Posted By Philip Cox,
Monday, November 01, 1999
Updated: Wednesday, August 29, 2012
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Electrical inspectors are a vital part of the electrical safety system. It is unlikely that one could adequately define the value of the service electrical inspectors provide to the public in preventing electric shock and damage to or loss of property through the enforcement of electrical safety regulations. Over 100 years ago it became clear that electrical safety guidelines were needed. Through the work of a dedicated group of individuals, a set of electrical rules was developed. Those rules soon became known as the National Electrical Code. A logical step following the development of rules was that of implementation. The electrical inspector fills that role. One readily recognizes that the development and adoption of laws is not what makes them work. The objectives of those adopted laws are achieved through enforcement. Just as police officers know how much traffic laws would be observed if they were not on patrol, electrical inspectors know that electrical safety rules will be violated by some intentionally or by others through a lack of knowledge. One only has to be on the road a short time before it becomes clear that many drivers don’t follow posted speed limits. When those drivers see a traffic officer or believe that one is in the vicinity, they slow their vehicle to a legal speed. In similar fashion, without electrical inspectors enforcing the electrical code, many will not follow those established rules.
It may seem strange to some, but manufacturers of electrical products and electrical contractors are among the strongest supporters of qualified electrical inspectors. Qualified electrical inspectors who are permitted to enforce the electrical code to protect the consumer also benefit manufacturers, designers, installers, and others within the industry. Proper enforcement of the Code is an important step in protecting the consumer against unsafe electrical products and installation practices. Manufacturers who strive to produce quality products that comply with established code rules can’t compete with those who make products that don’t comply. In addition, efforts by some to introduce imitations or counterfeit copies of some known electrical products into the market create a real safety concern. Electrical contractors who take the necessary steps to stay current with code provisions and follow adopted safety rules and associated laws in the conduct of their business are at an unfair disadvantage if their competitors don’t do the same. In both cases, without the adoption and enforcement of the electrical code, the consumer also loses.
Qualified electrical inspectors have an important voice that needs to be heard in the industry. They have a tremendous amount of collective knowledge and experience which can be used very effectively for more purposes than simply making inspections. One of the most important uses of that knowledge and experience is in the development of electrical safety codes and standards. However, before an individual or group can be heard and recognized as having credibility in the industry, they must demonstrate that they are worthy of that recognition. Qualified electrical inspectors who follow the basic principles upon which the IAEI stands have earned that respect through unbiased and fair interpretation and application of the code. The IAEI objectives are worthy in scope and desirable as goals. Those objectives include cooperating in the formulation of standards for the safe installation and use of electrical materials, devices, and appliances. It also promotes the uniform understanding and application of the National Electrical Code and other electrical codes and promotes cooperation between inspectors, the electrical industry, and the public. Another important objective is to collect and disseminate information relative to the safe use of electricity.
These objectives not only work to protect the consumer from hazards arising from the installation and use of electricity but also emphasize the necessity of segments of the electrical industry to work together in this effort. The electrical inspector’s main interest is that of the consumer. When considering code proposals, the electrical inspector can look clearly at the safety provisions as they affect the consumer without being influenced by unrelated factors. The electrical inspector’s role in code development is that he or she can look at code proposals and see the broad scope of factors that are associated with them. They can evaluate those factors and make a judgment that is felt to be in the best interest of the consumer. The qualified electrical inspector can sit as a member of a technical committee, listen to arguments for and against proposed changes, participate in the debate, and can make an unbiased decision based on the data presented. To do this, the inspector must not be directly tied to or in any way adversely influenced by outside interests. The founders of the IAEI developed a structure that ensures that electrical inspectors will be able to echo their voice without being controlled by any outside influence. Had IAEI members affiliated with interests other than electrical inspection been given the right to determine what positions the IAEI must take on code matters, it would have clearly destroyed the credibility of the inspection community in code development.
Those participating in code development who are not part of the code enforcement group recognize the importance of the unbiased position of electrical inspectors in code development and of the need to keep it that way. Influence of electrical inspectors by IAEI associate members and others within the electrical industry should be through informational means. This can be accomplished through reliable technical information, such as that generated through fact finding studies and field evaluations. The IAEI has an established procedure for initiating proposed code changes and for consideration of others in an effort to provide the inspector’s perspective and represent the consumer’s interest. The role of the electrical inspector is important for the industry and the public in general. The electrical inspector’s voice needs to be heard.
