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Variances in Exit Signs

Posted By Underwriters Laboratories, Tuesday, September 01, 1998
Updated: Thursday, August 23, 2012
Question: I’ve noticed that the color, size, and visibility of exit signs varies greatly. Do UL requirements permit these variances?

Answer: UL listed exit signs must meet the requirements outlined in UL 924, the Standard for Emergency Lighting and Power Equipment. These fixtures are intended for installation in accordance with the National Electrical Code, ANSI/NFPA 70, and the Life Safety Code, ANSI/NFPA 101. Some of the listing requirements are as follows.

Color — Typical exit sign letters are red, but UL also lists signs with green lettering. The requirement for a specific color may be included in state or local codes, but UL has no color requirements.

Letter Size—UL 924 requires that exit sign letters be at least 6 inches high. The 6-inch tall letters must be 2 inches wide (except for the 1): the stroke width must be 3/4 inch, and the spacing between the letters 3/8 inch. For larger signs, the letters must increase in size proportionally, to make them legible from a distance.

Luminance— The luminance specified in UL 924 and NFPA 101 is 0.06 ft. lamberts. This is based on research conducted by UL and industry in the mid-1970s that compared the visibility of internally illuminated signs and signs with self-contained energy sources.

Viewing Distance—All exit signs are evaluated for a viewing distance of 100 feet, based on the requirements in NFPA 101. In UL924, there are two tests that determine compliance with these viewing distance requirements—UL’s analytical luminance visibility test conducted on a photometer and a 100-foot observation visibility test. An exception to this rule allows manufacturers to request that their self-luminous or electroluminescent exit signs be tested at 50, 75, or 100 feet. UL would require these signs to be marked with the legible viewing distance.

Self-luminous Materials—Self-luminous signs are powered continuously by a self-contained energy source, other than a battery. An example is radioactive tritium gas. Because this gas has a limited life expectancy, UL requires a replacement date to be marked on the sign. This date must be visible after installation.

Photoluminescent Materials—Since photoluminescent materials require exposure to light for activation of their luminescent properties and have a limited life expectancy, they may be used only in combination with other methods of illumination.

Floor Proximity Egress Path Marking Systems—These path marking systems are intended for indoor installation in low level locations on floors, or on walls at or near the floor, to provide a clearly marked path of escape during emergency situations. Photoluminescent materials may be evaluated for their suitability as a floor proximity marking system in accordance with UL 1994, the Standard for Low Level Marking and Lighting Systems.

Exit Fixture Retrofit Kits—Exit fixture retrofit kits consist of parts and/or subassemblies intended for field installation in listed exit fixtures. They are intended to convert the light source of an exit fixture, i.e. incandescent to fluorescent.

The names of listed exit sign manufacturers appear in the product category Exit Fixtures (FWBO) in the Electrical Construction Materials Directory (Green Book). The guide card listing information also appears in the General Information Directory (white book). Exit fixtures are evaluated for installation at or near a ceiling or above a door, unless marked for use at floor level. Exit fixtures that are suitable for use at floor level have been subjected to additional tests for mechanical abuse. Exit fixtures should not be confused with "exit markers,” which are products that are UL classified for use in listed floor proximity egress path marking systems.


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Outlet boxes for Ceiling Fans

Posted By Underwriters Laboratories, Tuesday, September 01, 1998
Updated: Thursday, August 23, 2012
Question: For outlet boxes that support ceiling suspended fans, does UL consider out-of-balance fans?

Answer: Section 422.18(a)

Section 422-18(a) of the 1996 National Electrical Code permits "listed” ceiling fans that do not exceed 35 lbs. to be supported by outlet boxes identified for such use. Ceiling fans exceeding 35 lbs. are required to be supported independently of the outlet box, in accordance with Section 422-18(b).

Requirements for ceiling-suspended fan support for metallic outlet boxes are located in UL 514A, the Standard for Metallic Outlet Boxes. For nonmetallic boxes, the requirements can be found in UL 514C, the standard for Nonmetallic Outlet Boxes, Flush-Device Boxes, and Covers. Both of these standards specifically include testing of the boxes using out-of-balance fans.

For the ceiling-suspended fan support test, a ceiling fan (weighing or ballasted to weigh 35 pounds) is installed in accordance with the manufacturer’s installation instructions. Samples are tested in both the horizontal position and at an incline of 30 degrees. A 40 gm (1.4 oz) weight (imbalance) is placed on a single fan blade and a rigid-metal pipe is connected to position the lower edge of the fan blade 12 inches from the ceiling. The fan is operated at a specified speed for 24 hours to simulate an unbalanced fan and then the outlet box is examined for damage. Next, one of the fan mounting screws is loosened two full turns, and the fan is operated for an additional 24 hours.

A UL listed outlet box intended to support a ceiling-suspended fan is required to be marked "Acceptable for Fan Support.”


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Services for Multi-Occupancy Buildings

Posted By J. Philip Simmons, Tuesday, September 01, 1998
Updated: Friday, August 24, 2012

General

We published a photograph in the "Code Violations” section of the March/April 1995 IAEI News which shows 21 service disconnecting means that are grouped in the same location on the end of an apartment building. Four service laterals supply the building from a common transformer. The caption with the photograph indicates that the installation is in violation of Section 230-71(a) of the National Electrical Code® as there are more than six disconnects grouped at the same location.

After getting a few phone calls asking for clarification of the Code requirements as well as three letters indicating disagreement with our conclusion, we published a Closer Look article on services for multiple-occupancy buildings in our July/August 1995 IAEI News. The article was based on the rules in the 1993 NEC®.

We decided to take another look at these requirements since several changes have taken place in Code rules since that time. Some key definitions of terms used in Article 230 have changed as well. Where used in this article from this point on, terms that are defined in Article 100 are in italics. That is just to help remind us that these terms have precise meanings as used in the NEC®. As is true with many discussions of subjects related to any code, it is important to have a good understanding of definitions that apply to the installation under consideration. These terms have a specific meaning where used in the Code.

We will take a brief tour through some of the basic requirements in the Code that apply to the installation as well as review the applicable definitions. This discussion is based on the 1999 National Electrical Code®.

General Requirements

Section 230-2 has had a change of format as well as been rewritten to change the previous exceptions into positive language. There is now a general rule that requires, "A building or other structure served shall be supplied by only one service unless permitted in (a) through (d).” These subsections contain six of the former seven exceptions to the general rule. Language identical or similar to this has been in many editions of the Code. For the purpose of our discussion, we will assume that the multifamily dwelling under consideration does not have a fire wall that would qualify as a building separation as mentioned in Article 100. In addition, we will not cover Section 230-2(a) Special Conditions (applies to installations of services for fire pumps, emergency systems, etc.) or Section 230-2(d) Different Characteristics, as multifamily dwellings are generally supplied at one voltage level and system type.

Some Definitions

As an aid to the reader, we have underlined new words added to the definitions since the 1996 NEC® and struck through words that are deleted. The definition of "service” has been revised in the 1999 NEC® and is now defined in Article 100 as, "The conductors and equipment for delivering electric energy from the serving utility electricity supply system to the wiring system of the premises served.” These changes clarify that on-site power production such as solar photovoltaic systems, generators and power production facilities such as cogeneration systems are usually separately derived systems and not services. The term "service” is a broad term that includes the service drop, service lateral, service-entrance conductors (both overhead and underground system) and service equipment which includes the service disconnecting means. Of course, the definition of "equipment” is also a very broad general term that includes "material, fittings, devices, appliances, fixtures, apparatus, and the like used as a part of, or in connection with, an electrical installation.”

