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How Comfortable Are You?

Posted By Michael Callanan, Monday, May 01, 2000
Updated: Monday, February 11, 2013

The Hazards of Electricity


Figure 1. A good job safety will help determine the appropriate PPE for the task.

The electrical inspector and electrician are no different from any other craftsman. As they accumulate more experience and expertise in their field they begin to achieve a level of comfort with the tasks they perform on a regular and routine basis. Unfortunately, as the comfort level increases, the potential for complacency can begin to set in. In the electrical industry, there is no room for complacency. "Ordinary” and "routine” tasks, such as verifying voltage, taking current readings, and even visual inspection of live or energized parts, can subject the inspector and electrician to the multiple hazards of electricity.

For the past several years, an increasing amount of information has emerged concerning the multiple hazards of electricity. For many years, we assumed the electrical shock was the only hazard to be considered. Recent studies have shown that additional concerns must include the damaging effects of arc-blasts and arc-flashes. Consideration must be given to the devastating forces generated in arc-blasts when molten copper expands to 67,000 times its original value as it vaporizes. Likewise, arc temperatures can reach 35,000°F causing fatal burns at distances up to 10 ft and the pressure wave generated by the blast can reach upwards of 2,000 lbs/sq.ft., certainly enough pressure to rupture eardrums or even collapse the lungs. It is especially important to consider that as our electrical distribution systems continue to grow in size and capacity, the potential for higher and higher available fault currents and significantly greater arc-blasts and arc-faults rises sharply. For these reasons, electrical inspectors, electricians and anyone who works on or near energized circuits or equipment must be on constant guard that even the routine tasks they perform on a regular basis are done in a manner which affords the highest possible degree of personnel protection.

First Step: Are You Qualified?

The OSHA electrical safety-related work practices standard establishes guidelines for both the "qualified” person and the "unqualified.” Unfortunately, the OSHA General Industry, Electrical Standards, give very little direction in determining and defining what skills are necessary to be considered "qualified.” Fortunately, NFPA 70E-1995, Standard for Electrical Safety Requirements for Employee Workplaces defines a qualified person in Section 2-2.1. as one "trained and knowledgeable of the construction and operations of equipment or a specific work method, and be trained to recognize and avoid the electrical hazards that might be present with respect to that equipment or work method. Such persons shall also be familiar with the proper use of special precautionary techniques, personal protective equipment, insulating and shielding materials, and insulated tools and test equipment. A person can be considered qualified with respect to certain equipment and methods but still be unqualified for others.” As a first step, employers should evaluate all employee’s skills to determine if they have the necessary knowledge and training to perform work on or near energized electrical circuits and equipment. Those that have inadequate training or knowledge must maintain minimum approach distances in the direction of live parts to ensure their protection. At a minimum they should be trained in and be very familiar with proper approach distances for unqualified persons.

De-energize First


Figure 2. Potential high energy and available fault currents require specialized PPP.

To protect electricians and other workers exposed to the hazards of electricity, the Occupational Safety and Health Administration (OSHA) established guidelines which must be followed to protect against the damaging effects of electricity. In general, these are referred to as "Electrical Safety-Related Work Practices.” These work practices were originally developed under direction from OSHA in NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces. In general, both NFPA 70E and OSHA 1910 Subpart S, Electrical Standards require electrical circuits and equipment to be de-energized before work is performed on or near them. Note that "de-energized” is defined as being placed into an electrically safe work condition by locking out and tagging the circuit and equipment. For circuits 50-volts and above, work is not permitted to be performed on or near live parts unless the employer can demonstrate that de-energizing the circuit or equipment introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Note that the definition of "infeasible” does not include considerations such as cost or convenience. Infeasibility is intended to apply to equipment operational limitations. For example, measuring voltage or taking current readings is not possible with the circuit or equipment de-energized and would require that the task be performed with the circuit in an energized condition. Too often, accidents occur when electricians fail to de-energize or request that circuits be de-energized first. Typically, accident reports indicate that work was performed in an energized condition because it would have been "inconvenient” or would have cost too much to de-energize the circuit or equipment.

Working Hot


Figure 3. Routine tasks, such as current readings, require appropriate PPE.

When the employer demonstrates that it would create an additional or greater hazard or it is infeasible to de-energize the circuit, other safety-related work practices must be employed to protect workers when work on or near energized circuits or equipment must occur. These safety related work practices vary with the specific task and hazards associated with the job and must be suitable for the conditions under which the work is to be performed and for the voltage level of the exposed electric conductors or circuit parts. Typically they may include, the use of all necessary personal protective equipment, insulating blankets, shields or barriers, insulating tools and protective clothing. These provisions apply to all circuits and equipment which operate at 50-volts and over. Because of the multiple hazards associated with this type of work, a complete job safety analysis should be completed before performing the work. This analysis should include careful consideration of the specific hazards associated with the task. The proper selection of the appropriate safety-related work practices and personal protective equipment is determined from this analysis.

Conclusion

Unfortunately, many electricians today fail to follow or completely adhere to the OSHA regulations and NFPA 70E recommendations. Instead, they choose to ignore the minimum safety-related work practices and fail to utilize proper personal protective equipment. Frequently, electrical workers and other personnel who work on or near energized electrical circuits and equipment, without the necessary PPE become statistics, highlighting the dangers associated with electrical work. A decision to work circuits and equipment in an energized condition should only be made after a determination has been made that it would be infeasible or that it would create a greater hazard to de-energize. Once that demonstration has been made, a job hazard analysis should be performed to evaluate all of the possible hazards associated with the tasks. The specific safety-related work practices, personal protective equipment and insulating tools necessary for the task should be apparent after the job hazard analysis is complete. Finally, a job briefing should be conducted prior to beginning the task to ensure that each of the employees involved with the task understand his or her responsibilities and the hazards associated with those responsibilities.


Read more by Michael Johnston

Tags:  Featured  May-June 2000 

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Grounding Electrodes

Posted By IAEI, Monday, May 01, 2000
Updated: Monday, February 11, 2013

Definition


Figure 1. Grounding electrode

Grounding electrode.

A conducting element used to connect electrical systems and/or equipment to the earth. [See figure 1]

General

For many applications, grounding electrodes provide the essential function of connecting the electrical system to the earth. The earth is considered to be at zero potential. In some cases, the grounding electrode serves to ground the electrical system. In other instances, the electrode is used to connect noncurrent carrying metallic portions of electrical equipment to the earth. In both situations, the primary purpose of the grounding electrode is to maintain the electrical equipment at the earth potential present at the grounding electrode.

Another essential function of the grounding electrode is to dissipate over-voltages into the earth. These over-voltages can be caused by high-voltage conductors being accidentally connected to the lower-voltage system such as by a failure in a transformer or by an overhead conductor dropping on the lower-voltage conductor. Over-voltages can also be caused from lightning.

 

Figure 3. Grounding Electrode System

In Section 250-24(c), we find a requirement to connect the equipment grounding conductors, the service-equipment enclosures, and where the system is grounded, the grounded service conductor to a grounding electrode. The conductor used to make this connection is the grounding electrode conductor.

Grounding electrode system

The NEC in Section 250-50 requires that, where available on the premises at each building or structure served, all grounding electrodes including "made” electrodes be bonded together to form the grounding electrode system. This includes metal underground water pipes, metal frames of buildings, concrete-encased electrodes, and ground rings. The general requirement is that a bonding jumper must be installed between the grounding electrodes to bond them together. A grounding electrode conductor is run from the service enclosure to one of the grounding electrodes that are bonded together. The NEC also provides for the option of running a grounding electrode conductor to each grounding electrode individually. [See figure 3]

Where the interior metal water pipe is used as a part of the grounding electrode system or as a conductor to bond other electrodes together to create the "grounding electrode system,” Section 250-50 requires that all bonding take place within the first 5 feet from the point the water pipe enters the building. This section does not require that the interior water pipe be used for the purpose of interconnecting other electrodes to form the grounding electrode system. Any of the other electrodes, such as the metal frame of the building, concrete encased electrode or ground ring, can be used for the purpose of interconnecting the other grounding electrodes. Where these other electrodes are used for this purpose, no restrictions are placed on where the connections are permitted to be made or how far inside the building they are permitted. Section 250-68(a) requires grounding electrode conductor connections to grounding electrodes to be accessible except for connections to a buried, driven, or concrete encased electrode.

