fault Circuit-Interrupters, requires portable- type GFCIs used in the field to be provided with open neutral protection. However, permanently wired and installed GFCIs are not required to have open neutral protection.


Permanently connected GFCIs are more reliable since the connections are hard wired and usually are not disturbed after installation. Assuming the permanently installed GFCI is wired properly, it is very unlikely that the neutral would become damaged, rendering the device ineffective. Open neutral protection may or may not detect a miswired GFCI, and should not be relied upon to determine proper wiring.

Portable-type GFCIs used in the field are generally cord-connected. Open neutral protection is required since the cord is exposed and could be subjected to abuse.

Anyone can submit a proposal to revise a UL Standard. Proposals can be sent to your local UL Regulatory Services representative. It will then be forwarded to the appropriate UL Engineering Services staff. Or, visit the UL web site, and send your proposal or suggestion electronically to UL Regulatory Services.

Question: Open neutral for GFCI

Why is open neutral protection for ground fault circuit interrupters (GFCIs) necessary?


Switching contacts within a GFCI usually can be activated magnetically by the control circuit of the device, and mechanically by the test and reset buttons. In order for the unit’s control circuit to open these contacts, an imbalance of current (greater than 6 mA) between the ungrounded (hot) and grounded (neutral) conductor is required for Class A devices. Devices must be connected to a live circuit for the control circuit to operate when sensing such an imbalance in the electrical current.

Open neutral protection is required with temporary wiring methods since the cord is exposed and could be subjected to abuse.

Question: GFCI

Why doesn’t UL 943, the Standard for Ground Fault Circuit Interrupters, require visual indicators for proper wiring?


While UL 943 does not currently require visual indicators for the proper wiring of GFCIs, UL continues to develop requirements intended to prevent their miswiring. UL regularly forwards constituents’ questions and other comments on published Standards, as well as suggestions for developing new Standards, to the appropriate UL engineering staff.

Recent revisions to UL 943 now require that all installation instructions for GFCIs be standardized for consistency. Installation instructions are now essentially identical for all manufacturers. These instructions require specific methods for checking GFCI operation after installation to insure that devices are properly wired. In addition, the statements "Line and Load,” and "Hot and Wired,” are required to be marked on the product near the appropriate terminals. All of these requirements and others currently in place are intended to prevent GFCI miswiring.

Tags:  March-April 2000  UL Question Corner 

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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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Working in Dangerous Proximity to Overhead High Voltage Lines – OSHA, NESC, and the Law

Posted By David Young, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

There are two sets of rules for work in proximity of overhead high voltage lines: The rules for qualified persons and the rules for unqualified persons. There is no gray area.

Occupational Safety and Health Administration (OSHA) regulation 29CFR1910.269(x) defines a qualified person as "one knowledgeable in the construction and operation of the electric power generation, transmission, and distribution equipment involved, along with the associated hazards.” OSHA 29CFR1910.269(a)(2)(ii) also requires that qualified persons "…shall be trained and competent in the skills and techniques necessary to distinguish exposed live parts from other parts of electric equipment; the skills and techniques necessary to determine the nominal voltage of exposed live parts; the minimum approach distances specified in this section (1910.269) corresponding to the voltages to which the qualified employee will be exposed, and proper use of the special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools for working on or near exposed energized parts of electric equipment.”

A person qualified to work on overhead high voltage lines must be trained and competent in all of the above skills. If he or she is not, then they are unqualified with respect to work on overhead high voltage lines. A person may be a qualified crane operator or a qualified roofer, but they are not qualified with respect to work on overhead high voltage lines unless they comply with the above rules. Unqualified persons must stay away from overhead high voltage lines.

How Far Away?

OSHA29CFR1910.333(c)(3)(i)says that, "When an unqualified person is working in an elevated position near overhead lines, the location shall be such that the person and the longest conductive object he or she may contact cannot come closer to any unguarded, energized overhead line than the following distances: For voltage to ground 50kV or below – 10 feet (305 cm); For voltages to ground over 50kV – 10 feet (305 cm) plus 4 inches (10 cm) for every 10kV over 50kV.”

