Posted By David Young ,
Monday, March 01, 1999
Updated: Monday, August 27, 2012
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I spend a lot of time inspecting electric supply facilities for hazardous conditions and violations of the National Electrical Safety Code® (NESC®). Even when I’m on vacation, I don’t stop inspecting. I’ve shared the "problems” with my wife so often that now she points them out to me. I’ve driven all over the United States and find that no matter where I go, the hazards are out there, particularly in non-utility owned supply facilities.
The most common hazard I see with substations is access by non-qualified persons. A substation (electric supply station) by definition is an area within which electric supply equipment is located and the interior of which is only accessible to qualified persons. Qualified persons are people having adequate knowledge of the installation, construction and operation of apparatus and the hazards involved. The clearance requirements of the NESC inside a substation are not as stringent as the clearance requirements for electrical facilities located in areas accessible to the general public because the substation is not accessible to the general public. When we allow the general public (non-qualified persons) in substations, we are asking for trouble.
How do we keep non-qualified people out?
Photo 1. When the grade is raised next to a fence, the fence height must be adjusted
1. The surrounding fence must be at least 7-foot tall (Rule 110A1). Because chain link fences are so easy to climb, particularly by children, it is my opinion that a chain link fence without barbed wire is not an effective barrier. When the grade is raised next to a fence, the fence height must be adjusted. (See photo 1)
Remove Aids to Climbing:
If a parking lot is adjacent to a substation, appropriate barriers should be used to prevent vehicle parking and dumpsters up against the fence.
2. The fence and gates must extend to the ground. The roadway under gates must be reconditioned after heavy use. If the roadway under gates is subject to washout, the roadway should be paved. (See photo 2:)
Photo 2. The roadway under gates must be reconditioned after heavy use. If the roadway under gates is subject to washout, the roadway should be paved
3. Warn the public of the hazards inside the substation with appropriate WARNING signs in compliance with ANSI® standard Z535 at least at each entrance and on each side (Rule 110A1). ANSI Z535 recommends that the signs shall be so placed to alert the viewer in sufficient time to take appropriate action to avoid harm.
Photo 3. The second most common substation hazard is lack of grounding
4. Do not store materials within a substation even if such materials are stored well away from energized conductors and equipment (Rule 110B2). The storage of materials within a substation invites non-qualified personnel like material delivery personnel and thieves.
The second most common substation hazard is lack of grounding. (See photo 3)
The metal fence surrounding the substation and all non-current-carrying metals parts within the substation must be electrically bonded together and effectively grounded (Rule 123A). The grounding methods must be in compliance with Section 9 of the NESC. The grounding must be designed to limit tough, step and transferred voltages in accordance with industry practice. IEEE standard 80 is one source that may be utilized to provide guidance in meeting these requirements. To meet these requirements, usually a ground grid must be installed under the substation with grounding conductors bonding the fence and non-current-carrying parts to the grid.
The most common hazard I see with overhead lines is inadequate ground clearance or clearance to other structures, particularly signs.
Most overhead hazards are created by people who change the environment around the supply facilities, not by the people who originally constructed the facilities. For example, an overhead high voltage line was constructed to serve a large industrial company. The property between the utility service point and the customer’s substation was designated "employee parking.” The line was designed to meet the vertical clearance requirements of the NESC as stated in Rule 232 for an area accessible to truck traffic. Five years later, the management of the company decided to make the employee parking lot into a raw material storage area complete with a crane to move the materials. The use of the land under the overhead line was changed. The line should have been moved. (See Photo 4 & 5)
If you have general questions about the NESC, please call me at 302-454-4910 or e-mail me at email@example.com.
National Electrical Safety Code, NESC and IEEE are registered trademarks of the Institute of Electrical and Electronics Engineers. ANSI® is a registered trademark of the American National Standards Institute. ANSI Z535 is a publication of the National Electrical Manufacturers Association (NEMA).
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Posted By Underwriters Laboratories ,
Monday, March 01, 1999
Updated: Monday, August 27, 2012
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Question: Letter of findings
I recently encountered a lightning protection system on a water treatment plant that was certified by UL with what was referred to as a ‘Letter of Findings.’ The only type of lightning protection certification I’m familiar with is the UL Master Label. What is a ‘Letter of Findings,’ and what other types of certifications currently are available for lightning protection systems?
A Letter of Findings is one of the three basic types of certifications that Underwriters Laboratories Inc. provides for lightning protection systems. The others are the more recognized Master Label and Reconditioned Master Label certifications. Below is a description of each of these programs.
Master Label — The Master Label is a Listing Mark that covers a lightning protection system installed in accordance with UL 96A, Installation Requirements for Lightning Protection Systems. With any certification based on UL 96A, there are limitations regarding the types of buildings that may qualify for the protection system. Buildings that do not qualify for a Master Label include those that handle, process or store flammable or explosive materials and structures with incomplete lightning protection systems. A partially protected structure may include a church where only the steeple is protected or a school where only the chimney is protected. Other examples of partial protection include a series of interconnected buildings where only one building is protected and the others are not, or interconnected buildings where both buildings have systems, but only one system complies with the current Standard UL 96A.
Reconditioned Master Label – The Reconditioned Master Label is a Listing Mark that covers a lightning protection system that has been refurbished or expanded to comply with UL 96A. This type of certification is identical in scope with the Master Label, except that it applies only to refurbished systems.
Letters of Findings – UL’s Letters of Findings indicate that an isolated roof section or an entire building complies with either UL 96A, NFPA 780 or Army Material Command (AMC) Regulation 385-100. The requirement regarding the size of an isolated roof section states that firewalls or similar means of building separation must isolate the protected roof section from other sections of the roof.
The lightning protection system must also be complete within itself (i.e., have the proper number of down conductors and air terminals for its size, etc.).
The following Standards include the noted limitations:
UL 96A, Installation Requirements for Lightning Protection Systems – This Standard does not cover buildings that store or handle explosive materials, flammable liquids, or flammable gases. NFPA 780, Lightning Protection Code – This standard does not cover buildings that house explosive materials.
AMC-R 385-100, Safety Manual — This standard does not include limitations.
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UL Question Corner
Posted By Philip Cox ,
Monday, March 01, 1999
Updated: Monday, August 27, 2012
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Certification is an important step in the progression of becoming a truly professional, highly trained and skilled electrical inspector. The Canadian Section of the IAEI promotes an inspector certification program in Canada and operates separately from its counterpart in the United States. An article on the Canadian Electrical Inspector Certification Program is scheduled for a later date.
The IAEI participates in an electrical inspector certification program along with the Building Officials & Code Administrators International (BOCA). Several states also participate in the program. BOCA and the state of New Jersey had major roles in the development and implementation of the program. The Educational Testing Service of New Jersey worked with the sponsoring organizations in providing professional services in both development and administration. The Chauncey Group International presently serves as the administrator for the program. Other inspector certification programs also exist. The International Conference of Building Officials (ICBO) and the Southern Building Code Congress International (SBCCI) each have an inspector certification program. The IAEI and ICBO have a memorandum of understanding to recognize one another’s certification.