Grounding electrodes required to be used


Figure 4. Grounding electrodes that must be used

All of the identified grounding electrodes are required to be used where "available on the premises at each building or structure served.” The Code does not define what is meant by "available” nor does it require that the electrodes be made available where they are not. For example, if the building has encased concrete reinforcing rods when the electrical system is installed, it is not required that the rods be exposed for connection. On the other hand, the concrete reinforcing rods must always be used when "available.” Several electrical inspection agencies require that a concrete-encased grounding electrode be connected to the system before approval of the service for utility connection is granted. The grounding electrodes are not listed in an order of preference nor is it optional to choose which ones to use. [See figure 4]

Electrodes that must be used, in addition to any "made” electrodes that exist or are installed at the building or structure served, where "available” are as follows:

1. Metal Underground Water Pipe.Defined in Section 250-50(a) as "A metal underground water pipe that is in direct contact with the earth for 10 feet or more including any metal well casing that is effectively bonded to the pipe.” There is no minimum or maximum pipe size given. Types of metal, such as steel, iron, cast iron, stainless steel or even aluminum are not distinguished. Different types of water pipes such as for potable water, fire protection sprinkler systems, irrigation piping, etc., are also not defined. As a result, all of these metal underground water pipes must be used where "available at each building or structure served.”

Continuity of the grounding path of the water pipe grounding electrode or the bonding of interior piping systems cannot depend on water meters or on filtering devices or similar equipment. See Section 250-50(a)(1). Where a water meter or filtering equipment is in this metal water piping system, a bonding jumper must be installed around the equipment to maintain continuity even if the water meter or filter is removed.

2. Metal Frame of the Building.Section 250-50(b) requires the metal frame of the building to be used as a grounding electrode where it is effectively grounded. "Grounded effectively” is defined in Article 100 and means that the metal frame of the building is, "Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.”N


Figure 5.

To be an effective grounding electrode, the metal frame of the building must have a sufficiently low-impedance contact with the earth to pass current when called upon to do so and to maintain the electrical system at or near the electrical potential of the surrounding earth. The building steel can be connected to the earth by bolted or welded connection to reinforcing steel in foundations or footings that are in turn encased in concrete. Also, building structural steel may itself be encased in concrete that is in contact with the earth. In both of these cases, the concrete that encases the building steel or reinforcing steel must be in direct contact with the earth.

Certain back-fills such as gravel or vapor barriers may render the building steel an ineffective electrode. Building steel that is connected to concrete footings or foundations by only "J” bolts are not considered "effectively grounded” unless these "J” bolts are in turn connected to structural members such as reinforcing steel. The reinforcing steel needs to be near the base of the footing or foundation.


Figure 6. Size of bonding jumper for grounding electrode system

The structural steel should be tested with an earth resistance tester if in doubt about its resistance to ground and adequacy as a grounding electrode.

3. Concrete-Encased Electrodes.Section 250-50(c) defines this grounding electrode as one or more steel reinforcing bars or rods that are not less than 20 feet in length and ½ inch in diameter or 20 feet or more of bare copper conductor not smaller than No. 4. These electrodes must be located within or near the bottom of the foundation or footing and be encased by at least 2 inches of concrete. A single 20 ft. length of reinforcing bar is not required. Reinforcing bars are permitted to be bonded together by the usual steel tie wires or other effective means like welding. Where subjected to high currents such as lightning strikes, welding is preferred.

Reinforcing rods must be of bare, zinc galvanized or other electrically conductive coated steel material. Obviously, insulated reinforcement rods would not perform properly as a grounding electrode. Some complaints have been made that lightning surges, that are dissipated through this electrode, break out chunks of concrete where the surge exits the footing.

This grounding electrode is commonly referred to as the "Ufer ground” after H.G. Ufer who spent many years documenting its effectiveness. Additional information on the development and history of the concrete-encased electrode is available in the Appendix of Soares Book on Grounding.

Several electrical inspection agencies require that a concrete-encased electrode be installed or connected to the service prior to authorizing electrical service due to its effectiveness in most any climatic and soil condition.

4. Ground Ring.Section 250-50(d) recognizes a copper conductor, not smaller than No. 2 and at least 20 feet long, as a ground ring grounding electrode. The conductor must "encircle” the building or structure and be buried not less than 2½ feet deep. Ground rings often are installed at telecommunication central offices, radio and cellular telephone sites. Where available on the premises served, ground rings must be used as one or more of the grounding electrodes making up the grounding electrode system.

Supplemental electrode


Figure 7. Size of individual grounding electrode conductor

Section 250-50(a)(2) requires that where the only grounding electrode available and connected at the building or structure served is a metal underground water pipe, it be supplemented by another grounding electrode. Electrodes suitable to supplement the metal underground water pipe include: the metal frame of the building, a concrete-encased electrode, ground ring, other local metal underground systems or structures, rod and pipe electrodes, or plate electrodes. This supplemental grounding electrode is required since, often, metal underground water pipes are replaced by plastic water services or the system continuity is interrupted by nonmetallic couplings or repairs. The effectiveness of the water pipe grounding electrode would thus be lost.

Specific locations are provided where the supplemental grounding electrode is permitted to be connected. Where an underground metal water pipe is the only grounding electrode, the supplemental grounding electrode is permitted to be connected to only the grounding electrode conductor, the grounded service-entrance conductor, the grounded service raceway or to any grounded service enclosure. An exception to this requirement permits the bonding connection to the interior metal water piping in a qualifying industrial or commercial plant to be made at any location if the entire length of interior metal water pipe that is being used as a conductor is exposed.


Figure 8. Made electrodes

Often, changes, repairs or modifications are made to the metallic water piping systems with nonmetallic pipe or fittings or dielectric unions. In this case, it is possible to inadvertently isolate portions of the grounding system from the grounding electrode conductor. This is another in several steps that has been taken over recent years to reduce the emphasis and reliance on the metal water piping system for grounding of electrical systems.

With a change to the 1999 NEC, where the supplemental grounding electrode is of the rod, pipe or plate type, it is now required to meet the 25-ohm-to-ground rule in Section 250-56. This means that the supplemental grounding electrode must have a resistance of not more than 25 ohms or a second supplemental grounding electrode must be used. This has the effect of the system being served by only the supplemental grounding electrodes in case the underground metal water pipe grounding electrode is interrupted for any reason. [See figure 6]

Size of bonding jumper for grounding electrode system

The bonding jumper used to bond the grounding electrodes together to form the grounding electrode system must be sized in accordance with Section 250-66 based on the size of the ungrounded service-entrance conductor. The conductor that connects the grounding electrodes together is a bonding conductor and not a grounding electrode conductor. The bonding conductors are not required to be installed in one continuous length as grounding electrode conductors are. Also, the exceptions for sizing the grounding electrode conductor in Section 250-66 apply for the sizing of the bonding jumpers. [See figure 7]


Figure 9. Installation of made electrodes

For example, if the service-entrance conductor is 500 kcmil copper, the minimum size of bonding jumper is determined by reference to Section 250-66 and Table 250-66, including the rules in Sections 250-66(a), (b) and (c) are as follows:

  • To metal underground water pipe and metal frame of a building; No. 2 copper or No. 1/0 aluminum conductor. (From Table 250-66.)
  • To "made” electrodes as in Section 250-52(c) or (d) such as pipes, rods or plates; that portion of the bonding conductor that is the sole connection to the made electrode; No. 6 copper or No. 4 aluminum. The term "sole connection” means that the bonding conductor is not connected to the made electrode being considered and then another grounding electrode is connected to it. See Section 250-66(a).
  • To a concrete-encased electrode as in Section 250-50(c); that portion of the bonding conductor that is the sole connection to the concrete-encased electrode; No. 4 copper conductor. See Section 250-66(b).
  • To a ground ring as in Section 250-50(d); that portion of the bonding conductor that is the sole connection to the ground ring is not required to be larger than the ground ring conductor. See Section 250-66(c).

Note that aluminum is not permitted to be installed as a grounding electrode conductor where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum grounding conductors are not permitted within 18 inches of the earth. See Section 250-64(a).

No sequence for installing the bonding jumper or jumpers is given. However, the minimum wire size required to the various grounding electrodes must be observed. In addition, the point where the grounding electrode connects to the grounding electrode system must provide for the largest required grounding electrode conductor. For example, it would be a violation to connect a No. 4 bonding conductor from a concrete-encased grounding electrode a building steel grounding electrode which would require a 3/0 grounding electrode conductor. The installation would be acceptable if the 3/0 copper grounding electrode conductor connects to the building steel and a No. 4 copper bonding jumper extends to the concrete-encased electrode. In addition, the unspliced grounding electrode conductor is permitted to run from the service equipment to any convenient grounding electrode.

Alternately, individual grounding electrode conductors are permitted to be installed from the service equipment to one or more grounding electrodes rather than the electrodes being bonded together in a circular or "daisy-chain” manner. See Section 250-50. The minimum size of each grounding electrode conductor to the individual grounding electrode is shown in Figure 6-5. Note that a grounding electrode conductor is permitted to "supply” or "serve” any number of grounding electrodes but must be sized for the largest grounding electrode conductor required. For example, a bonding conductor is permitted to be run to a concrete-encased electrode and then to the underground metal water pipe. The bonding jumper must be sized for the largest grounding electrode conductor required for the grounding electrode or electrodes served.

Made electrodes


Figure 10.