In OSHA29CFR1910.333(c) (3)(i)(B), "”When an unqualified person is working on the ground in the vicinity of overhead lines, the person may not bring any conductive object closer to unguarded, energized overhead lines than the distances given in "…the above rule. Sometimes the minimum distances described above are referred to as "dangerous proximity.”

If work must be performed by unqualified persons within dangerous proximity of an overhead high voltage line, OSHA requires in 29CFR1910.333(c)(3) that "…the lines shall be deenergized and grounded, or other protective measures shall be provided before work is started.” "If protective measures, such as guarding, isolating, or insulating are provided, these precautions shall prevent employees from contacting such lines directly with any part of their body or indirectly through conductive materials, tools, or equipment.” The person requesting to work in dangerous proximity of an overhead high voltage line may perform the work only after the line is deenergized and grounded or preventative measures have been taken.

Employees of companies are required to comply with the OSHA regulations. An individual homeowner planning to paint his house near overhead high voltage lines is not required to comply with the OSHA regulations. In fact, he or she may not even know the regulations exist. To protect the general public, more than half the states have adopted "Overhead High Voltage Line Safety Acts.” Within these laws, the states incorporate some or all provisions of the OSHA regulations. The penalties range anywhere from $25 to $5000 per incident and imprisonment in some states. One particularly good addition to some laws is a provision that all overhead lines are to be considered energized and having a voltage of more than 750 Volts, i.e., High Voltage, unless written determination is obtained from the utility or company operating the line. Some laws go so far as to require the posting of warning signs on cranes and other equipment capable of coming in dangerous proximity of overhead high voltage lines. Contact your state government department of labor to see if there is an act in your state. Note: The Delaware Overhead High Voltage Line Safety Act goes into effect January 1, 2000.

Part 4 of the National Electrical Safety Code® (NESC®) covers work rules used by qualified employees. The NESC® rules and the OSHA regulations are very similar.

If you are looking for the OSHA regulations and you have Internet access, they are available FREE on-line

If you have general questions about the NESC®, please call me at 302-454-4910 or e-mail me

National Electrical Safety Code® and NESC® are registered trademarks of the Institute of Electrical and Electronics Engineers.

Read more by David Young

Tags:  March-April 2000  Other Code 

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Standby Power and Transfer Switching

Posted By Leslie Stoch, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

In this article, we will cover some of Canadian Electrical Code requirements for standby power and transfer switching. The National Building Code specifies the minimum requirements for emergency standby power supplies for different building sizes and classifications, for high-rise residential, commercial, industrial and commercial buildings depending upon size height and occupancy. It specifies the minimum electrical backup requirements for critical emergency facilities including fire alarm systems, fire pumps, elevators, lighting, exit signs, ventilation systems and emergency voice communications.

While the National Building Code explains what must be provided, the Canadian Electrical Code provides the rules for installing emergency backup power supply systems in buildings where they are required by the NBC. For the most part, these requirements are found in the CEC, Section 46, Emergency Systems, Unit Equipment and Exit Signs. Other sections of the CEC also contain rules for different aspects of standby power supplies and transfer switching.

When required by the NBC, Rule 46-202 specifies that an emergency power supply may consist of either:

  • A bank of storage batteries capable of maintaining at least 91 percent of their full voltage for the minimum time specified in the building code or at least ½ hour; or
  • An automatically started generator, having sufficient capacity to supply the required emergency loads.

Storage batteries must be rechargeable and equipped with a battery charging system. Due to their time and storage capacity limitations, battery systems are normally restricted to emergency lighting and exit sign applications. Larger buildings, required by the NBC to provide more than emergency lighting and exit signs (elevators, fire pumps, ventilation, etc.), are normally supplied by a fuel-driven emergency generator (diesel, gasoline, natural gas). The generator must have a sufficient supply of fuel to permit the generator to operate for the required period of time.