IAEI Bylaws include provisions that contain the basic framework for establishing and maintaining an inspector certification program. They are: "The IAEI is to establish an electrical inspector certification program designed to meet all IAEI program objectives and, if deemed necessary, to participate with other national, regional, state or provincial authorities, agencies or groups in established and recognized certification programs. The IAEI program is to have inspector certification modules consisting of not less than: (1) Electrical Inspector, General; (2) Electrical Inspector, One- and Two-Family; and (3) Electrical Inspector, Plan Review.” In following this provision of the IAEI Bylaws, the IAEI actively works within the established program to assist in providing the necessary administrative and technical support to help make the existing program meet its objectives.
One objective that the IAEI has not achieved is the implementation of a recertification program to help those who hold inspector certificates maintain an acceptable level of continued training. The provisions authorizing a recertification program are already in the IAEI Bylaws and plans are to take steps necessary to have the program operational by the end of the year. It is important for this service to be offered to members and others who hold certification issued by the IAEI. The IAEI education committee is scheduled to meet within a few months and one major item on the agenda is to provide recommendations on how to best implement the program. Additional provisions or guidelines need to be developed before the recertification program can be fully functional. In addition to meeting the need of IAEI certificate holders, it is important that the program implemented will also be recognized by other organizations which have recertification programs.
While the existing program used by the IAEI effectively tests a person’s knowledge of the Code, recommendations are coming from within the electrical industry to develop a more comprehensive inspector certification program. Concern expressed by industry is that one who has no experience in electrical construction can become certified through existing programs simply by learning Code rules well enough to pass the examination. The complaint is that knowing what the Code rules are does not necessarily mean that the individual either understands the significance of the rules or knows how to apply them. It is stressed that an electrical inspector must not only know electrical code rules, but he or she must also understand electricity and electrical systems in order to know how to evaluate what they see in an actual installation in the field and know how and when to properly apply Code rules. The issues raised by members of the electrical industry will need to be addressed in the near future.
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Posted By David Young,
Friday, January 01, 1999
Updated: Monday, August 27, 2012
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Tom had flash burns to his face. Fortunately, he had been wearing safety glasses. What was left of his right arm had to be amputated above the elbow. He was in the hospital for almost four months. The medical care and physical therapy would continue for years.
Tom said that he and two experienced linemen were called out on trouble at two o’clock in the morning. A relatively new housing development was completely out of telephone service. Upon arrival at the development, they found a water company crew up to their waists in a water main break. The break was at the entrance to the long drive leading back to the development. The water company excavation was next to the trench line where the telephone company’s three hundred pair cable entered the development. The water company crew indicated it might be twelve hours before they would be done with their repair. Tom’s foreman assumed that the water main break or the repair excavation damaged the telephone cable. Since the nearest termination pedestal on the development side of the break was almost five hundred feet away, the decision was made to cut the telephone cable on the development side of the water problem and splice in a new cable around the water problem to the source terminal pole on the other side.
The foreman attached the transmitter of a cable tracer to the telephone cable at the development side pedestal. With a sensor to pick up the signal, Tom marked the location of the cable on the ground.
The telephone company maps showed the telephone cable had been installed in joint trench with the electric and cable TV cables. Since the NESC® in Rule 353D requires the power company high voltage cables to be installed at a minimum depth of 30 inches, the backhoe operator carefully dug down about 24 inches. So as not to damage the other cables, Tom and the backhoe operator dug the next 6 inches by hand. They found three jacketed cables: one about 0.9 inches in diameter and two cables about 1.3 inches in diameter. The cables were only a few inches apart. One of the larger cables was clearly marked with a lightning bolt and wording that indicated that it was a power cable. The NESC® in Rule 350G requires all direct buried communication cables and jacketed supply cables installed after January 1, 1996 to be marked with an identification symbol at spacing not more than 40 inches. The symbol for power cables is a lightning bolt. The symbol for communication cables is a telephone hand set. The marking is only required on one side of the cable.
Since one of the two large cables was marked and the cable TV cable is usually smaller than the telephone cable, the unmarked large cable was assumed to be the telephone cable. Since the transmitter of the cable tracer was still attached to the telephone cable at the pedestal, they tried to confirm their assumption. Because the cables were so close together, the sensor could not confirm their assumption. Tom was directed to cut the unmarked large cable . . .
Fortunately for Tom, his partners knew CPR and a paramedic team was only a few minutes away.
The investigation found that the two large cables were both power cables energized at 7200 Volts. The marking on the cable Tom cut was on the underside. The fault current that vaporized Tom’s cable cutter was about 9,000 Amps. The smaller cable was the cable TV cable. It was marked with a blue stripe facing to the side. The telephone cable they were looking for was next to the cable TV cable in undisturbed soil. It was only marked with the manufacturer’s name.
The NESC® in Rule 352 requires a horizontal minimum separation of 12 inches between direct buried supply and communication cables to permit access to and maintenance of either facility. The cables involved in the accident had been installed in "Random Separation.” "Random Separation,” supply and communication cables buried together at the same depth with no deliberate separation, is allowed by the NESC® in Rule 354 but only where the applicable requirements of Rule 354D are met and all parties are in agreement. The joint trench agreement between the telephone and power company did not mention "Random Separation.” The two companies’ construction standards only showed the NESC® 12-inch minimum. The work orders that directed the crews to install the cables spoke only of joint trench. Obviously, the crews who installed the cables were in agreement. But were the companies in agreement? The cable tracer equipment could not distinguish between cables installed so close together. The NESC® in Rule 423E requires positive identification before cutting into a cable. Did the people responsible for purchasing the tracer equipment know about the "random separation”? Had the crews been properly trained as to the hazards of random separation? Were the companies in agreement?
Even when the cable is marked, it’s a 50/50 chance that the marking faces down. If companies are going to do "Random Separation” to reduce trench expense, the employees must be properly trained and have the proper tools to deal with cables in close proximity.
Tom took the power company to court and Tom won. OSHA fined the telephone company. How much money did the companies really save going with "Random Separation”? [See photo 1]
Test yourself:Which of the four cables is the three hundred pair telephone cable? [See photo 2]
From left to right, the first cable is a cable TV cable (diameter 0.93″). The second cable is a 25kV class high-voltage cable (diameter 1.26″). The third cable is a 15kV class high-voltage cable (diameter 1.10″”). The fourth cable is a 300 pair telephone cable (diameter 1.30”).
Read more by David Young
Posted By John E. Brezan,
Friday, January 01, 1999
Updated: Monday, August 27, 2012
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For those of us who have been using the NEC for a number of years, when looking at the 1999 edition, we see a whole new document. This will be a cycle of relearning what we already know. Along with the changes is the relocation of a lot of existing information. It would be impossible to explain all of the rework and changes in Article 240 in just one short article. It is my recommendation that you check the IAEI News for a seminar in your area and attend it.
But let us browse through Article 240, examining the highlights briefly:
Section 240-3(d) Small Conductors is a relocation from the bottom of Table 310-16. There is not anything new here; it was felt that the rule would be better served if this information were located in the Overcurrent Protection Article and that it would, possibly, be easier to find.
Section 240-3(e) Tap Conductors now have a definition of their own as used in 240. In the past there were major abuse and misuse of the rules for a tap conductor and there was not a clear statement as to what a tap conductor is. Hopefully the definition will clear up any questions as to what is a tap conductor.