Where the electrodes described in Section 250-50 are not available at the service location, a grounding electrode must be "made” or installed. The made electrode as provided for in Section 250-52 may be local metal underground systems or structures, driven pipes, or rods or buried plates conforming to the following requirements:

(b) Local systems.Local metallic underground systems as piping, tanks, etc. These objects must have the metal in direct contact with the earth. Protective coatings may render them ineffective as a grounding electrode.

(c)(1) Pipe electrodes.Pipe or conduit electrodes shall be not less than 8 feet in length nor smaller than ¾-inch trade size and if of iron or steel, shall be galvanized or metal-coated for corrosion protection.

(c)(1) Rod electrodes.Electrodes of steel or iron shall be at least 5/8 inch diameter. Rods of nonferrous metal or stainless steel that are less than 5/8 inch in diameter shall be listed and be at least ½-inch in diameter.

(d) Plate electrodes.Electrodes shall have at least 2 square feet of surface in contact with exterior soil. If of iron or steel, the plate shall be at least ¼-inch thick. If of nonferrous metal, they shall be at least 0.06 inch thick.

Note that underground metal gas piping systems are not permitted to be used as a grounding electrode. This does not eliminate the requirement that interior metal gas piping systems be bonded. For additional information on bonding of metal piping systems, see Soares Book on Grounding, 7th edition, Chapter 8.

Installation of made electrodes

Where practicable, made electrodes must be installed below permanent moisture level. This is a key ingredient in establishing an effective made electrode. They also are required to be free from nonconductive coatings such as paint and enamel. [See figures 8 and 9]

Rod and pipe electrodes must be installed so at least 8 feet is in contact with the soil. They must be driven vertically unless rock bottom is encountered. If rock bottom is encountered which prevents the rod from being driven 8 feet vertically, the rod is permitted to be installed at an oblique angle of not more than 45 degrees from vertical or it can be buried in a trench that is at least 2½ feet deep. See Section 250-52(c)(3).

The upper end of the rod must be flush with or below ground level unless the aboveground end of the rod and the grounding electrode attachment are protected from physical damage. This, of course, requires that a ground rod longer than 8 feet be used if any of the rod is exposed above ground level. For an eight-foot ground rod or pipe, the ground clamp must be listed for direct earth burial as the electrode must be driven to its full length.

Plate electrodes are required to be buried not less than 2 1/2 feet in the soil.

Section 250-10 requires that ground clamps or other fittings be approved (acceptable to the authority having jurisdiction) for general use without protection or be protected from physical damage by metal, wood or equivalent protective covering.

Common grounding electrode


Figure 11.

Section 250-58 of the NEC requires that a common grounding electrode be used for all alternating-current system grounding in or at a building. In addition, where more than one service supplies a building, the common grounding electrode must be used for all services. This section recognizes that where two or more grounding electrodes are bonded together, they are considered to be one electrode.

Interestingly, no distance between electrodes is given beyond which the electrodes do not have to be bonded together. Buildings of "large area” are permitted by Section 230-2(b)(2) to have more than one service. However, nothing in the Code defines the dimensions of a "large building.” Some inspection authorities use voltage drop of major feeders for guidance in determining when a building is one of "large area.” Where feeder conductors would have to be increased in size unreasonably to maintain voltage regulation, one or more additional services are permitted.

Section 250-58 requires the grounding electrodes for the multiple services be bonded together no matter how far apart they are in the same building. This is important so there is not more than one earth potential impressed on equipment in or on the building. Section 250-50 requires the bonding jumper(s) used for this purpose to be sized from Table 250-66 and be installed in accordance with Sections 250-64(a), (b) and (e). For additional information on installation of grounding electrode conductors, see Soares Book on Grounding, 7th edition, Chapter 7. It is permitted to use the steel frame of a building that is effectively grounded or a concrete encased grounding electrode to bond grounding electrodes from other services together.

Section 250-58 also requires that a common grounding electrode be used to ground conductor enclosures and equipment in or on the building and that the same grounding electrode be used to ground the system. This does not mean that one cannot use more than one grounding electrode. But, if more than one is used, then all the grounding electrodes must be bonded together to form a common grounding electrode. Where multiple grounding electrodes are bonded together as cited above, such multiple grounding electrodes become, in effect, a common grounding electrode system.

Earth return prohibited

No mention is made in the Code to providing a low-resistance, low-impedance, common grounding electrode for clearing ground faults. Reference to Figure 6-10 will show that the grounding electrode is in the earth return circuit. Even if the grounding electrode resistance to earth was very low, it would have little affect on the clearing of a ground fault, because the reactance of the earth and the soil resistance in the return circuit is very high. Where a parallel path exists through the earth and through a grounded service conductor, about 95 percent or more of the ground-fault current will return to the source over the grounded service conductor. A low-resistance, common grounding electrode is valuable, however, in holding equipment close to earth potential. It simply is not effective in clearing a line-to-ground fault.

Section 250-2(d) and 250-54 make it clear that grounding electrodes are not permitted to be used instead of equipment grounding conductors. The earth is not to be used as the sole or only equipment grounding conductor. However, grounding electrodes are permitted to supplement equipment grounding conductors.

If a ground fault should develop as shown in the upper drawing in Figure 6-11 where two separate grounding electrodes are used, the fault current flow will be through the service conductor then through the impedance of the ground fault, the grounding electrode conductor, the grounding electrode, the path through the earth to the grounding electrode at the transformer and finally through the grounding conductor to complete the circuit to the transformer. It would be a rare case where that circuit resistance would add up to less than 12 ohms (while the impedance would be higher). At best, therefore, the fault current would not reach a value high enough to operate a 15-ampere overcurrent device on a 120 volt-to-ground circuit. (120 ÷ 12 = 10 amperes).

Considering resistance only, the circuit shown has two grounding electrodes in series. Compared to the much lower resistance parallel path of the grounded circuit conductor, a resistance ratio between the two parallel paths is about 50 times for a 100 ampere service, to well over 100 times for the larger services. When impedance of the two paths is considered, the ratio will be higher. Thus, almost all the current from a line-to-ground fault will return to the transformer over the grounded service conductor.

Under normal operating conditions some unbalanced current will flow in the neutral. Some unbalanced neutral current will thus flow through the earth, but it will be small in comparison to that which will flow through the grounded service conductor.

Any belief that the circuit to the grounding electrode can be depended on to clear a ground fault is clearly erroneous no matter how large a grounding electrode conductor is used or how good a grounding electrode is. However, when the high-impedance earth path is short-circuited by installing the grounded circuit conductor as shown in the lower drawing in Figure 6-11, a low-impedance path is established as required in Section 250-2(d). This will allow a large current to flow over the equipment grounding and service-grounded conductor to allow the branch-circuit, feeder or service overcurrent device to clear the fault and thus provide the safety contemplated by the Code.

Resistance of grounding electrodes

There is no requirement in Article 250 that the grounding electrode system required by Section 250-50 (consisting of metal underground water pipe, metal frame of the building, concrete-encased electrode or ground ring) meet any maximum resistance to ground. No doubt it is felt that the grounding electrode system will have a resistance to ground of 25 ohms or less.


Figure 12. Supplemental electrode

The rules change for "made” electrodes. The Code states, in Section 250-56, that where a single rod, pipe, or plate electrode does not achieve a resistance to ground of 25 ohms or less, it shall be supplemented by one additional electrode. This means that where driven ground rods are utilized, two ground rods would be the maximum required under any condition. There is no requirement that additional made electrodes such as ground rods or plates be installed until the 25 ohm-to-ground resistance is obtained.

In general, metallic underground water piping systems, metallic well piping systems, metal frame buildings and similar grounding electrodes may be expected to provide a ground resistance of not over 3 ohms and, in some cases, as low as 1 ohm.

However, from a practical standpoint, no grounding electrode, no matter how low its resistance, can ever be depended upon to clear a ground fault on any distribution system of less than 1,000 volts.

If a system is effectively grounded as pointed out in the Code under Section 250-2(d), a path of low impedance (not through the grounding electrode) must be provided to facilitate the operation of the overcurrent devices in the circuit. See Chapter 11 of Soares Book on Grounding "”Clearing Ground-fault Circuits on Distribution Systems.”"

The lowest practical resistance of a grounding electrode is desirable and will better limit the voltage to ground when a ground fault occurs. It is more important to provide a low-impedance path to clear a fault promptly, for a voltage to ground can only occur during the period of time that a fault exists. Clearing a ground fault promptly thus will enhance safety. [See figure 12]

Even though the grounding electrode has low resistance, it is a part of a high-impedance circuit and plays virtually no part in the clearing of a fault on a low-voltage distribution system. This is due in part to being a higher resistance path through the earth than through the grounded service conductor. In addition, the remote path through the grounding electrode and earth is a high-impedance path compared to the circuit where the grounded service conductor is installed and routed with the ungrounded phase conductors.