Rule 46-204 stipulates that the emergency power supply must be switched automatically to ensure that the standby supply comes on promptly on failure of the regular power supply. This rule also requires that transfer switching equipment be installed in a location inaccessible to unqualified persons. This is to ensure that such equipment is not inadvertently disabled, resulting in failure of backup power for emergency facilities at the time when needed.

As indicated above, an emergency power supply must not become accidentally disconnected. For this reason, Rule 46-206 also specifies that the building emergency panel supplying critical loads such as the fire alarm system, elevators, emergency voice communications, etc., must not have a readily accessible main switch, fuses or circuit-breaker. Main overcurrent protection for the emergency panel should only be located in a locked room containing the emergency generator or batteries. Once again, this requirement also helps to guarantee continuity of the standby power supply when it is needed.

To ensure that operating personnel become aware when the emergency power supply is out of service, Rule 46-208 requires:

  • Audible and visible alarms that warn of problems such as failure of the battery charging system, or emergency generator problems such as overheating, low fuel or high bearing temperatures.
  • An audible alarm may be silenced, but a red trouble light must remain on to remind personnel that a problem still continues.
  • When the problem has been corrected, either an audible alarm must remind personnel to reset the trouble signal, or it resets automatically.

Section 32, Fire Alarm Systems and Fire Pumps in the Canadian Electrical Code also contains some special requirements for electrical backup and transfer switching for fire alarm systems and fire pumps.

To minimize the possibility of being accidentally disconnected from its backup supply, Rule 32-108 requires that a fire alarm system must be connected as close as possible to the terminals of the transfer switch where the fire alarm receives its emergency supply when other equipment is also supplied. Usually this means connection to an emergency electrical panel supplied from the regular and standby power sources.

To ensure that fire pumps are capable of operation during a fire, Rule 32-204 permits a separate electrical service for the fire pump(s). In an exception to other rules, this service may be located remotely from the main service. This helps ensure that any faults in the main electrical installation do not hinder operation of the fire pump(s). A remote location may also be necessary for placement of the fire pump service in relation to the emergency power supply and transfer switching.

To minimize the possibility of an electrical failure during disruption of the main electrical supply, Rule 32-206 requires that each fire pump have a dedicated automatic transfer switch, approved for fire pump service. In other words, each fire pump must have its own switch. The switch must be located either in a barriered compartment of the fire pump controller or in a separate enclosure adjacent to the controller and labelled to identify it.

Other sections of the Canadian Electrical Code contain requirements for backup power supplies and transfer switching. For example, Rule 14-612 in Section 14, Protection and Control requires that transfer switching between the regular and emergency standby power supplies must prevent the inadvertent interconnection of the normal and standby sources. This means that the transfer switch must disconnect the regular source before connecting the standby source during a power failure. An accidental interconnection between normal and standby supplies could create an electrical hazard to personnel working to correct the problems in the regular supply. Rule 6-106 in Section 6, Services and Service Equipment also stipulates that where a service is supplied by more than one system, the switching must be arranged to prevent systems from interconnection.

Uninterruptible power supplies (UPS) are used to provide computers and other sensitive or critical electronic equipment with a clean source of power and protection against voltage surges, frequency fluctuations, with temporary protection against power failures. Normally, the UPS has solid state transfer switching between the incoming and the uninterruptible power supplies to provide rapid switching when the inverter fails. Rules 14-700 to 14-704 of the CEC prohibit the use of this solid state switch as a disconnecting means. A separate circuit-breaker or disconnect switch must be used to isolate these systems from each other when personnel must work on the regular supply with the UPS in operation. This is intended to avoid electrical shock hazards due to leakage through the solid state devices or failure of the solid state switch.

As with previous articles, you should consult your local electrical inspection authority for a more exact interpretation of any of the above.