Read more by Philip Cox
Posted By David Young ,
Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
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Electrical inspectors involved with aerial high voltage facilities frequently have to determine whether electrical conductors are in compliance with the National Electrical Safety Code (NESC). In the July/August issue, I discussed level spans, spans where the conductor points of attachment on the structures are at the same elevation. Inclined spans, spans where the conductor points of attachment on the structures are not at the same elevation, are much more difficult to determine clearances. The minimum clearances of Table 232-1 must be met under three conditions of maximum sag; whichever produces the largest final sag.
- Conductor temperature 120° F.
- The maximum conductor temperature for which the line is designed to operate if greater than 120° F.
- Conductor temperature 32° F. with radial thickness of ice as specified in Rule 250B for the loading district concerned. To determine if a line conductor is in compliance with the NESC, we must know the sag of the conductor under the above three conditions. For example, let’s consider an inclined 900 foot span of 954 kcmil.
Conductor identification is important. If someone tells you the conductor is 954, that’s not enough information. There are two 954 AAC (All Aluminum Conductors): 37 strand and 61 strand. There are also five 954 ACSR conductors (Aluminum Conductor Steel Reinforced). In this example, we will use 954 AAC 61 strand identified by the Aluminum Association as "Goldenrod.” For simplicity, let’s assume the ground under the span is flat and level. The conductor height of attachment at one structure is 153 feet. At the other structure, the conductor height of attachment is 72 feet. Because the grade is not often flat and level, it is difficult to determine at which point the conductor is closest to the ground. For that reason, I usually take numerous conductor height measurements including one at mid-span. You will recall that for a level span, the conductor elevation is lowest at mid-span. For an inclined span, the lowest conductor elevation is shifted toward the structure with the lower height of attachment. In both cases, knowing the conductor height at mid-span is important for our calculations. It is best to measure the conductor heights relative to a common elevation. I usually use the ground level at the base of the pole of higher conductor attachment height. As you may have guessed, a transit or telescope level is a necessary tool when taking field measurements. For our example, let’s assume the conductor height above ground at mid-span is 52 feet and the conductor temperature at the time of measurement is 70°F.
To understand the calculations necessary to determine the ground clearance, one must have a clear understanding of the relationship between a level span and an inclined span. Mathematically, an inclined span is a piece of a level span.
Diagram B shows the conductor at a conductor temperature of 70°F. To determine the ground clearance C we must know the position of the conductor at maximum sag.
The following method is one way of calculating the desired clearance. This method is relatively easybut also inaccurate. The error in this method can be as much as 11%. If you use this method, I recommend that you multiply the calculated sag by 1.11 to correct for the error. If a more accurate clearance is necessary, the long method will have to be used.
The first step is to find the conductor horizontal tension H and the level span length S. (See diagram B.)
To find S and H we have two equations:
D= height of the conductor at mid-span above the level of the base of the pole with the higher conductor height attachment.
For our example D = 52′.
X = the height of attachment of the conductor on the pole with the lower height of attachment.
For our example X = 72′
Y = the conductor height of attachment on the pole with the higher height of attachment.
For our example Y = 153′
B = level span length
For our example B = 900′
cosh = hyperbolic cosine trig function
The two equations can only be solved by substitution. There is only one value of S and A that satisfies both equations. With the help of a computer S = 1200 and A = 1689.54. H can be calculated from A as follows:
H = A x W
where W is the conductor weight per foot in pounds. For 954 AAC "Goldenrod”
W = .8961 lb./foot
H = 1689.54 x .8961
H = 1514 lb.
Next, we need to calculate the conductor vertical tension V:
V =L x W
where L is the conductor length in feet.
L = 2A sinh (S/2A)
L = 1225.38′
V =1225.38′ x .8961
V = 549 lb.
The conductor total tension T is
The conductor average tension =
= 1562 lb
The average tension 1562 is the final tension limit we now put into a sag/tension computer program at 70°F. to determine the conductor’s maximum sag. One example of a sag/tension computer program is the Alcoa Sag 10®. The loading table for the heavy loading district should look something like this.
The ruling span should be set to 1200′. Running the program should give us the following:
From the above results, the conductor sag at 120°F is 109.31′. The conductor sag at 32°F. with 1/2 inch of ice (heaving loading district) is 107.13′. The conductor sag at maximum operating temperature for which the line was designed is 112.26′ (212°F). The maximum sag is 112.26′.