Section 240-6(c) Restricted Access Adjustable-Trip Circuit Breakers now defines what "”restricted”" means and what has to be done to comply with the rules of installation. There are three ways to restrict access and at least one of the three must be met.
Section 240-21 Location in Circuit has undergone some major rewrite that did not cause any code requirement changes but it will make life a lot easier when applying the rules. There is now a separation of taps to feeders from the requirement for transformer secondaries.
Section 240-21(a) has been divided into two parts: (a) Branch-Circuit Conductors specifies that for this tap one must comply with Section 210-19. The new (b) Feeder Taps lets one know that he is now in the rules for feeder taps. The wording in the 10 ft. tap rule was changed from "”the line side of the tap conductor shall not exceed 1,000 percent of the tap conductor’s ampacity”" to "”shall not exceed 10 times the ampacity of the tap conductor.”" This change was made simply to make the understanding of the math easier. If this still confuses some people you could always do the math both ways. The greatest change was really the rearrangement so that all of the 10 ft. rules are in one location and all of the 25 ft. rules are in another location and so on, you get the idea.
Under Section 240-21(b)(5) Outside Taps of Unlimited Length did not see any significant changes here. Perhaps the most significant event here is something that did not happen. There were a number of proposals and comments for the rule to allow termination in up to six disconnects instead of one. The main argument here was that it is felt, by the proposers and commenters, the number of disconnects should follow the same rules as Service Equipment. The panel felt that they should not be treated the same because of the differences between the two and that there would be less overloading of the tap conductors if limited to one disconnect rather than a maximum of six. I think this one will be back.
There is an interesting clarification to Section 240-21(g) Conductors from Generator Terminals that now spells out that the conductors are not protected from short-circuit or ground-fault under the specifications of 445-4. The conductor sizing has changed and the wording in 445-5 provides for additional language for the sizing of these conductors to afford them the protection needed.
These are some of the changes and relocation of information in 240, but there are some major changes that I did not even mention. I hope you found this to be of interest and may have even picked up some new information that will be of help to you in this new code cycle. I hope that I have an opportunity to present some more of Article 240 in the future such as the new Part H and the rewrite of Part I. Please do not feel as though you have to wait for me, let one of the IAEI Seminars introduce you to these exciting changes. Check your magazine for dates and places, and check with your local chapter.
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Posted By Underwriters Laboratories ,
Friday, January 01, 1999
Updated: Monday, August 27, 2012
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Question: GTO cable
Which GTO cable types may be used in electric signs? I am especially interested in the cable that looks like a coaxial cable.
The following GTO cable types may be used in electric signs (UXYT), and in field-installed skeleton tubing (UZBL):
GTO Cable (general information)
— GTO cable is listed under UL’s category Gas Tube Sign and Ignition Cable (ZJQX) in the General Information Directory (white book) and Electrical Construction Equipment Directory (green book). UL listed GTO cable is available in three voltage ratings, 5 kV, 10 kV, and 15 kV, which correspond to the UL designations GTO-5, GTO-10 and GTO-15, respectively. The voltage rating and corresponding designation are marked on the cable.
GTO cable used in electric signs is rated for a maximum service temperature of 60°C, unless marked otherwise. The construction of GTO cable consists of a single stranded conductor of a size ranging from No. 18-10 AWG, which is also marked on the cable. The insulation consists of a single layer, with or without an outer jacket.
GTO Cable With Braided Shield —
The GTO cable construction that resembles a coaxial cable includes a braided shield over the high voltage insulation and has an outer polymeric jacket over the shield. This cable construction is UL listed and has been evaluated for the same applications as GTO cable.
GTO Cable Used With GTO Cable
Sleeving — For UL Listed Signs (UXYT), listed GTO cable may be used only within the sign or other electrical enclosure, or installed using one of the wiring methods specified in UL 48, Standard for Electric Signs. In place of the specified wiring methods, exposed lengths of GTO cable used in dry and damp locations may be provided with UL Listed GTO Cable Sleeving (UYMR, Sign Accessories). GTO cable used with Listed GTO Cable Sleeving has not been evaluated by UL for use in wet locations or where concealed within a wall or attic, or above a suspended ceiling.
GTO Cable With Integral Sleeve —
Listed GTO cable marked "LISTED WITH INTEGRAL SLEEVE” is similar to GTO cable, except the insulation is thicker for the integral sleeve cable. GTO cable with integral sleeve is intended to be used in the same manner as GTO cable with Listed GTO Cable Sleeving. Supplemental cable sleeving is not required for integral sleeve GTO cable.
GTO Cable Used in Field-Installed
Skeleton Tubing – When used in fieldinstalled skeleton tubing, GTO cable must be installed in accordance with Part B of Article 600 of the NEC®. UL listed field-assembled systems, intended for permanent installation in accordance with Article 600, are covered under the category Field Assembled Skeletal Neon and Outline Lighting Systems (UZBL).
Question: Daisy-chaining outlet strips
I have been told that "daisy-chaining” outlet strips may present a potential fire and/or electrical hazard. I want to use two listed strips, each with integral 10A overcurrent protection, connected in series, to provide power to several pieces of computer equipment on adjacent desks. The total connected load would be less than 10A. Would you comment on whether (or why) this may be a fire hazard or dangerous practice, if all other requirements of the NEC® are followed? What other applications for extension cords might create a safety concern?
Extension cords (cord sets) and outlet strips (relocatable power taps) are not intended to be used as a substitute for fixed wiring in a structure. They provide power to portable appliances, such as personal computers and peripherals. Relocatable power taps are considered extensions of the branch circuit. Therefore, they are intended to plug directly into a branch-circuit outlet, not another relocatable power tap or an extension cord. Extension cords are intended to extend the power supply cord of electrical equipment, and should be marked with a current rating sufficient for the load of the connected equipment.
Use of these devices in series has not been evaluated by UL. Potential overheating can be one concern with a series arrangement. Other considerations in a seriesconnected arrangement include the reaction between overcurrent protective devices on relocatable power taps, increased voltage drop, potential tripping hazards, and an increased likelihood that additional available outlets may lead to an overload.
Extension cords attached to building surfaces with staples or other connectors may be subject to weathering, damage from the staples, mechanical abuse, strain or repeated flexing. In some cases, they may be placed under carpet, through holes in walls, floors, or ceilings: or through openings in partitions with sharp edges. These are conditions that can result in damage to the insulation or conductors, and increase the likelihood of an electric shock or fire. In these situations, the extension cords are a substitute for fixed wiring in a structure, which is prohibited by Section 400-8 of the NEC®.
While the NEC® doesn’t specifically reference series-connection of relocatable power taps or extension cords, this arrangement is prohibited by the Uniform Fire Code (UFC), Section 8508.3, and the 2000 International Fire Code (IFC)-Final Draft, Section 605.4.2. These Codes indicate that power taps are required to be directly connected to a permanently installed receptacle. Regarding extension cords, the UFC and IFC Final Draft each require that extension cords be plugged directly into an approved receptacle, power tap or multiplug adapter.