The term "service equipment” has also been revised in the 1999 NEC® and is now defined in Article 100 as, "The necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s), and their accessories, connected to the load end of service located near the point of entrance of supply conductors to a building or other structure, or an otherwise designated defined area, and intended to constitute the main control and means of cutoff of the supply.” As can be seen, the term "service equipment” includes the equipment, such as switches and fuses as well as circuit breakers, that are used as the "service disconnecting means.”

The term "service disconnecting means” is not defined in the NEC®, although the term is used in several articles including the definition of "service conductors” in Article 100. Section 230-70 contains several requirements for the service disconnecting means and also describes its purpose. The service disconnecting means is or are provided to "disconnect all conductors in a building or other structure from the service-entrance conductors.” It must be located "at a readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors.” Each service disconnecting means must be permanently marked to identify it as a service disconnecting means and must be suitable for the prevailing conditions, such as being suitable for a wet location.

The term "building” is defined in Article 100 as, "A structure that stands alone or that is cut off from adjoining structures by fire walls with all openings therein protected by approved fire doors.” The term "structure” is not defined in the Code although Section 230-21 mentions "such as a pole” when describing an "other structure.” Since the term is not defined in the Code, we can use a common dictionary definition such as "Something made up of a number of parts held or put together in a specific way.” So, we can apply the rules for installation of service equipment to any building or structure the service equipment is mounted in or on.

The term "service conductors” has also been revised in the 1999 NEC® and now is defined in Article 100 as "The supply conductors that extend from the service point street main or from transformers to the wiring system service equipment of the premises supplied.” The term "service conductors” is a broad term that includes: "service drop,” "service-entrance conductors, overhead system,” "service-entrance conductors, underground system” and "service lateral.” This change coordinates with Section 90-2(b)(5) which indicates that installations under the exclusive control of electric utilities for distribution of electric energy are not covered by the Code. However, these same conductors installed by the owner, contractor or electrician are covered by the Code.

As can be seen, the conductors supplied by an electric utility on the line side of the "service point” are now not considered by the Code to be "service conductors.” The term "service point” is defined in Article 100 as, "The point of connection between the facilities of the serving utility and the premises wiring.” This means that where the service point is at the building or structure such as the connection at the weatherhead for overhead services or at the meter socket for underground services, the service drop and service lateral are not covered by the Code.

The following definitions also apply to our discussion and should be understood:

"Service Drop: The overhead service conductors from the last pole or other aerial support to and including the splices, if any, connecting to the service-entrance conductors at the building or other structure.”

"Service-Entrance Conductors, Overhead System: The service conductors between the terminals of the service equipment and a point usually outside the building, clear of building walls, where joined by tap or splice to the service drop.”

"Service-Entrance Conductors, Underground System: The service conductors between the terminals of the service equipment and the point of connection to the service lateral.

(FPN): Where service equipment is located outside the building walls, there may be no service-entrance conductors, or they may be entirely outside the building.”

"Service Lateral: The underground service conductors between the street main, including any risers at a pole or other structure or from transformers, and the first point of connection to the service-entrance conductors in a terminal box or meter or other enclosure with adequate space, inside or outside the building wall. Where there is no terminal box, meter, or other enclosure with adequate space, the point of connection shall be considered to be the point of entrance of the service conductors into the building.”

Section 230-2

With these definitions in mind, let’s look at the requirements in Section 230-2 for installing a service for a multiple-occupancy building. The general rule in this section is that "A building or other structure shall be supplied by only one service unless permitted in (a) through (d).” Keep in mind that the term "service” includes, "The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.”

Figure 1. An overhead service to a building and an underground service to another.

Figure 1. An overhead service to a building and an underground service to another.

The general requirement is that only one service drop or service lateral is permitted to be run to a building or other structure where the service equipment is located. Why? Because the term "service” includes both the "service drop” and "service lateral” and Section 230-2 generally permits only one service to a building or structure. The service drop will connect to service-entrance conductors, usually "outside the building, clear of building walls.” (See the definition in Article 100.) As can be seen in the definition of "service lateral,” where the service lateral stays outside the building such as supplying weatherproof service equipment, there are no service-entrance conductors.

Structure with a Fire Wall

A structure with a fire wall that qualifies as a building separation is considered to be more than one building as determined by the number of fire-wall separations. For example, a structure with one fire wall will be two buildings. The fire resistance rating of a fire wall required to qualify as a building separation is not given in the NEC®. It is necessary to obtain the details on fire-wall construction to create a building separation from the applicable building code. Usually, a fire wall having not less that a two-hour rating is required to create a building separation. Each of these buildings is then generally permitted to have not more than one service to it. Have you seen a duplex (two-family dwelling) with two service drops or service laterals to it? This is a violation of Section 230-2 unless the structure has a qualifying fire wall.

A permanent plaque or directory is required where more than one service is permitted for any reason, be it fire-wall separation or one of the conditions of Section 230-2(a) through (d). This requirement applies where a building or structure is supplied by any combination of more than one service, feeder or branch circuit. The plaque or directory must be located at each service disconnect location and must indicate all other services, feeders or branch circuits supplying that building or structure and give the area served by each of them. See Section 230-2(e).

Section 230-2(b) Special Occupancies

(This previously was Section 230-2 Exception No. 3, Multiple-Occupancy Buildings.) This subsection permits additional services, by special permission, for (1) Multiple-occupancy buildings where there is no space for service equipment accessible to all occupants, and for (2) A single building or other structure sufficiently large to make two or more services necessary.

The term "multiple-occupancy building” is not defined in the NEC® but "multifamily dwelling” is. Traditionally, the term multifamily dwelling has been interpreted as a multiple-occupancy building in applying the rules of the NEC®. Building codes tend to use the term "occupancy” as referring to a class of users of a building such as "business use,” "educational use” or "residential use.” So, a multiple-occupancy building in building code terms would have more than one type of occupancy in the same building and might be referred to as a "mixed use or occupancy” building. The NEC® tends to use the phrase "multiple-occupancy building” as one having more than one occupant rather than more than one class of occupant.

Figure 2. Section 230-2(b)(1).


Section 230-2(b)(1)

Note that Section 230-2(b)(1) permits an additional service(s) where there is "no available space for service equipment,” that is "accessible to all the occupants.” It seems there will always be space available for a single main disconnecting means, or up to six disconnecting means grouped at one location that is be accessible to all the occupants. This service equipment does not have to be inside such as in an electrical equipment room but can be located outside on or adjacent to the building. The location rule is in Section 230-70(a) and reads, "The service disconnecting means shall be installed at readily accessible location either outside of a building or structure or inside nearest the point of entrance of the service conductors.” Economical design criteria may dictate not installing a single main service disconnecting means but that consideration is not given in this Code rule. In addition, there may not be space at one location for all the metering equipment and individual meter/mains needed for a large complex. However, this section does not address multi-metering or feeder disconnecting means for all the individual occupancies. The installation in the photograph at the beginning of this article fails the test for this subsection as there certainly appears to be space at the location chosen for the multiple services for a single service to be installed with meter/feeder supply to the individual dwelling units.

There is no requirement in this rule that the space available for the service equipment that is to be accessible to all the occupants must be inside the building. It is quite common to locate service equipment outside multiple-occupancy buildings as indicated in the photograph that accompanies this article. If there is space for the service equipment in a common area, including outside, that is adequate for the service equipment for the building, Section 230-2(b)(1) cannot be used. If space is available for the service equipment to serve all the occupancies, then, according to the main rule, only one service is permitted for the building with a maximum of six disconnects. (We will look at some other provisions a little later in this article.)

Where the main disconnecting means is or are located before the metering equipment, it is common for the serving utility to require that the service disconnecting means enclosure be locked to reduce the likelihood of tampering and theft of electrical energy. The authorization for more than one service in Section 230-2(b) must be by "special permission,” which is defined in Article 100 as "The written consent of the authority having jurisdiction.”