Read more by Leslie Stoch

Tags:  Canadian Code  March-April 2000 

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IAEI Re-certification of Electrical Inspectors

Posted By Philip Cox, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

The IAEI has participated in inspector certification programs for several years and is pleased to see a growing interest in certification by code enforcing organizations and electrical inspectors. Electrical inspectors in both Canada and the United states have become certified through programs developed and administered in each respective country. The goal is for all electrical inspectors to be certified.

This editorial will focus primarily on electrical inspector certification and recertification in the United States for the purpose of covering proposed changes that will affect the IAEI program in the US. Many inspection jurisdictions have progressed to the point of requiring proof of certification as an inspector before one is permitted to perform the duties of an inspector. Certification through a professionally developed examination process is an important tool in demonstrating an individual’s knowledge in those areas covered in the testing. Credit should be given to those individuals who have taken the required examinations because of their desire to better themselves. Those who have taken the certification exams because their employers recommend or require it should also be commended for successfully achieving certification. The general public certainly benefits from electrical inspector certification programs that effectively determine an applicant’s level of knowledge and ability.

Achieving an electrical inspector certification is certainly an accomplishment in which one can take pride. However, becoming certified is only one of many steps needed to adequately fill the role of a professional electrical inspector. Inspectors who become certified can’t afford to be satisfied with just that achievement. The demand to continue learning in order to keep up with the industry is too great for one to neglect opportunities to gain that knowledge. The IAEI provides many opportunities for certified electrical inspectors to continue their education. While electrical inspector certification has been promoted by the IAEI, training programs made available, and continuing education units (CEUs) awarded for training, those holding inspector certifications have not been required by the IAEI to be re-certified. The IAEI does recognize that recertification of inspectors is an important part of an effective certification program and is taking steps to fully develop and administer a re-certification program as authorized by the bylaws.

The IAEI Education Committee recommended that changes be made in the IAEI bylaws covering the inspector re-certification program and that the necessary steps be taken to properly implement the program. The proposed changes and recommendations were approved by the Board of Directors during its November 1999 meeting. Section 820(A) of the IAEI Bylaws states "The IAEI shall establish and maintain an inspector re-certification program to effectively measure the level of competency being maintained by those holding IAEI inspector certification(s).” That provision is expected to be implemented this year.

Those who hold one or more IAEI certifications in the classifications of Electrical General, Electrical 1 & 2 Family Dwelling, or Electrical Plan Review will need to re-certify within a three year period. The methods of obtaining the necessary re-certification include passing an approved written examination, obtaining not less than 2.4 CEUs of approved training, or a combination of those two methods. Of the total training, 0.8 CEU is to be in electrical code changes. Continuing education units are awarded on the basis of 0.1 CEU for each hour of approve training. As an example, 0.8 CEU is issued for participation in an 8-hour seminar on the Analysis of the NEC. Individuals who let their certification lapse for more than one year beyond the renewal date will be required to retest to regain their certification. Individuals holding inspector certification according to IAEI records will be notified as to the re-certification program and will be provided with details on what is needed to maintain their certification.

The IAEI is a strong supporter of electrical inspector certification and encourages all inspectors who have not already achieved that status to do so. Being an electrical inspector carries serious responsibility with it. The better trained an inspector is, the better that person can do his or her job. Certification not only gives the electrical inspector a feeling of accomplishment, it also proves to others that a certain level of knowledge has been demonstrated. The educational program offered by the IAEI is intended to provide a source of training that inspectors, potential inspectors, electricians, and others can rely on to help expand their knowledge. The IAEI is working as fast and effectively as it can to develop new educational material and to improve on existing products to aid people within the industry. It is believed that the educational program will be an asset to those needing to prepare for certification and to those who wish to maintain that certification. It is difficult for others to legitimately criticize an electrical inspector who has a well-rounded knowledge of electrical codes, standards, wiring methods, electrical products, and the electrical system in general and who performs his or her job in a professional manner.

Read more by Philip Cox

Tags:  Editorial  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.


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.


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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