Objectionable currents

The Code in Section 250-6 recognizes that conditions may exist which may cause an objectionable flow of current over grounding conductors such as the grounding electrode conductor, other than temporary currents that may be set up under accidental conditions. We should recognize that grounding conductors are not intended to carry current under normal operating conditions. They are installed for and are intended to carry current to perform some safety function.

The Code does not define what is meant by "objectionable” currents. Clearly, any current over a grounding electrode conductor that would prevent it from maintaining the equipment at the earth potential would be objectionable. Since every conductor has resistance, current flow through the conductor will produce a voltage drop across it. Any voltage drop on a grounding conductor that would create a shock hazard certainly would also not be acceptable.

Section 250-6(b) permits the following corrective actions to be taken where there is an "objectionable” flow of current over grounding conductors:

1. If due to multiple grounds, one or more, but not all, of such grounds may be discontinued,

2. The location of the grounding connection may be changed,

3. The continuity of the grounding conductor or conductive path between grounding connections may be suitably interrupted, or

4. Other means satisfactory to the authority enforcing the Code may be taken to limit the current over the grounding conductors.

The Code points out that temporary currents that result from accidental conditions such as ground-fault currents, that occur only while the grounding conductors are performing their intended protective functions, are not considered the "objectionable” currents covered in these sections.

Section 250-6(d) points out that currents that introduce noise or data errors in electronic equipment are not considered to be objectionable currents. Electronic data processing equipment is not permitted to be operated ungrounded or by connection to only its own grounding electrode.


Reprinted fromSoares Book on Grounding, 7th edition.

About IAEI: IAEI, as the keystone of the electrical industry, is a membership driven, not-for-profit association promoting electrical safety throughout the industry by providing premier education, certification of inspectors, advocacy, partnership and expert leadership in electrical codes and standards development.

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Lightning and Lightning Protection

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

A National Standard of Canada, CAN/CSA-B72-M87 Installation Code for Lightning Protection Systems provides guidance on lightning protection. The following information may be found in the standard.

Lightning

Lightning results from the build-up of an electrical charge on a cloud. When this charge has built up to a sufficient level, a lightning stroke to earth may result. The stroke is essentially an electric spark and it acts as a conductor for the electric charge stored in the cloud. Consequently, throughout the duration of the stroke, an electric current flows between the cloud and the earth. This current may reach many thousands of amperes. The damage caused by lightning is mainly due to the very high current flowing in the lightning stroke. Lightning damage can be tremendous when the current flows in a poor conductor or when an arc flows between conductors.

Lightning Protection

The fundamental principle in the protection of life and property against lightning is to provide a means by which a lightning discharge can enter or leave the earth without resulting damage or loss. A low impedance path must be offered that the discharge current will follow in preference to all alternative high impedance paths offered by building materials such as wood, brick, concrete or stone. When lightning follows the higher impedance paths, damage may be caused by heat and mechanical forces generated during the passage of the discharge.

Most metals, being good conductors, are virtually unaffected by either heat or mechanical forces if they are of sufficient size to carry the current that can be expected. The metal path must be continuous from the ground terminal to the air terminal. Care should be exercised in the selection of metal conductors to ensure the integrity of the lightning conductor for an extended period. A non-ferrous metal such as copper or aluminum will provide, in most atmospheres, a lasting conductor free of the effects of rust or corrosion.

Parts of structure most likely to be struck by lightning are those that project above surrounding parts, such as chimneys, towers, water tanks, spires, steeples, dormers and parapets. The edge of the roof is the part most likely to be struck on flat-roofed buildings.

Lightning Protection Systems

Lightning protection systems consist of three basic parts:

  • A system of air terminals and intercepting conductors on the roof or other elevated locations
  • A system of ground electrodes
  • A conductor system connecting the two systems by means of down conductors or ground terminals

Properly located and installed, these basic components ensure that the lightning discharge will be conducted harmlessly to ground.

Conductors are provided to interconnect metal bodies to ensure that such metal bodies are maintained at the same electrical potential so as to prevent "”sideflashes,”" "”sparkover”" or the development of dangerous voltages. Surge arresters may also be provided to protect electrical equipment from dangerous overvoltages due to direct strokes or induction.

Metal parts of a structure may be used as part of the lightning protection system. For example, structural metal framing that has sufficient cross sectional area to equal the conductivity of main lightning conductors and is electrically continuous may be used instead of separate down conductors.

The best time to design a lightning protection system is during the planning phase, and the best time to install is during construction. Installing lightning protection during construction also allows the use of many architectural features that may result in savings not possible with an exposed system installed after construction.

The installation of air terminals and intercepting conductors should be planned for all areas or parts likely to receive a lightning discharge. The aim is to intercept the charge immediately above the parts liable to be struck and to provide a direct path to earth, rather than to attempt to divert the discharge in a direction it would not be likely to take.

Conductors should be installed to offer the least impedance to the passage of the stroke current between the air terminals and the earth. The most direct path is best, and there should be no sharp bends or narrow loops. There should be at least two paths to ground, and more if practicable, from each air terminal.

Well made ground connections and ample contact with the earth are essential to the effective functioning of a lightning protection system. This does not necessarily mean that the resistance of the ground connection must be low, but rather that the distribution of metal in the earth or upon its surface in extreme cases should be such as to permit the dissipation of the stroke without damage.

Low resistance is desirable but not essential, as may be shown by a building on moist clay and a building on solid rock. In the first case, if the soil has low resistance, a ground connection made by extending the conductor 3m into the ground might be 15 to 200 ohms. Two connections on a small building may to be sufficient.

In the second case, it would be impossible to make a ground connection in the ordinary sense of the term because most rocks have high resistivity. In order to obtain effective grounding, other and more elaborate means are necessary. The most effective means would be an extensive wire network, which would be laid on the surface of the rock surrounding the building and to which the down conductors could be connected. The resistance to the remote earth of such an arrangement would be high, but at the same time, the potential distribution around the building, and the resulting protective effect, would be substantially the same as those of a building resting on conductive soil.

Grounding arrangements will depend upon the characteristics of the soil, from a simple extension of the conductor into the ground where the soil is deep and of high conductivity, to a buried network where the soil is of very poor conductivity. Where a network is required, it should be buried, as this improves its effectiveness.

When a lightning conductor system is placed on a building within or about which there are metal objects of considerable size within a metre or two of the conductor, there will be a tendency for sparks or sideflashes between the metal object and the conductor. To prevent this, interconnecting conductors should be provided.

Lightning currents entering protected buildings on overhead or underground power lines, telephone conductors or radio antennas are not necessarily restricted to associated wiring systems and appliances. Therefore, such systems should have protective devices.

In general, lightning protection systems grounding is done using copper or aluminum cables and ground rods, 1/2 inch copper or copper-clad steel, 5/8 inch galvanized steel or copper plates. Rods should be spaced to allow down conductors to be run down a building to the ground electrode as directly as possible. A metallic underground water system may be used for both electrical and lightning protection grounding. The lightning protection grounding should always be interconnected with a metallic water system if available outside the building.

As with previous articles, you should consult local authorities for a more precise interpretation of any of the above.


Read more by Leslie Stoch

Tags:  Canadian Code  May-June 2000 

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Installations and Inspections of Motors and Motor Circuit Protection

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

Overcurrent protection for motors and motor circuits is a little different than the rules for conductors as specified in Article 240, because motor loads have different characteristics than general lighting and other loads. Motor circuits draw a large amount of current at initial start-up, usually around six times the normal full-load current (FLA) of the motor. This large amount of current drawn at start-up is usually referred as the "inrush current,” although the Code term is "locked rotor current” (LRA) (see Figure 1).

Figure 1. This large amount of current drawn at start-up is usually referred as the "inrush current," although the Code term is "locked rotor current" (LRA)

Safe wiring installations of electric motors and motor circuits depend on the proper understanding and application of some basic requirements in Article 430 of the National Electrical Code, particularly those in Section 430-6 for general motor installations or inspections. Instead of the nameplate full-load current, Section 430-6 requires the tables in Article 430 to be used to size circuit conductors, branch-circuit short-circuit and ground-fault protective devices, and ampere ratings of disconnect switches (see Figure 2). The actual full load current for different motors of the same size and type may vary. The tables are used to ensure that if a motor needs to be replaced, the components of the motor circuit will not also need to be replaced. This requirement applies to general motor applications. The rules for torque motors and alternating-current adjustable voltage motors are different. The actual nameplate current is used to size these components of the circuit. This article deals with general motor applications.


Figure 2. Instead of the nameplate full-load current, Section 430-6 requires the tables in Article 430 to be used to size circuit conductors, branch-circuit short-circuit and ground-fault protective devices, and ampere ratings of disconnect switches

When installing or inspecting a motor circuit for the proper overcurrent protection, a systematic approach usually works best. The four elements of the installation typically examined include, (1) the branch-circuit sizing (conductors), (2) the overload protection, (3) the branch-circuit short-circuit ground-fault protective device, and (4) the rating of the motor disconnect (see Figure 3). These four elements are the major items of concern in the installation or inspection and certainly are not all-inclusive as installations differ.