It is not necessary for there to be a fire wall separation to grant the "special permission” provided for in this subsection. As provided in Section 230-2(b), "special permission” is to be based on space-for-equipment considerations. Six service disconnecting means are permitted to be installed for each additional service that is allowed by the authority having jurisdiction under Section 230-2(b)(1). The Code does not clearly require that the additional services installed as permitted by this subsection be installed at a location separate from the other service(s) to the building. For example, a building is supplied by a 120/240 volt, single-phase service with six service disconnecting means grouped at one location. A second service, this one a 208Y/120 volt, three-phase service, can be installed immediately adjacent to the single-phase service with an additional six service disconnecting means. See Section 230-71(a).

Section 230-2(b)(2)

(This previously was Section 230-2 Exception No. 5, Buildings of Large Area.) Again, by special permission, one or more additional services are permitted for "A single building or other structure sufficiently large to make two or more services necessary.”

No guidance on how to determine what qualifies as a "building of large area” is provided in the NEC®. Large industrial plants where lengthy feeder runs would cause excessive voltage drop which would require increasing the conductor size unreasonably is an example of where this exception may be applied. Other examples are large shopping centers or high-rise office buildings. The authority having jurisdiction is pretty much left on his own when attempting to determine when to allow this subsection to be applied.

Obviously, this subsection does not apply to the installation under review as there are no long feeder runs involved.

Section 230-2(c). Capacity Requirements

(This subsection is the former Exception No. 4 to Section 230-2.) This subsection permits additional services to a building or other structure for only the following reasons, all of which are related to the capacity or size of the service:

"(1). Where the capacity requirements are in excess of 2000 amperes at a supply voltage of 600 volts or less; or

(2). Where the load requirements of a single-phase installation are greater than the serving agency normally supplies through one service; or

(3). By special permission.”

Figure 3. Section 230-2(c).


Condition "(1)” does not apply to this installation as there is no service in excess of 2000 amperes. The total ampacity of the four service laterals do not come close to 2000 amperes. In fact, the size of the four service laterals could no doubt be smaller if they were combined to serve all the loads. This is due to the increased demand factors permitted by Article 220 for load calculations as the number of units served increases.

One disadvantage of installing the service lateral conductors in parallel (connected together at both ends to form a larger conductor) is the fault current available at the service equipment will be greater than where individual sets of conductors are run to separate service equipment enclosures.

Condition "(2)” does not apply as the four service laterals shown in the photograph are from the same transformer and thus obviously do not exceed the capacity of the serving utility’s transformer.

Condition "(3)” provides for special permission from the authority having jurisdiction. To apply in this situation, the special permission must relate to the capacity of the system. Condition "(3)” does not seem to apply to the installation under review as there is no need for an additional service based on capacity considerations. The single transformer installed by the utility has adequate capacity for the load.

Sections 230-2 and 230-40 Exception No. 2

Former Exception No. 7 to Section 230-2 has been incorporated into the opening paragraph of Section 230-2 as a new second sentence. The sentence reads, "For the purpose of Section 230-40, Exception No. 2 only, underground sets of conductors, size 1/0 and larger, running to the same location and connected together at their supply end but not connected together at their load end shall be considered to be supplying one service.” This sentence really does not allow an additional service in the strictest sense but allows several sets of service lateral conductors to be considered as supplying one service. (In reality, the service lateral does not supply the service but is a part of the service as defined in Article 100.)

Figure 4. Sections 230-2 and 230-40 Ex. No. 2.

As indicated, the second sentence of Section 230-2 is limited in application to Section 230-40 Exception No. 2. It reads, "Where two to six service disconnecting means in separate enclosures are grouped at one location and supply separate loads from one service drop or lateral, one set of service-entrance conductors shall be permitted to supply each or several such service equipment enclosures.”

As illustrated in Figure 4, a maximum of six service laterals, sized 1/0 or larger, that are connected together at their line end, but not at their load end, are considered to be supplying one service. The service laterals must be run to a common location at the building or structure served and are permitted to supply one, two or up to a total of six service disconnecting means. However, no more than six disconnecting means are permitted to be grouped at the location being served by these service laterals. See Section 230-71(a).

Section 230-40 Exception No. 1

Section 230-40 generally permits a service drop or service lateral to supply not more than one set of service-entrance conductors. When taken with the requirements of Section 230-2 and the definition of "service” in Article 100, this means that a building or other structure can generally be supplied by only one service drop that connects to one set of service-entrance conductors or it can be supplied by one service lateral that connects to one set of service-entrance conductors.

Exception No. 1 to Section 230-40 provides that, "Buildings with one or more than one occupancy shall be permitted to have one set of service-entrance conductors for each class of service run to each occupancy or group of occupancies.”

Figure 5. Section 230-40 Exception No. 1


Figure 5 illustrates an underground supply to a multiple-occupancy building. A similar procedure can be followed for an overhead supply. While only six units are shown due to space limitations, any number of units may be served in a similar manner.

The service lateral supplies a metering cabinet that does not contain service disconnecting means or overcurrent protection but has only meter sockets. A set of service-entrance conductors is run to each occupancy or to a group of occupancies. Of course, these service-entrance conductors do not have overload protection until they terminate in the service equipment. Where this scheme is selected, it is customary to run service-entrance conductors to the individual units rather than to a group of units. Up to six disconnecting means are permitted in or on each of the units, and of course, they must be grouped at the individual locations to comply with Section 230-71 (a).

Figure 6. Service lateral conductors are run from the utility transformer to a wireway mounted below the meter sockets.

Figure 6. Service lateral conductors are run from the utility transformer to a wireway mounted below the meter sockets.

As indicated in the Figure 6, service lateral conductors are run from the utility transformer to a wireway mounted below the meter sockets. Service-entrance conductors are spliced to the service lateral conductors within the wireway and run through the meter sockets to each unit. Service disconnecting means and overload protection is provided in or on each unit.

The location of the service disconnecting means must comply with Section 230-70 (a) which requires, "The service disconnecting means shall be installed at a readily accessible location, either outside of a building or structure or inside nearest the point of entrance of the service conductors.” Some inspection jurisdictions interpret, "nearest the point of entrance of the service conductors” to require the service equipment to be located in the stud space in the outside wall or back-to-back with the meter socket or point of entrance. Others interpret this rule to permit up to four feet of service-entrance conductors inside the building, while still others permit as much as 25 feet of service-entrance conductors to be installed inside the building. In some cases where these longer lengths of service-entrance conductors are permitted inside the building by local rule, a limited number of wiring methods are permitted. Be certain to verify the local rule or interpretation before beginning an installation.

Section 230-71(a)

This section provides that, "The service disconnecting means for each service permitted by Section 230-2, or for each set of service-entrance conductors permitted by Section 230-40, Exception Nos. 1 or 3, shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be no more than six disconnects per service grouped in any one location.” Since the term "location” is not defined in the Code, it will be applied based on its common dictionary meaning. As this word applies to the installation in the first photograph, most people probably would apply the meaning of "location” to be that all the service disconnects shown are at the same "location” since they are immediately adjacent to each other at the same end of the building. Most people would likely consider the services to be in different "locations” if a service were installed some significant distance apart such as at opposite ends of the building.

As we previously mentioned, more than six service disconnects are permitted at the same location only where more than one service is permitted to a building. For the installation under consideration, it does not appear that any of the rules in Section 230-2(a) through (d) apply so only one service with a maximum of six disconnecting means is permitted.

Section 230-72

This section generally requires the service disconnecting means permitted by Section 230-71 to be grouped.