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UL Question Corner
Posted By Philip Cox,
Friday, January 01, 1999
Updated: Monday, August 27, 2012
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Two major purposes of the IAEI is to promote the uniform understanding and application of the NEC and other electrical codes and to collect and disseminate information relative to the safe use of electricity. In trying to achieve these objectives or goals, the IAEI focuses on education for both its members and the electrical industry in general. This is a great challenge. The IAEI staff and other contributors work hard to develop and produce educational material of a quality that can be readily used as learning tools for self motivated people who learn well through their own study and for others who prefer the classroom setting. With every educational product developed, ongoing review of the material is made to see where improvements can be made to help users get more out of the material. Good education and the continual improvement of both knowledge and skills can have a positive effect on electrical safety. Knowledge and skill will not on their own make electrical installations better or the use of electrical systems safer. However, when a person has achieved those and works hard to properly apply them, the end result will be very beneficial.
IAEI sections, chapters, and divisions provide excellent opportunities for electrical training through the many conferences and training programs they conduct. These programs can complement the work done by the IAEI international office. In fact, some work closely with the IAEI international office in conducting either IO developed seminar programs or jointly developed educational programs. Coordinated joint effort involving chapters and the IO can work well in most cases and can be beneficial to IAEI members. Several chapters have effective education programs and, in addition to providing training, attract a number of new members. They do it through dedication to promoting good education and through well-organized and effectively run chapter programs. It takes strong and dedicated leaders in those chapters to do the job they do and they don’t hesitate to make the effort. All IAEI sections, chapters, and divisions working hard and achieving success are to be commended.
It is appropriate to single out one chapter that has consistently reached a high level of achievement. That chapter has shown support for the international office, used IAEI developed training materials, and has worked as a partner in promoting the IAEI education program. I doubt that anyone is surprised to learn that the Wisconsin Chapter has once again set the pace and provided a good example for other chapters. Wisconsin Chapter members have set their sights high for 1999. A number of seminars on the Analysis of the 1999 National Electrical Code® have been scheduled within the state of Wisconsin during the first part of 1999. As of the date of this writing, over 1,000 people have preregistered for those seminars. Those seminars will be conducted using the IAEI material and qualified chapter members will conduct the training. Chapter members have conducted training programs within the state of Wisconsin for several years in a row. Not many chapters can attract the number of people that the Wisconsin Chapter does, but the potential is there for reaching many people who need to get more involved and become more familiar with electrical safety.
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Posted By Leslie Stoch,
Sunday, November 01, 1998
Updated: Friday, August 24, 2012
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The Canadian Electrical Code has a long and precise definition for grounding as: "a permanent and continuous conducting path to the earth with sufficient ampacity to carry any fault current liable to be imposed upon it, and of sufficiently low impedance to limit the voltage rise above ground and to facilitate the operation of the protective devices in the circuit.” When we talk about grounding, we are usually thinking about electrical systems.
The code has an equally precise, but not as lengthy definition for bonding as: "a low impedance path obtained by permanently joining all non-current-carrying metal parts to assure electrical continuity and having the capacity to conduct safely any current likely to be imposed upon it. When we talk about bonding, we are usually thinking about metallic items such as equipment enclosures.
The rules for grounding and bonding are designed to protect people and property against the dangers of electrical shocks and electrically caused fires by:
- Limiting electrical circuit voltages
- Minimizing differences in potential between metal objects such as equipment enclosures and other grounded metal.
- Making sure that fuses and breakers trip promptly when they should
- Reducing voltage surges caused by lightning or other causes.
The code specifies that alternating current systems must be connected to ground for voltages up to 150 phase to ground or if the system carries a neutral.
The code also provides a number of exceptions where it would be more hazardous or impractical to operate with a grounded electrical system.
One such exception is electrical arc furnaces. It would be easy to see why a grounded system would be impracticable and maybe even hazardous, since the return currents might find many paths back to its source.
Another exception would be isolated circuits such as wiring to the underwater speakers in a swimming pool. If these circuits were to be grounded, under fault conditions, there could be a higher possibility of current flow in the water, which could be very hazardous to swimmers.
A third exception to the rule is an electric crane running across a hazardous area containing combustible fibres. It would be easy to see why it is advisable to eliminate some of the arcing and sparking risks so as to minimize the possibility of fire.
The code has some very specific requirements for grounding an electrical system. The system must be connected to ground once at the owner’s main substation or by the electrical utility and once again in the owner’s main service. The code prohibits any connections to ground once past the owner’s main electrical service.
This requirement has two main purposes:
- To maintain control of return currents by ensuring that they flow only on the insulated neutral conductor and not select random paths back to their source; and
- To ensure that the ground fault protection in switchboards will operate accurately and when it should.
The code is also very specific about bonding. It specifies that except for double insulated devices, metallic electrical equipment enclosures and raceways must be bonded to each other and to ground by a number of acceptable means such as bonding wires, busbars, metallic cable sheaths or metallic raceways.
The code prohibits using the identified circuit conductor (neutral) as the means of bonding electrical enclosures. Once again, there are several valid reasons:
- To maintain control of return currents as above; and
- To avoid grounding the electrical system past the main service, which the code prohibits; and
- To avoid the possibility that equipment enclosures could become live should the identified conductor (neutral) become accidentally disconnected.
The code goes to great lengths to ensure that all related metal objects remain at ground potential and do not develop differences in potential to each other. For these reasons, it specifies that in addition to the above, we must also bond to ground all of the following with a building:
- Metal water piping when not used for grounding the electrical system
- Metal gas piping
- Metal waste piping
- Metal supports for computer room floors
In dairy barns and other agricultural buildings where small differences in potential can have harmful effects upon animals (tingle voltages), all metal is bonded together, including all water piping, stanchions, water bowls, vacuum lines an even the floors where cattle are milked.
Another area where very small potential differences can be dangerous to people is that area in or around swimming pools. Section 68 of the code includes some very precise requirements for bonding all of the metal objects in the vicinity of a pool, in particular when electrical equipment is used within three metres of the pool.
As in previous articles, you should seek the advice of your local inspection authorities for an exact interpretation of any of the above as they apply in each province or territory.
Some Special Requirements
Several months ago in two previous articles, we covered some of the standard grounding and bond fundamentals contained in the Canadian Electrical Code. Now let’s look at some less usual or more interesting code requirements covering some special circumstances. It’s unlikely that all of our readers will have had an opportunity to employ all of the following.
One excellent example is Rule 18-132(2), which deals with bonding electrical equipment in Class I, Division 1 hazardous locations. Such locations contain dangerous concentrations of flammable gases or vapours which may ignite or explode on occurrence of a spark due to any cause including a loose bonding connection.
You may not be aware that Rule 18-132(2) has some exceptional bonding rules for electrical circuits, not only within the hazardous location, but also within the non-hazardous location from which the hazardous location circuit is supplied. (See Sketch A). This rule refers us to Rule 10-606(1)(a), (c) and (d), and Rule 10-60(2) (Means of Assuring Continuity at Service Equipment).
In simpler terms this combination of CEC rules tells us that circuit bonding in Class I, Division 1 hazardous locations and the rest of each circuit originating outside the hazardous location, must satisfy the special CEC conditions normally reserved for bonding on the supply side of customer main services.