Since the "shortcut” calculation was used, we need to multiply the maximum sag by 1.11 to correct for calculation error 112.26 x 1.11 = 124.6′.
The height of the conductor above the ground at maximum sag is 153 – 124.6 = 28.4′.
Unless the ground under the span is traversed by vehicles taller than 14′, requiring a clearance greater than 18.5′, the span is in compliance with the 1997 NESC. The more accurate calculation method takes three times longer and is beyond the scope of a short article.
About David Young: Dave is an electric power consulting engineer and president of Young Engineering, Inc. of Wilmington, Delaware. Dave has been working with and teaching all aspects of the NESC and electric power distribution for over 36 years. Dave teaches the electric power curriculum at the University of Delaware and is a member of the NESC Interpretations Subcommittee and the Overhead Line Clearances Subcommittee. Dave is also an inspector member of the IAEI.
Posted By Michael Johnston,
Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
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There are two important areas regarding protection of property and persons. Appropriately, these requirements are located in Chapter two of theNEC,labeled, Wiring and Protection. This article will focus on ground-fault circuit-interrupter (GFCI) protection for persons and ground-fault protection of electrical equipment (GFP). One must understand that there are two main differences between these two forms of protection.
Ground-fault protection of equipment and ground-fault circuit interrupters for personnel are essential to safety. Often people in the electrical industry learn about the requirements for ground-fault protection in electrical service equipment by the "school of hard knocks” when a ground-fault in a service not equipped with a ground-fault protection device endures the forces of a destructive ground-fault leading to a total burndown of their electrical service, or equipment, to their facilities. These situations lead to questions: How are we protected? How is the electrical equipment protected?” Why is this protection required? Too late, it seems, some in the industry learn that injuries, death, property loss, and downtime for facilities are all situations that can result when the required ground-fault protection for equipment or ground-fault circuit interrupters are not installed as required by theNational Electrical Code.
"The purpose of thisCodeis the practical safeguarding of persons and property from hazards arising from the use of electricity. ThisCodecontains provisions necessary for safety. Compliance therewith and proper maintenance will result in an installation that is essentially free from hazards.” These principal purposes are so critical to the users of theCodethat they are set forth in the initial section of theCode.NFPA Standard 70 is a minimum electrical requirements safety standard, which simply means that one must, at the very least, comply with those minimum requirements. See Section 90-1(a) and (b) of theNEC.
For the protection of personnel
Article 100 of theNECdefines ground-fault circuit interrupter as, "A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the overcurrent protective device of the supply circuit.”
GFCI protection is required to provide protection against the hazards of electric shock and electrocution. The Underwriters Laboratories (UL) requirements for Class A ground-fault circuit interrupters is that the device will open (trip) when the continuous 60 cycle differential current exceeds 6 mA, but shall not trip at less than 4 mA. One can see by these values of current at which GFCIs operate that these types of protective devices are sensitive to low-level current leakage (see figure 1).
The GFCI is a current sensing device that, in basic terms, monitors the current balance in the ungrounded (hot) conductor and the neutral (grounded) conductor (see figure 2). If the current in either conductor changes by more than 6 mA, the GFCI will open. In other words, the GFCI monitors the current coming and going through the sensors. In a ground-fault condition, the current is seeking all paths back to the source. The device trips. So when one is troubled by the alleged nuisance tripping of a GFCI, chances are the device is just doing its job. One should troubleshoot the circuit, not defeat the GFCI to eliminate the alleged nuisance tripping.
The requirement for ground-fault circuit interrupters first appeared in Section 210-22(d) of the 1971National Electrical Code,which read, "For residential occupancies all 120-volt, single-phase, 15- and 20-ampere receptacles installed outdoors shall have approved ground-fault circuit-interrupter protection for personnel. The effective date of this requirement shall be January 1, 1973.” The 1971 edition of theNECalso required GFCI protection for all construction site receptacles rated at 15- and 20-amperes, effective January 1, 1974. The requirements set forth in Section 555-3 for marinas and boatyards stated that ground-fault protection for shore power receptacles "may be provided with GFCI protection,” which was permissive at that time. Those rules have since become mandatory requirements. See Section 555-3 of the 1999NEC.
The requirements in Section 680-6 for receptacle location and GFCI protection was, and still is, a mandatory requirement. As theCodedeveloped over the years, more and more additional requirements for GFCI protection were made mandatory. In the 1999 edition of theNEC,the requirements for GFCI protected circuits and receptacles are widespread and numerous. These devices have certainly contributed to the personnel safety and protection as required by theNEC.