In a multiple-occupancy building, each occupant is required to have access to their service disconnecting means. The exception to Section 230-72(c) allows the service disconnecting means that supplies more than one occupancy to be accessible to authorized building management personnel only where the electric service and electrical maintenance for the building are under continuous building management supervision. This exception does not apply to the multiple-occupancy installation under consideration as all the service disconnecting means are on the outside at a common location and are accessible to each of the occupants.

Conclusion

As can be seen, there are many different ways to install the service for a multiple-occupancy building and be in compliance with the National Electrical Code. To do so requires a thorough reading and understanding of the definitions and Code requirements that apply. As you have probably observed, the installation shown in the photograph at the beginning of this article did not comply with the 1993 or 1996 NEC and would not comply with the revised rules of the 1999 NEC Article 230.

Serving utilities may have local requirements regarding serving multiple-occupancy buildings that must be complied with. These utilities often have booklets with diagrams that illustrate their distribution requirements.

Finally, the authority having jurisdiction may have local amendments to the Code that must also be complied with as well. These amendments will be in the form of legally adopted ordinances, regulations or statutes and are not simply an unwritten interpretation. These local amendments, if any, are also available in writing from the inspection agency.


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The Grounding Electrode

Posted By Peter Boden , Tuesday, September 01, 1998
Updated: Friday, August 24, 2012

Plastic piping is not only widely used in new installations, but also in repairing existing installations. Even in a metal water piping system, maintenance and repairs can introduce plastic pipe fittings that interrupt the electrical continuity of the system. These fittings can also significantly reduce the length of piping that can act effectively as a grounding electrode. Water utilities do not commonly install jumper wires around the fittings to maintain electrical continuity. In fact, many utilities discourage the connection of buried parts of metal water piping systems to the electrical system. They say that electric current through the pipe can hasten corrosion and affect the taste of the water.

With the introduction of nonconductive plastic water piping systems, the burden of providing a low-impedance connection to earth at the service equipment falls on other types of grounding electrodes. The research project described in this article is being conducted to evaluate buried electrodes to be sure they can handle the job.

Background

A project to evaluate grounding electrodes began a few years ago in Clark County, Nev. (Las Vegas area), when questions arose about the adequacy of certain types of grounding electrodes. Subsequently, the National Fire Protection Research Foundation (NFPRF) got involved, and a Technical Advisory Committee (TAC) was organized to gather information about the performance of grounding electrodes for building wiring systems. Today, the TAC is composed of representatives from NFPRF, NFPA, UL, electric utilities, industry associations, electrical contractors, municipalities, the U.S. Army, instrument manufacturers, grounding rod manufacturers, the entertainment industry, and possibly more, as the list of members grows. The organizations represented on the TAC are sponsoring the long-term research project on grounding electrodes. The project is being managed by the NFPRF.

The purpose of this grounding electrode research project is to develop data to evaluate the corrosive effects of various weather and soil conditions around the United States on different styles of grounding electrodes, and how an electrode’s ability to carry ground current is affected. The project includes long-term testing of grounding electrodes buried in the soil at a number of test sites. Presently, test sites are located near Staunton, Va., and Las Vegas. This spring, the third and fourth test sites are planned for installation in Texas and on UL’s campus in Northbrook, Ill. Other sites are proposed for New York, Montana, and central Florida (on the Disney property). The testing involved will explore grounding resistance and the electrical integrity of the electrodes over time in various types of soil to determine whether moisture content, pH levels and other soil conditions can corrode grounding rods enough to degrade grounding paths to an unacceptable extent.

Expected to take approximately 10 years or longer, the research project consists of monthly measurements of electrode-to-earth resistance; and measurements of soil moisture content, soil pH, temperature, precipitation and other environmental conditions. Some electrodes in the project carry a small current from a dc power supply. At the end of the testing period, the electrodes at each site will be exhumed and weighed to determine weight loss due to corrosion. The data will be sent to NFPRF for compilation and analysis. NFPRF will issue a comprehensive report that will show how the specific grounding electrodes fared at specific site locations. Recipients of the report will then be able to make judgements regarding the adequacy of specific electrodes in certain soils, and whether some grounding electrode systems may warrant further study. Changes in the codes covering grounding methods could result from the data obtained from this project.

Why are Electrical Systems Grounded? Why is Grounding Important?

According to Article 250 of the National Electrical Code® (1996 edition), systems and circuit conductors are grounded to limit voltages on a system with respect to earth and items that are in contact with the earth. The voltages can be caused by lightning, line surges or unintentional contact with higher voltage lines. Grounding the system also stabilizes the voltage on the system with respect to ground during normal operation. A low-impedance connection at the service equipment between the grounded conductor of the electrical system and earth can enhance the longevity of the electrical insulation and reduce the risk of electric shock. Concerns regarding electric shock include not only the effects of faults in electrical insulation, but also the voltage that can appear between "grounded” accessible parts under normal conditions. These voltages are usually low, but can be undesirable in areas where people have simultaneous access to the earth and to equipment grounding conductors that are, in turn, connected at the service equipment to the grounded circuit conductor of the system.

Figure 1. An example of an electric shock scenario created in part from a poor grounding connection to earth

Figure 1. An example of an electric shock scenario created in part from a poor grounding connection to earth

An example of an electric shock scenario created in part from a poor grounding connection to earth is shown in Figure 1. In the figure, an electrical product is in contact with the earth through a conductive part of the building structure, perhaps involving its mounting means. A low-impedance fault occurs between a live part inside the product to its enclosure. Fault current flows as indicated by the dashed line in the figure, but the impedance of the grounding electrode system limits the magnitude of the fault current. The opening of the overcurrent device is delayed, or the overcurrent device carries insufficient current to operate.

A person in contact with the earth and any conductive part connected to a grounding conductor of equipment plugged in or permanently wired anywhere in the building while the fault current flows could experience an electric shock, if the voltage dropped across the grounding electrode system is high enough. This situation is aggravated when the person is more susceptible to the effects of voltage across the body (for example, when the person is in a swimming pool or spa).

An overcurrent device, such as a circuit breaker, requires a minimum of 110 percent of its rated current to trip and open a circuit. For a 120-volt, 20-ampere circuit, for example, the total impedance of the loop carrying the fault current, according to Ohm’s Law, must not exceed 5.5 Ohms (i.e., 120V/22A). The grounding electrode contributes in part to this total value, and therefore, the resistance to earth of the grounding electrode system alone might have to be significantly lower than 5.5 Ohms.

What are Some Typical Grounding Electrode Designs

Article 250 of the National Electrical Code covers grounding. Grounding electrodes are described in Sections 250-81 and 250-83. Typical grounding electrode designs include rod and pipe electrodes, grounding plates, chemically charged electrodes, and concrete-encased electrodes.

What is The Scope of the Project and What Types of Electrodes Will be Installed at the Test Sites
The scope of the research project includes many types of grounding electrodes permitted by the NEC According to the scope, the project does not include water piping systems and building steel, nor does it include the grounding conductors and various equipment used to provide grounding paths elsewhere in a premise’s wiring system. Since the project focuses on building systems, it does not necessarily cover all types of electrodes used by electric utilities.

The Northbrook, IL., Site—Installation and Testing

The Northbrook, Ill., test site is similar to other test sites in the NFPRF study. A 100′ by 200′ test site will be located on the east side of the UL property. Within the test site, a total of 63 grounding electrodes will be buried. There will be two fields Ñ one passive (no electrical current applied), and one much smaller, active field (electrical current of approximately 5mA dc applied). The electrodes, installed horizontally or vertically in the site, will represent designs permitted by the NEC® in a number of configurations.