As a direct result, electrical circuits in Class I, Division 1 hazardous locations and all the way back to their points of supply are restricted to the following methods of bonding and bonding connections:
- Bonding conductors
- Threaded couplings and bosses
- Grounding bushings and bonding jumpers
- Standard locknuts and bushings are insufficient and you will notice that EMT connections are not anywhere included in this list.
I have always interpreted the above rules as applying to only a complete circuit from its source (such as a distribution panelboard or motor control centre) in a non-hazardous location to electrical equipment in a hazardous location. But do these rules also affect the feeders upstream from these sources and perhaps back to the main substation? I hope someone can enlighten me. The code is not very specific on that point and I’m afraid you’ll have to discuss that issue with your local inspector for the best interpretation.
Livestock buildings and facilities also necessitate special grounding and bonding practices. CEC Rule 10-402(4) specifies that livestock waterers must be bonded with minimum #6 AWG copper conductors. Also, Rule 10-406(5) requires that all metal such as water pipes, vacuum lines, water bowls and stanchions must be bonded with #6AWG copper conductors.
For what reasons do we need these heavy wire sizes and special precautions? Tingle voltages (between electrical objects and the floor or between metal objects) in livestock buildings or other areas may originate from electrical ground faults in farm buildings or neutral potentials from primary electrical distribution systems. Cattle are extremely sensitive to very small voltages and will refuse to drink if tingle voltages are present in water bowls. They may also withhold milk during milking. Both of these actions along with their general unease in the presence of tingle voltages may lead to deterioration in health and a loss of production.
For these reasons and for safety reasons the CEC has created some special grounding and bonding requirements in livestock areas where these conditions may exist. To supplement the effectiveness of these rules, a device sometimes called tingle voltage filter is also available.
I am sure you already know that Rule 10-204 specifies that AC services, when required to be grounded, must only be grounded once at the customer’s main electrical service and once again back at the transformer supplying the service. Rule 10-204(1)(d) also prohibits any grounding or bonding connections to the system neutral downstream from the main service.
This sub-rule has two important purposes. It is designed to prevent grounding and bonding conductors, metal piping, ducts, cable sheaths or building structures from becoming parallel paths for unbalanced neutral currents. It also ensures that electrical system ground fault protection will operate effectively to eliminate dangerous and destructive arcing ground faults when called upon.
If that be the case, how do we treat an emergency standby generator which must have its neutral interconnected with the building system neutral? If the generator neutral is interconnected with its case inside the machine, there is no choice but to provide multi-pole switching to disconnect the generator neutral simultaneously with the phase conductors. If the neutral is isolated from the case and if not separately grounded, the neutral does not require disconnection from the electrical system neutral, but a separate grounding conductor must be installed to the generator case.
As with past articles, please consult your local electrical inspector in each province or territory for an exact interpretation of the above as applicable.
Read more by Leslie Stoch
Posted By David Young,
Sunday, November 01, 1998
Updated: Friday, August 24, 2012
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The National Electrical Safety Code® (NESC®) occasionally references other standards. For example, ANSI Z535.1-1991 through ANSI Z535.5-1991 inclusive are referenced many times within the NESC. Most of these references are made in a NOTE: following a rule. Rule 015D explains that a NOTE: indicates material provided for information or illustrative purposes only. When a standard is referenced in a NOTE:, compliance with the standard is not mandatory.
One of the references to ANSI® Z535.1-1991 through ANSI Z535.5-1991 inclusive is within a rule. Rule 411D states that "all warning signs and tags required by Part 4 shall comply with the provisions of ANSI Z535.1-1991 through ANSI Z535.5-l991 inclusive.” In the very next sentence, Rule 411D requires "permanent warning signs shall be displayed in conspicuous places at all entrances to electric supply stations, substations, and other enclosed walk-in areas containing exposed current-carrying parts.” In this case, where a reference is made within a rule, compliance with the standard is mandatory.
So, What is ANSI Z535?
ANSI Z535 is the American National Standard for Safety Signs, Labels and Tags. It is the standard "for the design, application, and use of signs, colors, and symbols intended to identify and warn against specific hazards and other accident prevention purposes.” The standard consists of five publications labeled ANSI Z535.1-1991 through ANSI Z535.5-1991.
ANSI Z535.2-1991 is the standard for environmental and facility safety signs. The warning signs spoken of in Rule 4llD would be considered "facility safety signs.” For a safety sign to be effective in alerting people of a hazard, the message must be easily recognizable and highly conspicuous. To achieve this, ANSI Z535.2-1991 recommends that safety signs be designed with three elements.
The first element is a signal word to get the person’s attention, i.e., DANGER, WARNING, CAUTION or NOTICE. The signal word designates the degree or level of safety alerting.
The word DANGER should only be used in an imminently hazardous situation which, if not avoided, will result in death or serious injury. This signal word is the one to use on a sign located inside an enclosure containing exposed line parts as recommended by Rule 381G2 for pad-mounted equipment. The hazard is life threatening and immediate.
The word WARNING should only be used in a potentially hazardous situation which, if not avoided, could result in death or serious injury. This signal word is the one to use on a sign located on the outside of the entrance to an enclosed walk-in area containing exposed line parts as required in Rule 411D. The hazard is life threatening but is not immediate. There is a door between the person and the hazard.
The word CAUTION should only be used in a potentially hazardous situation which, if not avoided, may result in minor or moderate injury or property damage. This is not a life-threatening situation. This signal word is the one to use on a sign alerting people that a passageway does not have 7 foot head room as required by Rule 112B or a low ceiling in a parking garage.
The word NOTICE should only be used to indicate a company policy directly or indirectly related to safety of personnel or protection of property. This signal word is the one to use on the sign that informs personnel that "Hard hats are required in this area” or "Check oil when refueling your vehicle.”
The colors used to display the signal word must comply with ANSI Z535.1-1991, i.e., white letters on red for DANGER, black letters on orange for WARNING, black letters on yellow for CAUTION, and white letters on blue for NOTICE.
The second element is a symbol or pictorial to promote greater or more rapid understanding. The use of symbols and pictorials is very important in getting the message across since the general population’s reading and comprehension skills vary. ANSI Z535.3-1991 covers the requirements for safety symbols. Symbols should be tested to insure that the people to whom the sign is directed understand what the symbol means. Symbol comprehension varies with location. A symbol that passes the test in New York may not pass the test in Florida. A suggested procedure for evaluation of symbols is included in the standard.
The third element is the message text. The message should identify the hazard, i.e., High Voltage, the location of the hazard, i.e., Inside, how to avoid the hazard, i.e., Keep Out! and the probable consequences of not avoiding the hazard, i.e., Can shock, burn or cause death. This example is the message you might use on an electrical hazard WARNING sign. ANSI Z535.2-1991 also gets into letter style, letter size, viewing distance, sign placement, illumination and the use of bilingual signs.
ANSI Z535.4-1991 covers product safety signs and labels. These are the kind of labels you would expect to see on your new chain saw. ANSI Z535.5-1991 covers accident prevention tags for temporary hazards. The blocking tags required by Rule 442E and 444C must comply with ANSI Z535.5-1991 because they are in Part 4.