The motor nameplate information is important. The nameplate voltage and horsepower ratings are needed in order to use the tables in Article 430. The horsepower rating at the applied voltage is used with the appropriate table to determine the full-load current rating of the motor. This full-load current value must be used to size the conductors and the branch-circuit short-circuit and ground-fault protective device.


Figure 3.

Motor Branch-Circuit Conductor Sizing

As an example, a 115-volt, 1½ horsepower electric motor draws 20 amperes based on Table 430-148. Even though the motor nameplate (see Figures 4, 5, and 6) indicates it draws 18.6 amperes at 115 volts, the value in Table 430-148 must be used for sizing purposes, as required by Section 430-6(a).

The next component of the motor circuit is the branch-circuit conductor sizing. Part B of Article 430 covers the requirements for sizing the branch-circuit conductors for single motors and groups of motors. This is a single motor example, so looking at Section 430-22(a), a single motor used in a continuous duty application (three hours or more) is required to have an ampacity of not less than 125% of the motor full-load current as determined by Section 430-6(a)(1). Taking the value of 18.6 amperes and multiplying it by 125% results in an ampere value of 23.5 amperes. The minimum conductor size for this motor circuit, after any ampacity adjustments or correction factors have been applied, would need to be a minimum of 23.5 amperes. Using Table 310-16, the Code permits No. 12 THWN copper conductors for this installation, which would permit some cable assemblies, such as Type NM and others sized at No. 12, to be used for this application. There are other factors that can contribute to the size of the motor branch-circuit conductors, such as voltage drop in long runs and the application of ampacity adjustment factors for either number of current-carrying conductors in the same raceway or ambient temperature adjustment or both.


Photo 1. Thermaly protected motor

Motor Branch-Circuit Short-Circuit and Ground-Fault Protection

Branch-circuit short-circuit and ground-fault protective devices are required to be sized in accordance with the values given in Table 430-148 also. The branch-circuit short-circuit ground-fault device sizing requirements are located in Part D of Article 430. In Section 430-51 the Code says that these rules included in Part D amend or add to the requirements of Article 240. A few different types of branch-circuit short-circuit and ground-fault protective devices may be used for protecting motor branch-circuit conductors, motor control apparatus, and the motor against overcurrent due to short-circuits or grounds. Section 430-52(c) requires that the rating of the protective device that is used must not exceed the value calculated according to the percentage values given in Table 430-152. A look at Table 430-152 for a single- phase motor allows the following percentages.

• Nontime Delay Fuse 300%

• Dual Element Time-Delay Fuse 175%

• Instantaneous Trip Circuit Breaker 800%

• Inverse Time Circuit Breaker 250%

Basically these increases in percentages are to allow the motor to be started without causing the device to trip at locked rotor (starting) current. Where the values determined by the percentages in Table 430-152 do not correspond to the standard sizes or ratings of fuses, nonadjustable circuit breakers, or possible settings of adjustable circuit breakers, the next standard size, rating, or possible setting is permitted. The idea here is to provide a device that will afford the short-circuit and ground-fault protection and still be large enough to allow for the inrush current (locked rotor current) when the motor starts. If the motor locked rotor current is still great enough to trip the device at start-up, the percentage values given in Table 430-152 are again allowed to be increased to the maximum values given in the Exception No. 2 (a), (b), (c), and (d).


Figure 4.

Using a nontime delay fuse as the short-circuit ground-fault protective device for the 1½ horsepower, 115-volt motor, the device would be required to be sized at a maximum using the value 18.6 and multiplying that value by 300%, which results in a device rated at 55.8. Rounding up to the next higher standard size as permitted by Section 430-52(c)(1) Ex. No. 1, the short-circuit ground-fault protective device could be a 60 ampere nontime delay fuse and be in compliance with Section 430-52. This might appear as though the No. 12 conductors installed for the branch-circuit conductors would be unprotected. Remember, the rules in Part D of 430 amend the rules in 240 at this point. Do not expect the conductor to be protected at its ampacity, as it would be normally in Article 240.

There is still another level of protection to be provided in the motor circuit that completes the overcurrent protection for the motor and the motor circuit.

Motor and Branch-Circuit Overload Protection


Photo 2. Thermal heaters in a magnetic motor starter

Overload protection devices are intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure to start. An overload in an electrical motor circuit is an operating current that, if it persists for a sufficient length of time, would cause damage or dangerous overheating of the apparatus. Overload protection does not include protection against short-circuits or ground-faults. The combination of the overload protective device and the branch-circuit short-circuit ground-fault protective device provide the overcurrent protection for the motor and motor circuit.

Overload protection for motors can be provided in a few different forms. If the motor itself is a thermally protected motor, it is required to be marked with the words "Thermally Protected,” or an abbreviated marking "TP” (see Photo 1). If the motor is not marked to indicate it has integral thermal protection, then overload protection must be installed. Fuses, when sized properly, can serve as an overload protective device for a motor and motor circuit. Thermal heaters in a magnetic motor starter are another common method of overload protection (see Photo 2).


Figure 5.

Part C of Article 430 specifies overload protection requirements for motors, motor controllers, and motor branch-circuit conductors. Section 430-32 (a) requires that each continuous duty (three hours or more) motor rated at more than 1 horsepower must be protected by an overload protective device rated at no more than the following percentages of the motor nameplate rating. Using the values marked on the motor nameplate in Figure 6 the service factor of the motor is 1.15. This is the sizing that is accomplished by the use of the nameplate current value instead of the ampacity value in the table.

• Service factor not less than 1.15 125%

• Motor marked with

Temperature rise not over 40% 125%

• All other motors 115%

Modifications of these values above are allowed if the percentages are insufficient to start the motor or carry the motor load (see Section 430-34). These values in Section 430-32(a) are generally sufficient for general motor applications. Using the full-load current value marked on the motor, as per the requirements of Section 430-32(a)(1), the overload device would be sized at 125% of the value 18.6 amperes. The value, 18.6 amperes multiplied by 125%, results in a value of 23.25 amperes. An overload protective device should be selected not to exceed that value. The manufacturer of a motor starter or motor controller provides a thermal heater selection table with the controller to assist in selecting the properly sized overload device.


Figure 6.

Section 430-40 in the Code adds some additional requirements to be aware of. Overload protective devices for motor overload protection are generally not capable of opening short-circuits or ground-faults and thus these overload protective devices must be protected by fuses or circuit breakers with ratings or settings in accordance with Section 430-52 or by a motor short-circuit protector in accordance with Section 430-52. Many motor starters and controllers that utilize thermal overload devices also will specify a maximum rating of a fuse or circuit breaker to properly protect the overload device within its short-circuit capabilities. Pulling out the magnifying glass and reading the tiny print on the inside of the magnetic motor starter enclosure is critical to maintaining proper protection and complying with Section 110-10.

Disconnecting Means and Controller

The Disconnect. The rating of the disconnecting means for general motor installations is required to be in accordance with Part J of Article 430. Basically the disconnecting means must be capable of disconnecting the motor and controller from the circuit. The rating of the disconnecting means must have an ampacity of at least 115% of the full-load current rating of the motor, based on the appropriate table in Article 430. The disconnecting means must also have a horsepower rating at least equal to that of the motor or be of any of the other types listed in Section 430-109.


Figure 7. Overcurrent protection for motors

The Controller.A controller is defined as the device that is normally used to start and stop a motor by actually breaking the motor-circuit current. A control device connected to a motor control circuit is not a motor controller. A motor starter and a properly rated (HP) contactor are two forms of motor controllers. Other devices also are permitted to serve as motor controllers. Ratings of the controller or motor starter must be in accordance with Part G of Article 430. Section 430-82 requires each controller to be capable of starting and stopping the motor it controls and to have the capability of interrupting the locked-rotor current of the motor. Section 430-83 details the required controller ratings.

Summary

The total overcurrent protection for a motor, motor branch circuit, and motor control apparatus is provided by the combination of the motor branch-circuit short-circuit and ground-fault protective device (fuses, circuit breakers, or motor circuit protectors) in accordance with Part D of Article 430, used in combination with an overload protective device meeting the requirements of Part C of Article 430 (see Figure 7). Section 430-55 allows a single branch-circuit short-circuit and ground-fault protective device to provide the combined protection when the rating of the branch-circuit short-circuit and ground-fault protective device is set or rated so as to also provide overload protection in accordance with the ratings or settings required in Section 430-32 or 430-34. The Code also provides some references to Example No. D8 in Appendix D which may also assist the inspector and installer in more clearly understanding these requirements for overcurrent protection of motors and motor circuits.


Read more by Michael Johnston

Tags:  Featured  May-June 2000 

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Does UL review installation instructions?

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

Question: Installation instructions

Does UL review installation instructions that are provided with UL certified products? Are they evaluated to make sure they comply with model codes? What codes are they evaluated for compliance with?