Fifty-seven electrodes will be buried in the passive field. Some of these electrodes are specifically designed and manufactured as grounding electrodes, while others are simply various types of pipes and rods permitted by the NEC® for grounding use. Electrodes include solid reinforcing bars, metal rods, galvanized steel pipes, copper conductors, concrete-encased electrodes, plates and chemical electrodes. In the passive field, 30 electrodes will be vertically driven or buried in augered holes up to 1-1/2 feet in diameter and 10-feet deep. The remaining 27 electrodes will be buried in horizontal trenches 4-feet deep.

The chemical electrodes for installation in the passive field are hollow copper tubes with small predrilled weep holes along their lengths. The tubes are filled with salts. When the salts come in contact with moisture, an electrolytic solution is formed, promoting good electrical contact with the earth. Some of these chemical electrodes will be encased in bentonite, a material that also promotes good electrical contact with the earth.

In the active field, a current of approximately 5mA dc will flow through vertical electrodes of two types — 5/8″ copper-clad steel rods, and 3/4″ trade-size galvanized pipe. The current represents the dc component of the electrical ground current in a hypothetical scenario.

Direct current flows through the grounding paths of building wiring systems when products are used that rectify the load current. Electrode deterioration/corrosion is caused by electrolysis from direct current flowing through grounding electrodes. Load currents flow through grounding paths between the service equipment and the utility transformer because the grounded circuit conductor is connected to earth at more than one point. The NEC® requires one of these grounding points to be connected at the service equipment of a building to limit the voltages on the system with respect to earth. Utilities ground systems along entire power distribution networks for the same reason.

The importance of the active site is to illustrate how direct current can accelerate the corrosion of grounding electrodes, in contrast to the electrodes in the passive fields that do not have dc current flowing through them. The active site will also show the distribution of the corrosion along the length of the electrode. If, for example, the corrosion is concentrated near the point of connection to the grounding electrode conductor, early failure of the grounding electrode may result with minimal loss of material, leaving most of the electrode in the soil, but disconnected from the electrical system.

What Measurements Will be Used to Evaluate an Electrode in the Test Site?

All electrodes will be weighed prior to burial and again at the end of the project after they’ve been exhumed and cleaned. This process will determine the amount of electrode material lost to the earth by way of electrolysis and corrosion. Weight retention is an important factor indicating the ability of an electrode to provide a sound, low-impedance connection to earth over time.

In addition, the electrode-to-ground resistance will be periodically measured during the course of the project. The probes for the instrumentation and each electrode in the project will have buried leads terminating in a junction box where resistance measurements can be taken. Records of variable environmental conditions will be collected. Figure 2 illustrates the resistance measurement method.

How Are Grounding Electrodes Installed at the Northbrook Site?

The installation equipment includes a backhoe and a power auger. The backhoe is used to dig 12 trenches approximately 80-feet long for horizontal burial of 27 electrodes. Smaller trenches will accommodate instrumentation leads. The power auger is used to drill holes for vertically positioned electrodes, except that driven ground rods are installed by forcing them into the ground without the use of an excavated hole. Following site excavation, the electrodes are placed in the trenches and holes. A copper lead is connected to each electrode, and terminates in a junction box where the resistance measurements from electrode-to-earth will be taken during the term of the study. Prior to backfilling, the leads will be placed in plastic flexible conduit that serves as mechanical protection when the trenches and holes are filled with earth. The conduit also permits easier replacement of damaged leads during the course of testing, if necessary.

Figure 3. This figure illustrates the trenching detail.

Figure 3

Figure 3 illustrates the trenching detail.

Summary

The results of this study will provide valuable information for builders, designers, utilities, code authorities and others who build, install, inspect or maintain electrical systems. Future construction is likely to introduce more plastic water pipe, and thus, new designs of grounding electrodes for electrical systems may be required. These new designs for grounding electrodes — a simple concept — must respond to the complicated challenges of corrosion, electrolysis and accelerated deterioration that have become important problems to solve in our increasingly complicated environment.

For more information on this project, call Peter Boden at UL in Northbrook, Ill., at (847) 272-8800, ext. 42011; or e-mail him at bodenp@ul.com. Call Walter Skuggevig at UL in Melville, N.Y., at (516) 271-6200, ext. 22312; or e-mail skuggevigw@ul.com. Or write Doug Brown at the National Fire Protection Research Foundation, 1 Batterymarch Park, Quincy, MA 02269; call (617) 984-7281; or e-mail dbrown@nfpa.org.

Note: [1] K. Michaels. "Earth Ground Resistance Testing for Low-Voltage Power Systems,” IEEE Trans. on Industrial Applications, pp. 206-212, Vol. 31, No. 1, Jan./Feb. 1995.


Copyright © 1998 by On the Mark. Reprinted with permission.

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The Grounding Electrode

Posted By Peter Boden , Tuesday, September 01, 1998
Updated: Friday, August 24, 2012

Plastic piping is not only widely used in new installations, but also in repairing existing installations. Even in a metal water piping system, maintenance and repairs can introduce plastic pipe fittings that interrupt the electrical continuity of the system. These fittings can also significantly reduce the length of piping that can act effectively as a grounding electrode. Water utilities do not commonly install jumper wires around the fittings to maintain electrical continuity. In fact, many utilities discourage the connection of buried parts of metal water piping systems to the electrical system. They say that electric current through the pipe can hasten corrosion and affect the taste of the water.

With the introduction of nonconductive plastic water piping systems, the burden of providing a low-impedance connection to earth at the service equipment falls on other types of grounding electrodes. The research project described in this article is being conducted to evaluate buried electrodes to be sure they can handle the job.

Background

A project to evaluate grounding electrodes began a few years ago in Clark County, Nev. (Las Vegas area), when questions arose about the adequacy of certain types of grounding electrodes. Subsequently, the National Fire Protection Research Foundation (NFPRF) got involved, and a Technical Advisory Committee (TAC) was organized to gather information about the performance of grounding electrodes for building wiring systems. Today, the TAC is composed of representatives from NFPRF, NFPA, UL, electric utilities, industry associations, electrical contractors, municipalities, the U.S. Army, instrument manufacturers, grounding rod manufacturers, the entertainment industry, and possibly more, as the list of members grows. The organizations represented on the TAC are sponsoring the long-term research project on grounding electrodes. The project is being managed by the NFPRF.

The purpose of this grounding electrode research project is to develop data to evaluate the corrosive effects of various weather and soil conditions around the United States on different styles of grounding electrodes, and how an electrode’s ability to carry ground current is affected. The project includes long-term testing of grounding electrodes buried in the soil at a number of test sites. Presently, test sites are located near Staunton, Va., and Las Vegas. This spring, the third and fourth test sites are planned for installation in Texas and on UL’s campus in Northbrook, Ill. Other sites are proposed for New York, Montana, and central Florida (on the Disney property). The testing involved will explore grounding resistance and the electrical integrity of the electrodes over time in various types of soil to determine whether moisture content, pH levels and other soil conditions can corrode grounding rods enough to degrade grounding paths to an unacceptable extent.

Expected to take approximately 10 years or longer, the research project consists of monthly measurements of electrode-to-earth resistance; and measurements of soil moisture content, soil pH, temperature, precipitation and other environmental conditions. Some electrodes in the project carry a small current from a dc power supply. At the end of the testing period, the electrodes at each site will be exhumed and weighed to determine weight loss due to corrosion. The data will be sent to NFPRF for compilation and analysis. NFPRF will issue a comprehensive report that will show how the specific grounding electrodes fared at specific site locations. Recipients of the report will then be able to make judgements regarding the adequacy of specific electrodes in certain soils, and whether some grounding electrode systems may warrant further study. Changes in the codes covering grounding methods could result from the data obtained from this project.