ANSI Z535 has just recently been revised (1998). The significant change in the new edition is the addition of a safety alert symbol, an exclamation point inside a triangle, to the left of the DANGER, WARNING and CAUTION signal words involving personal injury. The safety alert symbol should not be used on a CAUTION sign intended to prevent property damage.
Read more by David Young
Posted By J. Philip Simmons ,
Sunday, November 01, 1998
Updated: Monday, August 27, 2012
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Bonding (Bonded):"The permanent joining of metallic parts to form an electrically conductive path that will ensure electrical continuity and the capacity to conduct safely any current likely to be imposed.” N
Bonding jumper, Main:"The connection between the grounded circuit conductor and the equipment grounding conductor at the service.”N
Main bonding jumper
The main bonding jumper is one of the most critical elements in the safety grounding system. This conductor is the link between the grounded service conductor, the equipment grounding conductor and in some cases, the grounding electrode conductor. The primary purpose of the main bonding jumper is to carry the ground-fault current from the service enclosure as well as from the equipment grounding system that is returning to the source. In addition, where the grounding electrode conductor is connected directly to the grounded service conductor bus, the main bonding jumper ensures that the equipment grounding bus is at the same potential as the earth.
Figure 5-1. Main bonding jumper
For a grounded system, Section 250-28 requires that an unspliced main bonding jumper be used to connect the equipment grounding conductor(s) and the service-disconnect enclosure to the grounded conductor of the electrical system. The connection is required to be made within the enclosure for each service disconnect.
An example of this is where two or more service disconnecting means in individual enclosures are grouped at one location. This type of installation often is made with a wireway or a short section of busway installed downstream from the metering equipment. In other cases, a wireway or short section of busway is installed ahead of metering and is supplied by a service lateral or service-entrance conductors. Sets of service-entrance conductors supply each of the service disconnecting means. Service disconnecting means are installed from the wireway or auxiliary gutter. (If there are nipples between the disconnecting means and the metal or nonmetallic trough, the trough meets the definition of a wireway from Article 362 rather than an auxiliary gutter from Article 374.) Section 250-28 requires a main bonding jumper be installed in each service disconnect enclosure. As previously mentioned, Section 250-24(b) requires that the grounded service conductor be brought to each service disconnecting means and be bonded to the enclosure. The main bonding jumper is the means to accomplish this requirement.
Figure 5-2. Main bonding jumper—multiple enclosures
The rules are a little different where more than one service disconnecting means is in a common enclosure. This equipment usually consists of listed switchboards, panelboards or motor control centers.. Where more than one service disconnecting means is located in an assembly listed for use as service equipment, Section 250-28 Exception No. 1 permits the grounded service conductors to be run to a single grounded conductor bus in the enclosure and then be bonded to the assembly enclosure. This means that only one main bonding jumper connection is required to be installed from the common grounded conductor bus to the assembly enclosure. The sections of the assembly are bonded together by means of an equipment grounding conductor bus or by being bolted together.
Exception No. 2 to Section 250-28(b) permits alternate means for bonding of high-impedance grounded neutral systems. See Chapter Four of the IAEI Soares Book on Grounding for methods and requirements for grounding high-impedance grounded neutral systems. Also see NEC® Sections 250-36 and 250-186 for the specific requirements and allowances.
The main bonding jumper is permitted to consist of a wire, bus, screw or other suitable conductor. It must be fabricated of copper or other corrosion-resistant material. Aluminum alloys are permitted where the environment is acceptable. In addition, where the main bonding jumper consists of a screw, it must have a green finish that is visible with the screw installed. This green finish assists in identifying the bonding-jumper screw from the other screws that are on or near the neutral bus. See Sections 250-28(a) and (b).
Figure 5-3. Main bonding jumper for listed assembly
Functions of Main Bonding Jumper
The main bonding jumper performs three major functions:
- Connecting the grounded service conductor to the equipment grounding bus or conductor and the service enclosure.
- Providing the low-impedance path for the return of ground-fault currents to the grounded service conductor. The main bonding jumper completes the ground-fault return circuit from the equipment through the service to the source as is illustrated in Figure 5-4.
- Connecting the grounded service conductor to the grounding electrode conductor. Under certain conditions given in Section 250-24(a)(4), it is permitted to connect the grounding electrode conductor to the equipment grounding terminal bar rather than to the terminal bar for the grounded service conductor. This scheme is common on larger switchboard services and is necessary for proper operation of certain types of equipment ground fault protection systems. See Chapter 15 of the IAEI Soares Book on Grounding for additional information on this subject.
Size of main bonding jumper in listed enclosures
Where listed service equipment consisting of a switchboard, panelboard or motor control center is installed, the main bonding jumper that is provided with the equipment is rated for the size of conductors that would normally be used for the service. The method for sizing of the main bonding jumper in listed service equipment is found in Underwriters Laboratories Safety Standard for the equipment under consideration and is verified by the listing agency. Therefore, if a main bonding jumper that is a bus bar, strap, conductor, or screw is furnished by the manufacturer as part of the listed equipment, it may be used without calculating its adequacy. Section 384-3(c) requires the equipment manufacturer to provide the main bonding jumper.
Figure 4. The main bonding jumper completes the ground-fault return circuit from the equipment through the service to the source
Size of main bonding jumper at single service-disconnect or enclosure
Since the main bonding jumper must carry the full ground-fault current of the system back to the grounded service conductor (which may be a neutral), its size must relate to the rating of the service conductors which supply the service. The minimum size of the main bonding jumper is found in Table 250-66 as required by Section 250-28(d). This relationship is based on the conductor’s ability to carry the expected amount of fault current for the period of time needed for the overcurrent device to open and stop the flow of current.
For example, where 250 kcmil aluminum service-entrance conductors are installed, the main bonding jumper is found to be No. 4 copper or No. 2 aluminum by reference to Table 250-66.
The size of the main bonding jumper does not directly relate to the rating of the service overcurrent device. Do not attempt to use Table 250-122 for this purpose. Table 250-122 gives the minimum size of equipment grounding conductors for feeders and circuits on the load side of the service.
Sizing of main bonding jumper for parallel service conductors
Figure 5-5. Main bonding jumper at single disconnect
Where service conductors are installed in parallel, (connected together at each end to form a larger conductor) the total circular mil area of the conductors connected in parallel for one phase are added together to determine the minimum size main bonding jumper required. See Section 250-28(d). For example, where three 250 kcmil conductors are connected in parallel per phase, they are treated as a single 750 kcmil conductor. By reference to Table 250-66 the main bonding jumper, if aluminum service-entrance conductors are used, is 1/0 copper or 3/0 aluminum.
Where the service-entrance conductors are larger than the maximum given in Table 250-66, Section 250-28(d) requires the main bonding jumper to be not less than 12½ percent (0.125) of the area of the largest phase conductors.
This is illustrated by the following example:
Three 500 kcmil copper conductors are installed in parallel as service-entrance conductors.
3 x 500 kcmil = 1500 kcmil.
1500 x .125 = 187,500 circular mils.
Since a 187,500 circular mil conductor is not a standard size, we next refer to Chapter 9, Table 8 to find the area of conductors.
The next conductor exceeding 187,500 circular mils is a No. 4/0 AWG conductor which has an area of 211,600 circular mils. It is always necessary to go to the next larger size conductor since the 12½ percent size is the minimum size permitted.