For the protection of equipment
Now compare ground-fault protection (GFP) for equipment. Once again one must look to the definitions in Article 100, where ground-fault protection of equipment is defined as, "A system intended to provide protection of equipment from damaging line-to-ground fault currents by operating to cause a disconnecting means to open all ungrounded conductors of the faulted circuit. This protection is provided at current levels less than those required to protect conductors from damage through the operation of a supply circuit overcurrent device.”
First, consider the service equipment. Section 230-95 requires ground-fault protection for all solidly-grounded wye electrical services of more than 150 volts to ground, but not exceeding 600 volts phase-to-phase for each service disconnect rated 1000 amperes or more (see figure 3). These provisions do not apply to fire pumps or continuous industrial processes where a non-orderly shutdown would result in increased or additional hazards. The maximum operational current setting for these devices is 1200 amperes. The maximum time delay is one second for ground-fault currents equal to or greater than 3,000 amperes. These equipment protection requirements are a result of a history of destructive burndowns of electrical equipment operating at these voltage levels.
An electric arc generating a tremendous amount of heat is readily sustained at these higher voltage levels. Conductive ionized gases produced by these arcs contribute to the electrical explosion and can go from a phase-to-ground fault to a phase-to-phase short circuit and result in destructive magnetic and thermal forces that cause the equipment to literally melt down (see photos 1 and 2). TheNECdoes not require ground-fault protection for services at voltage levels less than 150 volts phase-to-ground; however, it is permitted to be installed on those services.
There are basically two types of ground-fault protection (GFP). The zero sequence type, which may have more than one form, and the residual type, sometimes referred to as a neutral ground strap type. Both are designed to protect downstream equipment from destructive arcing burndowns (see figures 7, 8, and 9). It should be pointed out here that ground-fault protection equipment will not protect equipment from line-to-ground faults on the line side of this ground-fault protective device.
Ground-fault protection of equipment first became a requirement in theNational Electrical Codein the 1971 edition of Section 230-95. The requirement for ground-fault protection in health care facility electrical feeders downstream of a GFP service disconnecting means was introduced into the 1975 edition of theNEC.The main reason for this second level of protection downstream is to localize a destructive ground-fault condition to the device in trouble and keep the continuity of service to the facility and prevent a total blackout condition at the facility. This feeder protection is required to be 100 percent selective so that where a ground-fault occurs downstream from the feeder overcurrent device, only the feeder overcurrent device will open and the service and feeder main will remain closed. The coordination is achieved by theCoderequiring a six cycle, or greater, separation between the tripping times of both levels. These ground-fault protective devices are required to be tested, in accordance with the instructions provided with the equipment for the particular type of ground-fault protection used, when first installed. This is a performance test that includes injecting a current and measuring current and time, and is not just the pressing of a test button on the equipment. Testing is also required for service and feeder ground-fault protection devices in other than health care facilities. A written record of these test results shall be made available to the authority having jurisdiction. See Sections 230-95(c) and 110-3(a) and (b).
Observance of minimum standard required
TheNational Electrical Codeis a minimum standard, so we need to do at least that much. With that in mind, one can conclude that GFP is required for all service disconnecting means at the voltage rating of over 150 volts phase-to-ground but not exceeding 600 volts phase-to-phase. Looking a bit further, if we had an electrical service rated at 1600 amperes and 480/277 volts and the "six disconnect” rule were being applied as allowed by Section 230-71, meaning six disconnects rated at 400 amperes each, there would be no requirements for GFP to be installed for the service; installation would be optional. Obviously the system could benefit if a ground-fault protective device were installed as a main disconnect for the service, but this would not be required by theNEC.Ground-fault protection for equipment is required by Section 215-10 for feeder disconnects, 230-95 for service disconnects and in health care facilities, 517-17 for second level protection and coordination where a service disconnect is a ground-fault protective device.
Making the comparison between ground-fault circuit interrupters and ground-fault protection of equipment by carefully using the definitions and rules set forth in theNational Electrical Code,one can easily see the differences between the two types of protection. Ground-fault protection of equipment protects equipment, and ground-fault circuit interrupters protect people. Both types are equally important in maintaining the spirit and content of theCode.We hope this helps clear up some of the confusion between the requirements and reasons for both types of ground-fault protection. These two topics are covered more extensively in theIAEI Soares Book on Grounding,7thEdition.