Why are Electrical Systems Grounded? Why is Grounding Important?

According to Article 250 of the National Electrical Code® (1996 edition), systems and circuit conductors are grounded to limit voltages on a system with respect to earth and items that are in contact with the earth. The voltages can be caused by lightning, line surges or unintentional contact with higher voltage lines. Grounding the system also stabilizes the voltage on the system with respect to ground during normal operation. A low-impedance connection at the service equipment between the grounded conductor of the electrical system and earth can enhance the longevity of the electrical insulation and reduce the risk of electric shock. Concerns regarding electric shock include not only the effects of faults in electrical insulation, but also the voltage that can appear between "grounded” accessible parts under normal conditions. These voltages are usually low, but can be undesirable in areas where people have simultaneous access to the earth and to equipment grounding conductors that are, in turn, connected at the service equipment to the grounded circuit conductor of the system.

Figure 1. An example of an electric shock scenario created in part from a poor grounding connection to earth

Figure 1. An example of an electric shock scenario created in part from a poor grounding connection to earth

An example of an electric shock scenario created in part from a poor grounding connection to earth is shown in Figure 1. In the figure, an electrical product is in contact with the earth through a conductive part of the building structure, perhaps involving its mounting means. A low-impedance fault occurs between a live part inside the product to its enclosure. Fault current flows as indicated by the dashed line in the figure, but the impedance of the grounding electrode system limits the magnitude of the fault current. The opening of the overcurrent device is delayed, or the overcurrent device carries insufficient current to operate.

A person in contact with the earth and any conductive part connected to a grounding conductor of equipment plugged in or permanently wired anywhere in the building while the fault current flows could experience an electric shock, if the voltage dropped across the grounding electrode system is high enough. This situation is aggravated when the person is more susceptible to the effects of voltage across the body (for example, when the person is in a swimming pool or spa).

An overcurrent device, such as a circuit breaker, requires a minimum of 110 percent of its rated current to trip and open a circuit. For a 120-volt, 20-ampere circuit, for example, the total impedance of the loop carrying the fault current, according to Ohm’s Law, must not exceed 5.5 Ohms (i.e., 120V/22A). The grounding electrode contributes in part to this total value, and therefore, the resistance to earth of the grounding electrode system alone might have to be significantly lower than 5.5 Ohms.

What are Some Typical Grounding Electrode Designs

Article 250 of the National Electrical Code covers grounding. Grounding electrodes are described in Sections 250-81 and 250-83. Typical grounding electrode designs include rod and pipe electrodes, grounding plates, chemically charged electrodes, and concrete-encased electrodes.

What is The Scope of the Project and What Types of Electrodes Will be Installed at the Test Sites
The scope of the research project includes many types of grounding electrodes permitted by the NEC According to the scope, the project does not include water piping systems and building steel, nor does it include the grounding conductors and various equipment used to provide grounding paths elsewhere in a premise’s wiring system. Since the project focuses on building systems, it does not necessarily cover all types of electrodes used by electric utilities.

The Northbrook, IL., Site—Installation and Testing

The Northbrook, Ill., test site is similar to other test sites in the NFPRF study. A 100′ by 200′ test site will be located on the east side of the UL property. Within the test site, a total of 63 grounding electrodes will be buried. There will be two fields Ñ one passive (no electrical current applied), and one much smaller, active field (electrical current of approximately 5mA dc applied). The electrodes, installed horizontally or vertically in the site, will represent designs permitted by the NEC® in a number of configurations.

Fifty-seven electrodes will be buried in the passive field. Some of these electrodes are specifically designed and manufactured as grounding electrodes, while others are simply various types of pipes and rods permitted by the NEC® for grounding use. Electrodes include solid reinforcing bars, metal rods, galvanized steel pipes, copper conductors, concrete-encased electrodes, plates and chemical electrodes. In the passive field, 30 electrodes will be vertically driven or buried in augered holes up to 1-1/2 feet in diameter and 10-feet deep. The remaining 27 electrodes will be buried in horizontal trenches 4-feet deep.

The chemical electrodes for installation in the passive field are hollow copper tubes with small predrilled weep holes along their lengths. The tubes are filled with salts. When the salts come in contact with moisture, an electrolytic solution is formed, promoting good electrical contact with the earth. Some of these chemical electrodes will be encased in bentonite, a material that also promotes good electrical contact with the earth.

In the active field, a current of approximately 5mA dc will flow through vertical electrodes of two types — 5/8″ copper-clad steel rods, and 3/4″ trade-size galvanized pipe. The current represents the dc component of the electrical ground current in a hypothetical scenario.

Direct current flows through the grounding paths of building wiring systems when products are used that rectify the load current. Electrode deterioration/corrosion is caused by electrolysis from direct current flowing through grounding electrodes. Load currents flow through grounding paths between the service equipment and the utility transformer because the grounded circuit conductor is connected to earth at more than one point. The NEC® requires one of these grounding points to be connected at the service equipment of a building to limit the voltages on the system with respect to earth. Utilities ground systems along entire power distribution networks for the same reason.

The importance of the active site is to illustrate how direct current can accelerate the corrosion of grounding electrodes, in contrast to the electrodes in the passive fields that do not have dc current flowing through them. The active site will also show the distribution of the corrosion along the length of the electrode. If, for example, the corrosion is concentrated near the point of connection to the grounding electrode conductor, early failure of the grounding electrode may result with minimal loss of material, leaving most of the electrode in the soil, but disconnected from the electrical system.

What Measurements Will be Used to Evaluate an Electrode in the Test Site?

All electrodes will be weighed prior to burial and again at the end of the project after they’ve been exhumed and cleaned. This process will determine the amount of electrode material lost to the earth by way of electrolysis and corrosion. Weight retention is an important factor indicating the ability of an electrode to provide a sound, low-impedance connection to earth over time.

In addition, the electrode-to-ground resistance will be periodically measured during the course of the project. The probes for the instrumentation and each electrode in the project will have buried leads terminating in a junction box where resistance measurements can be taken. Records of variable environmental conditions will be collected. Figure 2 illustrates the resistance measurement method.

How Are Grounding Electrodes Installed at the Northbrook Site?

The installation equipment includes a backhoe and a power auger. The backhoe is used to dig 12 trenches approximately 80-feet long for horizontal burial of 27 electrodes. Smaller trenches will accommodate instrumentation leads. The power auger is used to drill holes for vertically positioned electrodes, except that driven ground rods are installed by forcing them into the ground without the use of an excavated hole. Following site excavation, the electrodes are placed in the trenches and holes. A copper lead is connected to each electrode, and terminates in a junction box where the resistance measurements from electrode-to-earth will be taken during the term of the study. Prior to backfilling, the leads will be placed in plastic flexible conduit that serves as mechanical protection when the trenches and holes are filled with earth. The conduit also permits easier replacement of damaged leads during the course of testing, if necessary.

Figure 3. This figure illustrates the trenching detail.

Figure 3

Figure 3 illustrates the trenching detail.

Summary

The results of this study will provide valuable information for builders, designers, utilities, code authorities and others who build, install, inspect or maintain electrical systems. Future construction is likely to introduce more plastic water pipe, and thus, new designs of grounding electrodes for electrical systems may be required. These new designs for grounding electrodes — a simple concept — must respond to the complicated challenges of corrosion, electrolysis and accelerated deterioration that have become important problems to solve in our increasingly complicated environment.

For more information on this project, call Peter Boden at UL in Northbrook, Ill., at (847) 272-8800, ext. 42011; or e-mail him at bodenp@ul.com. Call Walter Skuggevig at UL in Melville, N.Y., at (516) 271-6200, ext. 22312; or e-mail skuggevigw@ul.com. Or write Doug Brown at the National Fire Protection Research Foundation, 1 Batterymarch Park, Quincy, MA 02269; call (617) 984-7281; or e-mail dbrown@nfpa.org.