Follow a similar procedure for determining the minimum size main bonding jumper required for other sizes of parallel service-entrance conductors.
Figure 5-6. Main bonding jumper for parallel runs
Bonding of service conductor enclosures
Special rules are provided for bonding enclosures on the line side of the service disconnecting means. This is due to the fact that this equipment does not have overcurrent protection on its line side such as feeders and branch circuits have. Fault current of sufficient magnitude must flow during a short period of time to allow the fuse on the line side of the utility transformer to open. The level of fault current and particularly the duration the current may flow could be much larger than would flow in a feeder or branch circuit as there is not an overcurrent device in series with the conductor.
The basic rule is that all metallic enclosures that contain a service conductor must be bonded together. The bonding ensures that none of the equipment enclosures can become isolated electrically and become a shock hazard should a line-to-ground fault occur. The bonding also provides a low impedance path for fault current to flow in so the fuse or circuit breaker on the line side of the electric utility transformer will open.
Sizing of equipment bonding jumper on line (supply) side of service.
Equipment bonding jumpers on the line side of the service and main bonding jumper must be sized to comply with Table 250-66. This is required by Section 250-102(c). For example, where 250 kcmil copper conductors are installed as service-entrance conductors, Table 250-66 requires a No. 2 copper or 1/0 aluminum bonding jumper.
Where the sum of the circular mil area of the service-entrance phase conductors exceeds 1100 kcmil copper or 1750 kcmil aluminum, the equipment bonding conductor must be not less than 12½ percent (0.125) of the area of the ungrounded phase conductors.
Figure 5-7. Size of equipment bonding jumper on line side of service
Sizing of equipment bonding jumper for parallel conductors
Two methods are provided for bonding service raceways that are installed in parallel. The first method is to add the circular mill area of the service-entrance conductors per phase together and treat them as a single conductor. The bonding jumper size is determined from Table 250-66 and is connected to each conduit bonding bushing in a "daisy-chain fashion.” This method often results in an equipment bonding jumper that is quite large and difficult to work with.
For example, if five 250 kcmil copper conductors are installed in parallel for a phase, the equipment bonding jumper for bonding the metal raceways must not be smaller than 3/0 copper.
This is determined as follows:
Five x 250 kcmil = 1250 kcmil.
1250 kcmil x .125 = 156,250 circular mils.
Figure 5-8. Size of equipment bonding jumper on line side of service
The next larger conductor found in Chapter 9, Table 8 is 3/0 with an area of 167,800 circular mils.
In this case, a 3/0 copper equipment bonding conductor must be connected from the grounded service conductor or equipment grounding bus to each metal raceway in series (daisy-chain fashion from one raceway to another).
A more practical method of performing the bonding for services supplied by multiple raceways may be to connect an individual bonding conductor between each raceway and the grounded service conductor terminal bar or equipment grounding bus. This is permitted by Section 250-102(c). This will usually result in a smaller equipment bonding conductor which is easier to install.
Again, using the example above and referring to Table 250-66, the minimum size equipment bonding conductor for the individual raceways containing 250 kcmil copper service-entrance conductors is No. 2 copper or 1/0 aluminum. A properly sized equipment bonding jumper is installed from the terminal bar for the grounded service conductor or from the equipment grounding terminal bar to each conduit individually.
Different conductor material
Section 250-28(d) provides instructions on sizing the main bonding jumper or equipment bonding jumper on the supply side of the service where different conductor materials are used for the service-entrance conductors and the bonding jumper. The procedure involves assuming the phase conductors are of the same material (copper or aluminum) as the bonding jumper and that they have an equivalent ampacity to the conductors that are installed. This is illustrated as follows:
Assume aluminum phase conductors and a copper bonding jumper are installed.
Three 750 kcmil Type THW aluminum conductors are installed.
From Table 310-16, 385 amperes x 3 = 1155 amperes. The smallest type THW copper conductor that has an equivalent rating is 600 kcmil with an ampacity of 420.
Next, determine the total circular mil area of the copper conductors.
Three x 600 kcmil = 1800 kcmil.
1800 kcmil x .125 = 225 kcmil.
The next standard size is 250 kcmil copper which is the minimum size bonding jumper permitted to bond equipment at or ahead of the service equipment in this example.
Bonding service equipment enclosures
The Code requires that electrical continuity of service equipment and enclosures that contain service conductors be established and maintained by bonding. The items required to be bonded together are stated as follows in Section 250-92(a):
(1) The service raceways, cable-trays, cablebus framework or service cable armor or sheath.
(2) All service equipment enclosures containing service conductors, including meter fittings, boxes or the like, interposed in the service raceway or armor.
(3) Any metallic raceway or armor which encloses the grounding electrode conductor. (This subject is covered in detail in Chapter 7 of this text.)
An exception to this requirement for bonding at service equipment is mentioned in Section 250-92(a)(1). It refers to Section 250-84 which has rules on underground service cables that are metallically connected to the underground service conduit. The Code points out that if a service cable contains a metal armor, and if the service cable also contains an uninsulated grounded service conductor which is in continuous electrical contact with its metallic armor, then the metal covering of the cable is considered to be adequately grounded.
Figure 5-9. Bonding service equipment enclosures
Use of neutral for bonding on line side of service
Section 250-94(1) permits the use of the grounded service conductor (may be the neutral) for grounding and bonding equipment on the line side of the service disconnecting means. This is also permitted by Section 250-142(a)(1). (Two other applications of this bonding are explored in later chapters of the IAEI Soares Book on Grounding.) Often, connecting the grounded service conductor to equipment such as meter bases, current transformer enclosures, wireways and auxiliary gutters is the most practical method of bonding these enclosures.
Usually, self-contained meter sockets and meter-main combination equipment are produced with the grounded conductor terminals or bus (often a neutral) bonded directly to the enclosure. The enclosure is then effectively bonded by the connection of the grounded circuit conductor to these terminals. No additional bonding conductor connection to the meter enclosure is required. Current from a ground fault to the meter or meter-main enclosure will return to the source by the grounded service conductor (may be a neutral) and, hopefully, will allow enough current to flow in the circuit to operate the overcurrent protection on the line side of the utility or other transformer.
Figure 5-10. Use of neutral for bonding on line side of service
In addition, meter enclosures installed on the load side of the service disconnecting means are permitted to be grounded (bonded) to the grounded service conductor provided that:
(a) Service ground-fault protection is not installed; and
(b) The meter enclosures are located near the service disconnecting means. (No distance is used to clarify what is meant by the word "near.”), and
(c) The size of the grounded circuit conductor is not smaller than the size specified in Table 250-122 for equipment grounding conductors. See Section 250-142(b) Exception No. 2.
Means of bonding at service equipment
The methods for bonding at service equipment are outlined in Section 250-94. These requirements for bonding are more restrictive at services than downstream from the service. The reason this is so important is service equipment and enclosures may be called upon to carry heavy fault currents in the event of a line-to-ground fault. The service conductors in these enclosures have only short-circuit protection provided by the overcurrent device on the line side of the utility transformer. Only overload protection is provided at the load end of the service conductor by the overcurrent device. This is one of the reasons the Code limits the length of service conductors inside of a building.