Answer

Yes, during Listing and Classification investigations, UL does evaluate the manufacturer’s installation instructions provided with UL certified products. Many UL Standards specify numerous requirements for the content of the installation instructions. UL reviews installation instructions to verify proper content and determine if the product can be installed and operated as intended in accordance with the applicable UL Standards, which are compatible with the installation codes. For electrical products, the National Electrical Code (NEC), NFPA 70, is the applicable code.

The Guide information that proceeds each product category in the UL Product Directories usually identifies the specific(s), as well as articles and sections of the Code when appropriate.


Question: Class 2 circuits

What are the marking requirements for Class 2 circuits for information technology equipment?

Answer

The basic Standard used to investigate products in this category is UL 1950, Standard for Safety of Information Technology Equipment, Including Electrical Business Equipment. Class 2 circuits are marked or identified in the installation instructions with the intended circuit type, cable type or circuit voltage. Otherwise, all nontelecommunication-type output connectors of UL Listed Information Technology Equipment (ITE) are supplied by limited power circuits defined in UL 1950 and recognized as such in the NEC, Section 725-41(a)(4).


Question: Snow melting cable

Recently I came across some snow melting cable for roofs that was C-UL Listed. What does this Mark mean?

Answer

The C-UL Mark with letter "C” outside the circle at the 8 o’clock position means that UL has evaluated samples of the product to Canadian national standards. The UL Mark for Canada may also appear adjacent to the traditional UL Mark, which represents evaluations to national recognized safety standards of the United States.

When both Marks appear on the product, samples of the product were determined to comply with the national safety requirements of both countries. UL has also introduced a Mark that indicates compliance with both Canadian and U.S. requirements, the C-ULUS Mark.


Question: Light fixtures

Is a ceiling-mounted recessed-HID (high-intensity discharge) light fixture installed inside of a shower considered to be in a damp or wet location? What effects would a shower have on the fixture itself as HID lights are known to run hot?

Answer

The AHJ needs to determine whether a location is considered to be damp or wet. The NEC, Article 100 provides guidance for defining a damp or wet locations under the section heading "Locations.” For additional information, see Section 410-4(a), for installation requirements of fixtures in damp or wet locations. The ambient temperature of the shower is unlikely to cause a ceiling mounted recessed fixture to overheat. However, consideration should be given to the location relative to the likelihood of water splashing on the fixture.

A UL Listed wet location fixture is subjected to a rain and/or sprinker testing, as well as a thermal shock test to determine that the lens of the fixture does not break when subjected to splashing water.

Tags:  May-June 2000  UL Question Corner 

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CE Marking – Is the Inspector Being Fooled?

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

Inspectors play a key role in the implementation of the North American Safety System and the job is not easy. In addition to being the enforcer of the electrical installation code, the inspector must also determine if electrical products are acceptable for use. To do this, the inspector typically relies on some method of conformity assessment. Recently, there has been a significant increase in questions from inspectors about CE Marking and its acceptability as a method of conformity assessment in North America. Typical questions include… What is CE? Is it like UL? Who is CE? Am I supposed to be accepting CE? The material in this paper is intended to shed some light on such questions.

OK, So what is CE?

Nineteen countries1 in Europe decided that in order to have "free movement of goods and to promote a common level of safety” among their countries, these goods must have a mark that indicates conformity to the "common level of safety.” This marking is the CE Marking. Keep in mind that this "common level of safety” is for these European countries and is laid out in the European Union (EU) Directives. The CE marking is also applied to products outside the electrical industry, including toys, furniture, etc. The CE marking has nothing to do with North America or the North American safety system.

Where is the CE lab located?

CE is not an entity, a mark of conformity to a standard, a quality mark, or a certification organization logo. It is a marking for the EU authorities. There is no laboratory, certification agency, etc., associated with CE Marking. It is simply a marking applied to indicate conformance to a common set of essential requirements for nineteen countries in Europe.

How does a product get a CE Marking?

In general, the manufacturer can apply the marking and by doing so is self-declaring that the product meets the essential requirements laid out in the EU directives. In essence, this is a form of supplier’s declaration of conformity (SDoC) to the directives. The process for the manufacturer to determine compliance can vary from one product to the next and can be complicated, depending on the product. There are many excellent documents available to manufacturers that discuss the specifics of this process.

How does the CE Marking impact products sold in North America?

It doesn’t. The existence of the CE Marking has no bearing on whether or not the product complies with the appropriate standards in North America. The marking only indicates that the product meets the essential requirements mandatory in the member states of the involved European Community and allows free movement in the community. There are many products that meet the appropriate North American standards and are listed by acceptable laboratories. These products may also carry a CE Marking. However, these are separate issues. The product must meet the appropriate North American Standards to carry the listing mark (UL, CSA, ETL, ANCE, etc.). Separate from that, the same product may also be designed to meet the essential requirements of the EU directives and would be permitted to have the CE Marking for that reason.

As an electrical inspector, should I be accepting CE Marking on electrical products as suitable for installation in North America?
No. Unfortunately, inspectors are being told differently in the field. Many are being told that CE Marking is equivalent to UL (or similar marks of conformity) and that they must accept that CE Marking on the product installed in North America. This has led to significant and varied discussions on the issue. The bottom line is that CE Marking means nothing for products installed in North America and statements to the contrary only add confusion. The inspector needs some method of assurance that the product meets the appropriate North American standards.

Since the product is acceptable in Europe, isn’t it acceptable in North America?
No. First, keep in mind that CE Marking only indicates that the product can move freely throughout the European Community, it does not guarantee that it will be accepted locally in those countries. Furthermore, the standards in Europe and North America are different. The approaches to an Electrical Safety System are also different. With different safety systems and different product standards being used, you simply cannot assume that a product installed in one system can be safely applied in the other.

What should I be looking for?
Inspectors should look for what they always have. Some method of conformity assessment that is acceptable to your locality and will provide you with the confidence that the product meets the product standards appropriate for North America. For many, the typical method of conformity is third-party certification (listing) by a certification agency accepted in your locality. By having products that meet the product standards accepted in the country where you are inspecting, you can alleviate much of your concern as to whether the product is compatible with the electrical installation code being enforced.

1 There are sixteen countries actually in the EU, but three additional countries (Iceland, Norway and Liechtenstein) have also agreed to the CE Marking scheme.


Read more by Jim Pauley

Tags:  Featured  May-June 2000 

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I Thought It Was Dead

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

Assuming electrical circuits or equipment is dead or de energized can be a costly mistake. Phrases such as "I thought it was dead” have been used following an incident where an electrical shock or electrocution occurred. It pays to check it out. Every electrician who has worked for any length of time in the trade understands what can happen when a mistake occurs during work on energized or live parts of an electrical system. Electricians are taught in safety training classes to test the circuit to see if it is energized and to turn the power off before working on electrical equipment. Additional safety practices stipulate that the circuit should be locked out so that it cannot be inadvertently re-energized. For those situations where power cannot be shut off, proper safety equipment and procedures are to be used.

How many times have electricians intentionally worked on energized electrical systems when they easily could have been turned off? How often is safety equipment used when electricians have to work circuits hot? Experience has taught that many workers in the electrical construction field knowingly take risks by working on electrical equipment or circuits while they are energized when they could have readily been de-energized. When asked about it, a variety of answers are given as to why they do it. Excuses range from "I didn’t have time to go turn it off” to "If you can’t work it hot, you don’t need to be in the field.” Unfortunately, these types of attitudes are all to prevalent in the industry. An element of pride and good workmanship is the knowledge ability to work safely without taking risks that can have life long lasting effects. Electric shock and electrocution can ruin not only the lives of the individual(s) involved in the incident, but has devastating effects on families.

It is worth the time to go to the toolbox or truck to get a volt meter to verify that equipment to be worked on is not energized. It is a good practice to verify that the voltage tester is working. Test it on a known live circuit before testing the circuit being worked on. Relying on what appears to be or on someone’s word is dangerous. On one occasion where portable stage equipment was being connected to a weatherproof fusible 200-amp switch, the electrician making the connection did not have a volt meter handy and proceeded to connect the portable cable to the bottom lugs of the switch. Two raceways emerged from the ground and entered the switch from the bottom. A set of ungrounded conductors from one raceway connected to the top lugs of the switch and the conductors in the other raceway connected to the bottom lugs. The electrician opened the switch and began the process of connecting the portable cable to the bottom lugs. During the process, the electrician inserted the wrench in the slot and began loosening the hex screw in the wire terminal. As his hand brushed lightly against the side of the switch enclosure, he received an unpleasant shock. After checking, it was found that the switch was fed from the bottom, resulting in the lugs being worked on and the open blades being energized. The individual made an incorrect assumption that because the switch was supposed to be fed from the top lugs that it had actually been installed correctly. It could have been a severe lesson to learn had the metal wrench come in contact with the metal enclosure or if the individual made good contact with a grounded object while holding the wrench to the lug.