Note: [1] K. Michaels. "Earth Ground Resistance Testing for Low-Voltage Power Systems,” IEEE Trans. on Industrial Applications, pp. 206-212, Vol. 31, No. 1, Jan./Feb. 1995.


Copyright © 1998 by On the Mark. Reprinted with permission.

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Beinvenidos Mexico

Posted By IAEI, Tuesday, September 01, 1998
Updated: Friday, August 24, 2012

No doubt the changes México has had in the last six years related to the electrical inspectors are enormous.

The fact is that since the publication of the Ley Federal Sobre Metrología y Normalización (Federal Law of Metrology and Standardization) in the Diario Oficial de la Federación (official government journal) on July 1, 1992, to the latest modifications on May 20, 1997, the situation has changed dramatically.

Before the publishing of the law, the Federal Government, through some of its Secretariats, was the sole enforcement agency as far as inspecting the safety of all electrical installations, which kept all actions centered in México City, creating several problems such as follow-ups, reports, the "safety" concept, etc. Sometimes this enforcement was made trough the Comisión Federal de Electricidad (CFE, a utility) which left a lot to be desired and far under the standards we are now getting used to.

Then on July 1, 1992, the inspection of the electrical installations became privatized, creating a new figure called Unidad de Verificación de Instalaciones Eléctricas (UVIE) or as it is known elsewhere, Electrical Inspectors. It was obvious that to become an UVIE, you had to be experienced in the trade besides several other conditions that were valid at the time. This action gave birth to 709 UVIE, which started to work on January 1,1994, throughout the country.

As everybody knows, on that very same date, January 1,1994, the North American Free Trade Agreement (NAFTA or in Spanish, TLCAN) signed by México, the United States of America and Canada came into effect.

As a consequence of the above mentioned law and NAFTA, on October 10,1994, the Diario Oficial de la Federación published the Norma Oficial Mexicana (Mexican Electrical Code) NOM-001-SEMP-1994, which is, in its context, 98.17% of the NEC 93 ®, and within its context, has many other requirements.

This way, we have in our country, México, an inspection system for the electrical installations that the UVIEs put into effect in order to enforce the ordinances of our NOM-001-SEMP-1994.

On May 20,1997, a new change was brought into the country by modifying the Ley Federal Sobre Metrología y Normalización. The UVIEs in México must, in order to comply with the law, be accredited from now on by an "Accreditation Entity."

This entity will do so based on an auditorship procedure, much the same as the ISO 9000 one, that is made throughout the world. Besides analyzing the technical, material and human resources and capabilities related to the service they intend to offer to the public, they will need a quality and a procedure manual for the job they are going to do.

The UVIEs and the Entities of Accreditation must comply with the law that establishes, among other things:

1. Adjust to the rules, procedures and methods established in the NOM-001-SEMP-1994 and any other applied to the electrical installations.2. No discrimination when offering the service (sort of like equal opportunity)3. Avoid conflicts of interests.

These new enforcements will make the services the UVIEs offer their customers be done in a professional manner, with excellent quality and avoiding conflicts of interests that exist presently when the electrical inspector is, at the same time, a designer and/or a contractor.

For any information on this subject or any other related to the subject, my E-Mail address is: psanhua@tij.cetys.mx


Definitivamente los cambios que en los últimos seis años se han tenido en México en relación con los Inspectores Eléctricos, han sido enormes.En efecto, desde el inicio de la Ley Federal Sobre Metrología y Normalización cuando se publicó en el Diario Oficial de la Federación el 1º de julio de 1992, a las últimas modificaciones del 20 de mayo de 1997, la situación ha cambiado en forma dramática

.Antes de la aparición de la mencionada Ley, el Gobierno Federal por medio de algunas de las Secretarías de Estado, era el encargado de vigilar que las instalaciones fueran lo suficientemente seguras, lo cual, al tener centralizada la acción correspondiente en la ciudad de México, creaba ciertas dificultades, sobre todo de seguimiento, control, reportes, el concepto de "seguridad", etc, etc. En otras ocasiones esta vigilancia se hizo por medio de la Comisión Federal de Electricidad (CFE) lo cual ocasionaba una vigilancia no adecuada a los requerimientos a los que estamos ya acostumbrándonos.

A partir pues, del 1º de julio de 1992, se procedió a la "privatización" de las Inspecciones de instalaciones eléctricas, creando una nueva figura llamada Unidad de Verificación de Instalaciones Eléctricas (UVIE), o como se conoce en otras partes del planeta, Inspectores Eléctricos. Era claro que para ser UVIE, era necesario contar con experiencia en instalaciones eléctricas además de un sinfín de otras condiciones que en su momento se hicieron valer. Esto dio como consecuencia que a partir del día 1º de enero de 1994, y unos meses después, 709 UVIEs empezaran a funcionar en el país.

Como es de todos conocido, en esta misma fecha, 1º de enero de 1994, el Tratado de Libre Comercio de América del Norte (TLCAN), firmado por México, Estados Unidos de América y Canadá, entró en vigor.Como consecuencia del mismo TLCAN y de la Ley anotada, el 10 de octubre de 1994 se publicó en el Diario Oficial de la Federación la Norma Oficial Mexicana, NOM-001-SEMP-1994, misma que en un contenido se parece al 98.17 % del NEC 93®, aunque cuenta con disposiciones adicionales.

De esta forma, tenemos en vigor en México, un sistema de inspecciones a las instalaciones eléctricas que las UVIEs realizan con el fin de vigilar el cumplimiento de la NOM-001-SEMP-1994.

Con la modificación a la Ley Federal Sobre Metrología y Normalización del 20 de mayo de 1997, un nuevo cambio se presenta en el país, dado que las UVIEs acreditadas en México, tendrán que, para apegarse a la propia Ley, ser "acreditados" por una "Entidad de Acreditación", misma que lo hará con base en un procedimiento de auditorías similares a las que en el mundo se conocen como las auditorías tipo ISO 9000. Esto implicará, además de analizar la capacidad técnica, material y humana, en relación con los servicios que pretende prestar, contar con un manual de calidad y uno de procedimientos para el desempeño de sus funciones.

Tanto las Entidades de Acreditación, como las UVIEs deberán ajustarse a las reglas que la Ley ordena y que entre otras cosas marca lo siguiente:

Ajustarse a las reglas, procedimientos, métodos establecidos en la NOM-001-SEMP-1994 y las que se apliquen a las instalaciones eléctricas.No discriminar al prestar el servicio solicitado.

Evitar los conflictos de interés.Estas nuevas disposiciones harán que el servicio que presten las UVIEs a los usuarios de sus servicios se haga de una forma más profesional, con calidad y evitando el conflicto de intereses que se presenta, como sabemos cuando el Inspector Eléctrico realiza a la vez funciones de proyectistas y/o contratistas al mismo tiempo.

Para cualquier información sobre el tema, he de agradecer sus correos electrónicos a mi dirección: psanhua@tij.cetys.mx

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Substation Grounding

Posted By Leslie Stoch , Tuesday, September 01, 1998
Updated: Friday, August 24, 2012
Substation GroundingNot long ago I wrote an article on Substation grounding for Electrical Business that raised the issue of whether one should interconnect the building reinforcing steel with the station ground electrode. A reader responded with the question of whether the best approach might be to ignore the rebar bonding. The reader is well justified in wondering whether the best approach might be to ignore the rebar bonding. The reader is well justified in wondering whether there is not an easy answer to his question as there may not be a precise answer for every possible situation.