Read more by Michael Johnston
Posted By Philip Cox,
Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
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What are the guidelines for determining whether or not a person is a good electrical inspector? Qualifications for becoming an electrical inspector vary greatly. Some jurisdictions may not require any electrical training and experience to be hired as an inspector. Others require a minimum of journeyman or master electrician license, formal degrees in electrical engineering, or years of field experience. While there may not be an established list of qualifications one can use to determine if a person is qualified to be an electrical inspector, there are some characteristics which are very important for doing the job.
Commitment:The level of commitment an inspector has becomes evident within a short period of time. When a person either believes in the job nor is committed to its objectives, it can usually be recognized. The job of an electrical inspector is too important to be left in the hands of one who is interested only in putting in forty hours a week. The responsibility of helping provide for safe electrical installations for those living within the inspection jurisdiction is too serious to ignore. Fortunately, many dedicated electrical inspectors go beyond the required job responsibilities and give much needed service both to the public and to the industry. The demand upon an electrical inspector’s time is generally not limited to a 40-hour week. Most successful electrical code questions spend a lot of time after normal work hours teaching classes, giving presentations to groups, answering electrical code questions and doing other things to help improve his or her community.
Thirst for Knowledge:Generally speaking, the higher one climbs on the ladder of knowledge, the better one can see. The horizon is broadened and many things can be seen more clearly. It is refreshing to see inspectors who love to talk code at every opportunity, read extensively, attend educational forums and participate in other related electrical inspector activities. Training and skills necessary to be a good electrical inspector do not come quickly or easily. One must work hard to gain an acceptable level of expertise and be very diligent about staying proficient. The thirst for knowledge is a motivating force that drives many individuals to go beyond what is required and to do what is necessary in order to become the best they can be.
Positive Attitude:Approaching the responsibility of enforcing electrical safety rules with a positive attitude is beneficial for all affected parties. This is often reflected by the inspector projecting an image of working to verify compliance with established safety rules rather _han having a negative attitude of trying to find something wrong with a job. Listening to an inspector talk with a contractor or engineer provides a good insight into the attitude the person has in relation to the job. One main point that needs to kept in mind is that to see a job done correctly, the electrical inspector should workwithinstallers, designers and manufacturers’ representatives, etc., butforthe consumer or general public_
Fairness in Applying the Code:Rules should be interpreted and applied uniformly to all involved. The inspector is a type of law enforcement official and as such, has the responsibility of enforcing both the letter and intent of the adopted law. Those who make up their own rules, enforce provisions for which there is no established law or make decisions in direct conflict with adopted rules should seriously reconsider potential repercussions of those actions. There have been occasions when people have complained of unfair and unequal enforcement when, in fact, the work was not in conformance with the electrical code, and the inspector was simply doing a good job. In order to guard against problems in this area, inspectors should work very hard to ensure there is not even a hint of uneven enforcement.
Competency:Designers and installers have a greater level of confidence in the electrical inspector when they know he or she is very capable of inspecting a job, evaluating its compliance with safety code rules and making sound judgments on field conditions. The decisions inspectors often must make can dramatically impact the affected parties, and the responsibility for making those decisions is a heavy load to bear. This is one of the reasons an inspector must not only have an excellent knowledge of applicable code rules but also must understand the electrical system. Some people discount the importance of requiring inspectors to have a good working knowledge of the fundamental principles of electricity, but that knowledge is necessary for understanding how a system operates and how it will be affected by specific conditions. Understanding installation methods is also important. Unless one has worked in the trade, it is more difficult to fully comprehend field situations and to evaluate them according to written rules. Without field experience, it is more difficult to see the whole picture.
Consistency:Consistency interpreting and applying electrical code rules is very significant to users of the code. Whenever an installer does electrical work within an inspection jurisdiction, rules should be applied the same to all jobs regardless of which inspector looks at the installation. This is a serious challenge for chief electrical inspectors and supervisors. When an inspection department consists of a large number of inspectors, establishing and maintaining a common level of understanding of code rules and enforcement procedures is difficult. Inconsistencies in situations sometimes occur because of rules in theNational Electrical Code®that are not precise in nature. An example of this is Exception No. 5 to Section 230-2(a). Exception No. 5 permits more than one service to a building of large area where special permission is given. For consistency, both the definition of a "building of large area” and the use of "special permission” should be understood and used by the authority having jurisdiction. If it is left up to individual inspectors to interpret this rule without any established policy or guidance, undesirable inconsistencies could easily occur. Without basic guidelines to determine what constitutes a large building, individual inspectors may have widely different opinions on the matter. It should be clear to both inspectors and installers as to how a rule is applied every time a job is done.