Another assumption was made by an individual that could have resulted in a serious injury or a fatality. He was attempting to find out why a lighting fixture installed in a suspended ceiling of an office building would not work properly. The circuit feeding the fixture was enclosed in electrical metallic tubing and was connected to the fixture junction box by a compression-type connector. The EMT was secured to steel bar joists above the suspended ceiling. The electrician climbed a ladder and removed a lay-in ceiling tile in order to access the fixture and grasped the steel bar-joist with one hand to steady himself. With the other hand he loosened the screws on the junction box with a screwdriver and started to remove the cover. Upon contact with the fixture junction box, he received a shock running through both arms and across his chest. Fortunately, he was able to quickly remove his hand from the fixture housing and was stable enough to maintain his balance on the ladder. Upon investigation, it was found that the fixture junction box and housing was energized because the ungrounded hot conductor had separated from the wire connector and was in contact with the side of the junction box. The fault current path had been interrupted because an EMT connector was loose and did not make good contact with the metal raceway. Without a reliable path for fault current, the overcurrent protective device did not operate and the fixture housing remained at a potential of 120 volts to ground. No attempt was made to first disconnect power from the circuit before working on the fixture.

One electrician related a story of an installation where it was assumed that because a circuit breaker handle was in the off position, the circuit was not energized. Two individuals were installing a circuit for an electric range in an existing dwelling. The cable was run and because of the pressure to complete the job and get to another location, one electrician began connecting the cable to a receptacle at the range location. The other electrician began making the connection to the circuit breaker at the panelboard. After working the circuit breaker handle and leaving it in the off position, he installed it in the panelboard and began connecting the conductors. Upon connecting the conductors to the circuit breaker, the worker installing the range receptacle received a painful shock. It was discovered that the circuit breaker had been left unprotected in the back of a work truck and had been exposed to rain. The moisture had corroded the circuit breaker contacts and they remained closed even after the handle had been turned to the "off” position.

There is no substitute for knowing how to safely work with electricity and closely following safety procedures. It is not a test of courage to intentionally touch live parts or to work equipment hot. Working circuits hot does not create heroes. Heroes are made when they demonstrate wisdom in avoiding hazards where possible. It sometimes helps to bring the point home by listening to those who have been severely shocked by making contact with live parts or who were badly burned as a result of the flash of an electrical fault. It definitely pays to check out the equipment to make sure it is not energized before working on it.


Read more by Philip Cox

Tags:  Featured  May-June 2000 

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South Florida Building Code Accepts NEIS

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

The Miami-Dade County [Florida] Building Code and Product Review Committee became the first governmental entity in the country to accept NECA’s National Electrical Installation Standards™ for regulatory use, in November 1999. The Committee adopted the first three published NEIS™ into the South Florida Building Code as official references for methods of construction.

NECA’s South Florida Chapter spearheaded the first NEIS regulatory adoption, acting in cooperation with the IBEW and leading area electrical inspectors, who have been strong supporters of NECA’s quality standards for electrical construction.

"The chapter sponsored creation of a Chief Electrical Inspectors Council several years ago,” explains chapter manager Walter Bost. "That’s where we first started promoting the idea of having Metro-Dade and Broward counties endorse the NECA installation standards. Getting them included in the building code just seemed to make sense, the same way that NEC Article 800 references other industry standards as guides for neat and workmanlike installation.”

Bost credits three individuals in particular with helping turn the concept into a reality. Eddie Woodward of Anchor Electric Company Inc., president of NECA’s South Florida Chapter; Art Fernandez, business agent of IBEW Local 349; and John Travers, chief electrical inspector for the City of Hialeah all played major roles in the adoption process.

Fernandez serves on the Construction Products Approval Board, and Woodward testified in favor of adopting the National Electrical Installation Standards. Travers is former president of the Florida Chapter of International Association of Electrical Inspectors.

"I believe this just shows that we can accomplish a lot when our industry pulls together on an important issue,” Bost observes. "With NECA, IBEW and IAEI all singing from the same page, we had a strong three-fronted campaign supporting the cause of electrical construction quality and safety in South Florida.”

The Miami-Dade County Building Code and Product Review Committee action last month covers the first three NECA standards published—301 on fiber optics, 400 on switchboards, and 500 on commercial interior lighting. NECA’s South Florida Chapter has already proposed that the most recent standards (100 on electrical symbols and 502 on industrial lighting) be adopted as well.

"From this point forward, we plan to recommend that Miami-Dade County approve each new NEIS standard as they’re published,” says chapter manager Walter Bost.


NEIS in Print

There are currently five published National Electrical Installation Standards. For more information, contact NECA Codes and Standards at (301) 215-4521 tel, (301) 215-4500 fax, orneis@necanet.org. Or visit our website atwww.neca-neis.org.

• ANSI/NECA 100-1999, "Symbols for Electrical Construction Drawings” (ANSI)

• NECA/FOA 301-1997, "Standard for Installing and Testing Fiber Optic Cables”

• NECA 400-1998, "Recommended Practice for Installing and Maintaining Switchboards (ANSI)”

• NECA/IESNA 500-1998, "Recommended Practice for Installing Indoor Commercial Lighting Systems (ANSI)”

• NECA/IESNA 502-1999, "Recommended Practice for Installing Industrial Lighting Systems (ANSI)”


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Tags:  Featured  May-June 2000 

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Electrical Safety

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

May is the month designated to promote electrical safety. It is appropriate to set aside a time to emphasize this important issue. Too many accidents happen because individuals either fail to understand the hazards involving the misuse or abuse of electricity or they choose to ignore safety guidelines. One would think that members of the electrical industry, especially installers and maintenance personnel, would be familiar with electrical hazards and take the necessary steps to avoid them. Unfortunately, that is not always the case. It seems that familiarity breeds complacency. It is not uncommon to hear the phrase, "electricians are their own worst enemies.” The consuming public generally has little knowledge of electricity and either doesn’t know how to recognize electrical hazards or has been given wrong information by well-meaning but untrained people.

Electrically related fires and electric shock take too heavy a toll both in life, personal injury, and property damage. When we see the abuse of the electrical system, especially in those areas where no electrical inspections are required, it is surprising that there are not even more problems. This is partly the result of the quality of electrical products available and because of the existing electrical safety system in place in most areas. Manufacturers producing electrical equipment and materials based upon good safety and performance standards, the evaluation of those products by third party testing laboratories, and the verification of compliance with safety rules by qualified electrical inspectors all contribute significantly to the safety of users of electricity.

It seems that electrical inspectors need to become more active in promoting electrical safety in their local areas and becoming more visible in the public eye. Firefighters are recognized for the good work they do not only in fighting fires to protect the citizens they serve, but also in promoting fire safety through the local news media and in public appearances. Electrical safety can also be promoted by electrical inspectors through these types of venues. The IAEI has actively promoted electrical safety since it was established. However, it cannot reach many areas in need of safety education that can be accessed by the local electrical inspector. Several things are needed for this effort to be successful. The first is commitment by local inspectors. The inspector must also have support of the governing body to be successful. Unless the mayor, city manager, building official, or other appropriate authority recognizes the importance of promoting electrical safety and supports the electrical inspector in actively conveying that message to the public, the inspector’s efforts are severely handicapped. All too often, inspectors are seen by the public only during an inspection or when a local disaster occurs. In many cases, governmental officials of an inspection jurisdiction hear of the electrical inspector only when a complaint is filed by someone who does not want to comply with safety code rules. Unfortunately, those officials generally don’t hear about the good job inspectors are doing through the proper enforcement of electrical safety code rules to help provide an environment for the public to live and work in safely.

An individual electrical inspector can be very effective in conveying to the public the importance of electrical safety if he or she is willing to put forth the effort and the governmental agency supports it. An important part of this is that the electrical inspector needs to be prepared to meet the public and to promote safety in a professional manner. This includes having a comprehensive understanding of electricity, the dangers involved with it, and a knowledge of the proven safety practices that should be used. It also involves the ability to convey the message to the public in a way that can be readily understood. The National Electrical Safety Foundation (NESF), the Consumer Products Safety Commission (CPSC), and other organizations effectively promote electrical safety, but none is as capable of conveying the message to the consumer as the qualified electrical inspector. Those inspectors are the best and most direct link to the consuming public and they have the responsibility of serving those within their jurisdictions. The objective is to use the talent and skill of qualified electrical inspectors in making the public aware of the need for the adoption and enforcement of good electrical safety codes.

Read more by Philip Cox

Tags:  Editorial  May-June 2000 

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The Propagation of Surge Protective Devices

Posted By Deborah Jennings-Conner, Wednesday, March 01, 2000
Updated: Monday, February 11, 2013

Do you have a computer sitting on your desk? If so, chances are you have a surge protective device under your desk. As the use of products vulnerable to transient voltage surges and spikes continues to increase, the propagation of surge protection devices continues to increase. This article will focus on several types of surge protective devices, what they are, how they are tested, and the importance of markings, instructions and proper usage of the devices.