The Canadian Electrical Code, Rule 36-302(6) addresses the reader’s query as follows:

(6) The reinforcing steel members to be found in building foundations and concrete platforms shall be permitted to be included as part of the station ground electrode design provided that:

(a) No insulating film separates the concrete from the surrounding soil; and

(b) The maximum expected fault current magnitude and duration will not result in thermal damage to the steel members or the concrete structure; and

(c) The steel members are connected to the rest of the station ground electrode with not less than 2 copper conductors of not less than No. 2/0 AWG in such a way that should one grounding conductor be damaged, no single metal structure or equipment frame may become isolated; and

(d) The ground electrode design is made assuming that the concrete resistivity is greater than or equal to that of the surrounding soil.

In addition to this long list of conditions, the Appendix B reference to this rule also contains this note: "ANSI/IEEE Standard No. 80 should be consulted for conductor sizing to prevent thermal damage to the rebar during fault conditions.

You may also have noticed that this sub-rule is permissive, meaning that rebar bonding is not a code requirement and the designer can choose to connect or not make use of this option. However, as indicated, the code does allow reinforcing bars in concrete pads or building foundations to be included in the station ground electrode design. Measurements have shown that interconnecting reinforcing steel with the station ground electrode usually reduces the station’s grounding resistance. Consequently the station’s ground potential rise is reduced as well. This may be a practical and cost-effective way to improve the GPR, especially when insufficient property is available to expand the station ground electrode any further.

IEEE Standard 141 tells us that steel reinforcing bars in foundation piers in buildings usually consist of groups of four or more vertical members. These vertical members are wired to the horizontal members in the footings at the base of each pier. Measurements have shown that such piers may have an electrode resistance of about half the resistance of a ground rod driven to the same depth in earth. Large buildings would have many such piers, offering good opportunities to use them for this purpose.

This reference indicates that connection to such piers has good potential to improve substation grounding. In regions where the ground resistivity is high, a better grounding resistance can often be accomplished by including the reinforcing steel in the final design.

A question you should ask yourself: By reference to the IEEE No. 80 Standard, does the Canadian Electrical Code intend that the designer achieve the required 5000 volt maximum GPR before interconnection with rebar steel? There may be a wide range of opinions on this subject and we would welcome our readers’ views. In conclusion:

a) Ignoring rebar bonding does not violate the code. This may be a convenient way to improve the substation GPR.

b) Reinforcing steel in electrical equipment pads and building foundations may be used as part of the ground electrode design.

c) When interconnecting rebars with the station ground electrode, pay special attention to the requirements of this rule. This should include discussing with the electrical inspection authorities to obtain their interpretation of Rule 36-302(6)(a) to (d).

d) Finally, must the ground electrode design in every case satisfy the code on its own before interconnection?

As in all cases, for an exact interpretation of any of the above, you should consult locally with the electrical inspection authorities in each province or territory as applicable.

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Overhead Line Design from Scratch—Part 4

Posted By David Young, Tuesday, September 01, 1998
Updated: Friday, August 24, 2012

In Part 3 (July/August issue) we determined the height of our structures and the strength of our structures and foundations to comply with the National Electrical Safety Code® (NESC®). The last step in the design of a high-voltage overhead line up to 50 kV to comply with the NESC is to design the guying. Stringing conductors between poles puts tension on the poles. Though poles and foundations can be designed with enough strength to hold the tension, guying is much more economical.

Dead-End Guying

To understand the principles of anchor guying, let’s look at a simple example. Consider a single 250 foot span of 477 AAC "Cosmos" conductor strung between two poles at a stringing tension of 1410 pounds at 60 degrees F. The conductor is dead-ended on each pole at a height of 35 feet.

Figure 1

To prevent the poles from breaking at or near the ground line and to prevent the poles from kicking out (tipping over due to foundation failure), we need to install an anchor guy on each pole

The "longitudinal load" or force on the pole the anchor guy must hold is the final tension of the conductor under the conditions specified by Table 250-1 for the applicable loading district. Referring back to the heavy loading district Sag/Tension table for 477 AAC presented in Part 2 (May/June issue), the final tension of the conductor at 0 degrees F. with 1/2 inch of ice and 4 lb./sq. ft. wind pressure is 3601 pounds. Let’s assume we will install our anchor in the ground about 20 feet from the pole. This distance is called the guy lead.

Figure 2

The tension in the guy wire and the anchor is the longitudinal load on the pole due to the conductor "H" divided by the cosine of the angle "A".

The angle "A" is the arctangent of the guy attachment height divided by the guy lead.

A = tan-1(35'/20')

A = 66.9 degrees

T = 3601 lb / cos 66.9

T = 3601 / .496

T = 7260 pounds

For grade B and C construction, the rated strength of the guy wire and the anchor must exceed the guy tension "T" multiplied by the appropriate safety factors specified in Rules 261 C and B, respectively. For a discussion of Safety Factors, see my article "Strong Enough to be Safe" in the January/February issue. The rated strength of the anchor must take into consideration the type of soil in which it is installed.

The vertical force "V" on the pole due to the anchor is the tension in the guy wire "T" multiplied by the sine of the angle "A".

V = T x sin(A)

V = 7260 x sin 66.9

V = 7260 x .867

V = 6300 pounds

For grades B and C construction, the vertical strength of the pole and its’ foundation must exceed the vertical force on the pole "V" multiplied by the appropriate safety factor specified in Rules 261A and B, respectively, or the pole may buckle or sink. The shorter the guy lead, the higher the tension in the guy wire and anchor and the higher the vertical force on the pole. Using a shortlead increases the probability of the anchor slipping (pulling out) or the pole sinking, either of which will cause the pole to lean and the conductors’ sag to increase. The increased sag may cause a clearance violation.

Multi-Span Lines

All the spans of a multi-span line are usually strung at the same time and at the same tension. If all spans are not the same length, this stringing technique complicates the design. If each span is designed separately, the tension of the conductors in each span is a function of the span length, i.e. the longer the span, the higher the tension. If we string the different length spans at the same tension, what stringing tension do we use and what final tension doe we use in the guying calculation? The tensions we are looking for are the tensions of the "Ruling Span". The Ruling Span is a weighted average span. The calculation method gives longer spans more effect on the average. The Ruling Span is the square root of the sum of each span cubed divided by the sum of the spans.

Figure 3

Where S1 through SN are the individual span lengths and S13 = S1 x S1 x S1

The conductor tension we use for the guying calculation must come from a sag/tension calculation based on the Ruling Span length.

Let’s look at a common example of a three-phase vertical dead-end structure shown below:

Figure 4

In this example, the top three conductors are 477 AAC phase conductors. The bottom conductor is a 1/0 ACSR neutral conductor. The longitudinal load of the phase conductors is divided between the top two guys. The bottom guy holds the longitudinal load of the neutral. The tension in the top guy and its anchor is the longitudinal load of each phase conductor multiplied by three, divided by two and divided by the cosine of the angle "A".

T = H x 3
2 x cos(A)

A = tan-1(40/35)

A = 54.2 Degrees

T = 3601 x 3
2 x .658

T = 8209 Pounds

The rated strength of the top guy wire and its anchor must exceed the tension "T" multiplied times the appropriate safety factor specified in Rules 261C and B, respectively. The tension in the middle guy wire is calculated similarly except that angle B is different. The tension in the lower guy wire is the longitudinal load of the neutral conductor divided by the cosine of the angle C. The tension in the anchor closest to the pole is the sum of the tensions in the bottom two guy wires.


Read more by David Young

 Attached Files:
sep_oct1998.pdf (319.06 KB)
nov_dec1998.pdf (306.44 KB)

Tags:  Other Code  September-October 1998 

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