Figure 5-11. Methods of bonding service equipment
Bonding of these enclosures is to be done by one or more of the following methods from Section 250-94:
(1) Bonding to the grounded service conductor through the use of exothermic welding, listed pressure connectors such as lugs, listed clamps, or other listed means. These connections cannot depend solely upon solder.
(2) Threaded couplings and threaded bosses in a rigid or intermediate metal conduit system where the joints are made up wrench-tight. Threaded bosses include hubs that are either formed as a part of the enclosure or are supplied as an accessory and installed according to the manufacturer’s instructions.
(3) Threadless couplings and connectors are permitted where they are made up tight for rigid and intermediate metal conduit and electrical metallic tubing and metal-clad cables.
(4) Other approved devices such as bonding-type locknuts and bushings.
Bonding jumpers are required to be used around concentric or eccentric knockouts that are punched or otherwise formed so as to impair an adequate electrical path for ground-fault current. It is important to recognize that concentric and eccentric knockouts in enclosures such as panelboards, wireways and auxiliary gutters have not been investigated for their ability to carry fault current. Where any of these knockout rings remain at the conduit connection to the enclosure, they must always be bonded around to ensure an adequate fault-current path.
Figure 5-12. Bonding fittings
The Code states here that "Standard locknuts or bushings shall not be the sole means for the bonding required by this section.” This statement does not intend to prevent the use of "standard” locknuts and bushings, it is just that they cannot be relied upon as the sole means for the bonding that is required by this section. "Standard” locknuts are commonly used outside the enclosure on conduit that is bonded with a bonding bushing or bonding locknut inside the enclosure. Standard locknuts are used to make a good, reliable mechanical connection as required by Section 300-10.
Parallel bonding conductors
Section 250-102(c) requires that where service-entrance conductors are paralleled in two or more raceways or cables and the equipment bonding jumper is routed with the raceways or cables, the equipment bonding jumper must be run in parallel.
In this case again, the size of the bonding jumper for each raceway is based upon the size of the service-entrance conductor in the raceway by referring to Table 250-66.
Grounding and bonding of remote metering
Figure 5-13. Parallel bonding conductors
As mentioned before, Section 250-92(a) requires all equipment containing service conductors to be bonded together and to the grounded service conductor. This includes remote (from the service equipment) meter cabinets and meter sockets.
Grounding and bonding of equipment such as meters, current transformer cabinets and raceways to the grounded service conductor at locations on the line side of and remote from the service disconnecting means increases safety.
This equipment should never be grounded only to a grounding electrode such as a ground rod. Figures 5-14 and 5-15 show why. If a ground-fault occurred at this line-side equipment, and it is not bonded as required, the only means for clearing a ground fault would be through the grounding electrodes and earth. Given the relatively high impedance and low current-carrying capacity of this path through the earth and high resistance of grounding electrodes such as rods, little current will flow in this path. This leaves the equipment enclosure(s) at a dangerous voltage above ground potential just waiting to shock or possibly electrocute a person or animal that may contact it. The voltage drop across this portion of the circuit can easily be calculated by using Ohms Law. (Resistance times the current gives the voltage.) There are many records of livestock being electrocuted while contacting electrical equipment that was improperly grounded. Sections 250-2 and 250-54 require that the earth not be used as the sole equipment grounding conductor or fault-current path.
The most practical method for grounding and bonding this line-side equipment is to bond the grounded service conductor to it. As can also be seen in Figures 5-14 and 5-15, a ground fault to the equipment will have a low impedance path back to the source through the grounded service conductor. This will allow a large current to flow in the circuit to cause the overcurrent protection on the line side of the transformer to clear the fault.
Supplementary grounding electrodes
In accordance with Section 250-54, it is permissible to install a grounding electrode at the remote meter location shown in Figures 5-14 and 5-15 to supplement the grounded service conductor. This Code section refers specifically to grounding electrodes supplementing the equipment grounding conductors. Some electric utilities require a grounding electrode at meter equipment installed remote from service equipment such as on poles. The Code in Section 230-66 makes it clear that individual meter socket enclosures are not to be considered service equipment. The same is true for metering equipment installed in remote current-transformer enclosures. As mentioned earlier, it is critically important that these meter enclosures be properly bonded as they contain service conductors.
This additional grounding electrode will attempt to keep the equipment at the earth potential that exists at the meter location. In addition, the electrodes at the remote meter and at the service location are bonded together by the grounded service conductor installed between the metering and service equipment. This brings the installation into compliance with Section 250-58 which requires a common grounding electrode or where two or more electrodes are installed, they must be bonded together.
As previously stated, these grounding electrodes should never be used as the only means for grounding or bonding these enclosures or to carry fault current.
More extensive discussion of this subject is found in Chapter Six of the IAEI Soares Book on Grounding.
Bonding of multiple service disconnecting means
Installation of multiple services as permitted by Section 230-2(a) through (d) and installations of services that have multiple disconnecting means can take several forms. Additional services are permitted by Section 230-2 for:
(a) Fire pumps, emergency, legally required, standby, optional standby or parallel power production systems.
(b) By special permission, for multiple occupancy buildings where there is no available space for service equipment that is accessible to all occupants, or, for a single building or structure that is large enough to make two or more services necessary.
(c) Capacity requirements; where the service capacity requirements exceed 2,000 amperes at 600 volts or less, where load requirements of a single-phase installation is greater than the serving utility normally provides through a single service, or by special permission (related to capacity requirements).
(d) Different characteristics of the services such as different voltages, frequencies, or phases, or for different uses, such as for different rate schedules.
The basic rule for sizing of the equipment bonding jumper for bonding these various configurations is found in Section 250-102(c). This section requires that the bonding jumpers on the line side of each service the main bonding jumper be sized from Table 250-66. Also, the size of the bonding jumper for each raceway is based on the size of service-entrance conductors in each raceway. As discussed earlier, conductors larger than given in Table 250-66 are required for larger services. Since different sizes of service-entrance conductors may be installed at various locations, the minimum size of the equipment bonding conductor and main bonding jumper is based on the size of the service-entrance conductors at each location.
For example, the appropriate size of bonding jumper for the installation in Figure 5-16 with the assumed size of conductors is as follows: (all sizes copper)
|Service-Entrance Conductor||Bonding Jumper|
|a. 500 kcmil in service mast||1/0|
|b. 1000 kcmil in wireway||2/0|
|c. 300 kcmil to 300 ampere service||No. 2|
|d. 3/0 to 200 ampere service||No. 4|
|e. No. 2 to 125 ampere service||No. 8|
A practical method for bonding the current transformer enclosure and wireway (sometimes referred to as a "hot gutter”) is to connect the grounded service conductor directly to the current transformer enclosure or wireway. This may be done by bolting a multi-barrel lug directly to the wireway and connecting the neutral or grounded service conductors to the lug. Be sure to remove any nonconductive paint or other coating that might insulate the connector from the enclosure.
As previously discussed, the grounded service conductor must also be extended to each service disconnecting means and be bonded to the enclosure.
Excerpted from Chapter 5 of the IAEISoares Book on Grounding, 7th Edition
Read more by J. Philip Simmons