With the expanding surge protective devices industry comes a learning curve for code authorities, for electricians, for consumers and for the safety testing agencies. The code authorities begin to see an influx of different types of surge protective devices in the field and need to determine the proper usage and installation. Electricians need to know how to correctly install permanently connected surge protective devices. Consumers have to educate themselves as to which surge protective devices to purchase to meet their surge needs as well as how to install and use them safely. The safety testing agencies have to expand their knowledge base of surge protective devices to keep pace with the state of the art designs.


Figure 1.

What is a surge protective device and what does it do? A surge protection device is a device composed of any combination of linear or non-linear circuit elements intended for limiting surge voltages on equipment by diverting or limiting surge current. A surge protective device prevents continued flow of follow (power) current and is capable of repeating these functions.

Surge protective devices are available in many varieties, one of which is the transient voltage surge suppressor commonly referred to as TVSS. A TVSS may be permanently installed or may be of the cord-connected or direct plug-in style. Each type of TVSS is intended for use on the load side of the main service disconnect in circuits not exceeding 600 volts rms. The main service disconnect is considered to be the first overcurrent protective device between the distribution transformer secondary and the service entrance.

The basic safety standard used to test TVSS is UL 1449, the Standard for Safety for Transient Voltage Surge Suppressors. UL 1449 addresses construction items such as enclosure requirements, minimum wiring size for internal wiring as well as field wiring, acceptable spacings between circuits of opposite polarity or to metal walls of an enclosure, proper grounding means, and suitability of mounting to name a few. UL 1449 also contains testing, marking and installation instruction requirements.


Figure 2.

As well as containing the basic safety tests known to many UL electrical safety standards such as Leakage Current, Normal Temperature, Dielectric Voltage Withstand, etc., UL 1449 also contains a Measured Limiting Voltage Test. The measured limiting voltage is the maximum magnitude of voltage, measured at the output (leads, terminals, or receptacle contacts) of the TVSS during application of a test impulse of specified waveshape and amplitude. The Measured Limiting Voltage Test includes a duty cycle to determine that the surge suppression components can repeatedly limit the transient voltage surge test waveform without degradation.

During the Measured Limiting Voltage Test, it is verified that the average measured limiting voltages do not exceed the suppressed voltage rating marked on the product when subjected to a standard 1.2 by 50 microsecond (Figure 1), 8 by 20 microsecond (Figure 2) combination standard test waveform with the peak values shown in Figure 3. A component that is being subjected to the Measured Limiting Voltage Test is shown in Figure 4.

Within recent years UL 1449 was revised to include Abnormal Overvoltage Tests for TVSS devices. These abnormal overvoltage tests were added because surge protective devices may be located in areas with very high transient currents and/or long or short term abnormal overvoltages for which they may not be designed. When surge protective devices are exposed to abnormally high surge currents or to abnormal overvoltages, the surge protective component attempts to limit the abnormal by conducting or turning on. Since the surge protective components may not be designed for this purpose, means should be taken to make sure the TVSS can withstand the abnormal overvoltages or high surge currents without risk of fire or electrical shock.

The Surge Current Test in UL 1449 subjects TVSS devices to high transient currents and the Abnormal Overvoltage Tests in UL 1449 subject TVSS to abnormal power overvoltages. The Full Phase Voltage High Current Abnormal Overvoltage Test anticipates essentially a double overvoltage with the availability of follow current ranging from 200 amperes to 25,000 amperes or even higher fault current up to 200,000 amperes if the manufacturer requests. The Limited Current Abnormal Overvoltage Test anticipates the same double overvoltage but with a limited current ranging from 5 amperes to 0.125 amperes. Each test causes the TVSS to react differently. High current is the fast acting overvoltage whereas the limited current is a slow burn situation. Products manufactured bearing the UL Mark must pass the Surge Current and the Abnormal Overvoltage Tests of UL 1449 without increased risk of fire or electric shock.


Figure 3.

With TVSS devices it is important to read and follow all installation/operating instructions provided with the unit. At a minimum, the instruction manual for a permanently installed TVSS is required to contain instructions for installation including the minimum and maximum wire length and gauge sizes, the ampacity of the circuit the device is intended for use on, and the internal wiring methods showing location and routing. Instructions for mounting as well as an explanation of the purpose and function of any indicator features employed on the TVSS such as lights, audio indicators or the like should also be provided.

It is also important to understand and adhere to markings on TVSS devices. Permanently installed TVSS are required to be marked with the electrical ratings including the operating voltage rating, the ac power frequency and for certain devices, the load current rating. Permanently installed TVSS have a connection diagram and some TVSS devices may indicate a fault current rating as well as a requirement for the use of an externally connected fuse or breaker. When this marking is required by UL, it is important for consumers as well as code authorities to understand this marking means the TVSS device was tested at the specified fault current with the fuse or breaker installed during the Full Phase Voltage High Current Abnormal Overvoltage Test and the fuse or breaker operated during the test. Therefore, it is very important to make sure this marking is not ignored during installation of the TVSS.

Another marking of importance is the Suppressed Voltage Rating. The suppressed voltage rating is the rating or ratings selected by the manufacturer from the range of 330 volts peak to 6000 volts peak based on the measured limiting voltage determined during the Measured Limiting Voltage Test. 330 volts peak is the lowest and 6000 volts peak is the highest rating that may be marked on the TVSS based on the let-through voltage measured during the Measured Limiting Voltage Test. It is important to understand that a 330 volts peak suppressed voltage rating is not necessarily considered the "”best”" rating and higher peak voltage ratings are not necessarily considered "”worse”" ratings. TVSS devices should be chosen based on analysis of the transient problems that the TVSS is being used for and not only for the lowest suppressed voltage rating marked on the device. Many times a transient problem on a power system may require a combination of surge protective devices installed at critical locations within the power system.

Another type of surge device is the surge arrester. Low voltage surge arresters rated up to 999 volts ac are intended to afford protection against surge related damage to secondary distribution wiring systems and/or to downstream equipment. Surge arresters rated 1000 volts ac or higher are intended to afford protection against surge related damage to wiring systems and/or downstream equipment. Surge arresters are for use on alternating current power circuits and are intended to be installed in accordance with Article 280 of the National Electrical Code.


Figure 4. Device under test

The basic standard used to investigate metal oxide surge arresters is ANSI/IEEE C62.11, Standard for Metal-Oxide Surge Arresters for AC Power Circuits. Other types of surge arresters are investigated using IEEE C62.1-1989, Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuits.

The types of low voltage surge arresters include secondary, metal-oxide, valve and distribution light duty. The types of higher voltage surge arresters include station, intermediate, distribution (heavy, normal and light duty).

Surge arresters are subjected to aging tests as well as surge tests. With surge arresters as with TVSS devices is it important to read and follow all installation instructions and markings.

Surge arresters are intended for installation on the line side of the main overcurrent protection. It is important to understand that a surge arrester should not be installed on the load side of the main overcurrent protective device unless it has also been investigated as a TVSS. The reason for this is that the surge arrester has not been subjected to the Abnormal Overvoltage Tests of UL 1449 which, as noted above, are critical tests for usage of a surge protective device on the load side of the main overcurrent protective device.

The surge protective device which may be the type that is most widely recognized by consumers is the cord-connected or direct plug-in TVSS device that is typically found under your desk or behind your television or stereo. These TVSS devices are also referred to as relocatable power taps, power taps or current taps. These taps with internal surge components provide surge protection to multiple outlets, phone jacks, or cable jacks so that vulnerable products such as your personal computer, printer, scanner, fax machine, phone, cable connection etc. can be provided with surge protection from one convenient location.

Relocatable power taps or current taps are intended to be plugged directly into a branch circuit receptacle of the proper plug configuration. Power taps and current taps are not intended to be plugged into an extension cord or into another power tap or current tap nor have they been investigated for this purpose. All power taps and current taps with a ground pin are intended to be plugged into a properly grounded branch circuit receptacle and the ground pin should never be removed. In addition, the cords of power taps are not intended to be used as mounting means nor should the cord be routed under or around desk walls or through openings in doors, windows, ceilings or the like.

In summary, the propagation of surge protection devices will continue to increase. This article has provided some insight into types of surge protective devices, some of the safety tests they must undergo as well as installation locations. More information concerning surge protective devices can be found in the UL General Information for Electrical Equipment Directory or in the UL Electrical Equipment Construction Directory under the categories Transient Voltage Surge Suppressors (XUHT), Surge Arresters (OWHX), Surge Arresters, 1000 Volts and Higher (VZQK), Relocatable Power Taps (XBYS) or Current Taps (EMDV).

If you have questions concerning this article or surge protective devices contact Deborah Jennings-Conner at 919-549-1603 or Deborah.Jennings-Conner@ us.ul.com.


Read more by Deborah Jennings-Conner

Tags:  Featured  March-April 2000 

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