Good Judgment:Every inspector has to make judgments in the field because of conditions or situations that do not clearly fall under a code rule. The inspector should consider all aspects of the situation before making any decision on this type of matter. Consideration should be given to how the decision impacts on the job being inspected as well as other jobs. In addition, how it will affect all parties involved and how it relates to the purpose of the NEC® in "… the practical safeguarding of persons and property from hazards arising from the use of electricity” is important. There is no substitute for an inspector’s good judgment in evaluating electrical installations and applying code rules. Section 90-4 of the NEC® provides needed flexibility for inspectors. This provision assigns the responsibility of interpreting the code, approving materials and equipment, granting special permission and allowing alternate methods to the authority having jurisdiction. Every job does not neatly fit into conditions described by the code. Neither is it practical to write code that will cover every variation that could possibly occur. When the flexibility covered in Section 90-4 is used, it should be done with proper regard for the gravity of the responsibility.
Common Sense Approach:There are those who apply electrical code rules strictly by the letter and there are those who enforce both by the letter and by intent. This may appear confusing to some, but inspectors need to understand the reasons behind safety rules and to enforce them in a logical manner. Rules properly interpreted and applied in a logical manner will provide a good level of safety. An example is the application of the term "wrenchtight” where following rules for bonding. The rule does not specify the type of wrench, the amount of pressure to be applied, or any specific details or conditions. To skilled installers and inspectors, this term is readily understood as to its intent. Qualified inspectors who understand both the letter and intent of the code are familiar with electrical products and installations methods, know the difference between "wrenchtight” as applied to a run of 3/4-inch conduit as that for a run of 6-inch conduit. If one interpreted wrenchtight to allow the use of any type of wrench, the selected tool may very well be inadequate to do the job. A wrench used to tighten a threaded coupling on a small diameter raceway may not be appropriate to tighten a coupling on a 4-inch conduit even though the wrench may be adjustable to grip the larger conduit. The purpose and intent of the code are very much a part of the enforcement of electrical safety rules.
Dependability:One characteristic that most inspectors demonstrate is dependability. This involves keeping one’s word and being reliable. In turn, because inspectors traditionally feel strongly in this area, they expect those they associate with to live by the same standards. During visits to some inspection jurisdictions, it became readily evident in many cases that not only contractors but also the inspector’s superiors had a high level of trust in the electrical inspector’s ability and conduct. They apparently were very confidant that the inspector would do what was needed, and they could depend upon it being done in an acceptable fashion.
The Ability to Listen:Listening properly can solve a lot of problems and help eliminate misunderstandings. It is a learned skill in most cases. When people enter into conversations with their minds made up, or don’t want to hear what is being said, there is little chance of solving problems. Being able to effectively communicate is a skill vital to a professional electrical inspector. The inspector is frequently involved in discussions with manufacturers, designers, installers and property owners. In order to understand specific needs or positions taken by others, one should listen to what is said, have an open mind on the matter, digest that information and evaluate the situation without bias.
The Ability to Work With People:One can be the best technically qualified person available and still be a relative failure as an electrical inspector. Whether one realizes it or not, the inspector must be able to communicate effectively with people in order to succeed. In reality, it is one of the most important skills for an inspector. It is difficult to deal with an individual during a hostile confrontation. It takes a lot of patience and understanding to effectively work out this type of situation. One can expect these situations to arise from time to time because of the very nature of law enforcement. Misunderstandings, differences of opinions and many other factors result in conflicts with inspectors. The effectiveness of the inspector can depend a great deal upon his or her ability to solve these problems. Many such confrontations can be resolved and the involved parties reach a cordial understanding. Unfortunately, if these confrontations can’t be resolved, the inspector may end up with an adversarial form of enforcement.
A combination of these traits and associated technical training and experience should help produce an individual highly qualified in the profession of electrical inspection. Being truly professional as an electrical inspector in both conduct and performance not only brings greater respect to the industry but makes it easier for other members of the electrical community to do their jobs.
Read more by Philip Cox
Posted By Leslie Stoch,
Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
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The 1998 Canadian Electrical Code has made some more changes in the rules for underground conductor ampacities. Rule 4-004 contains the requirements for maximum ampacities for single and multiple copper and aluminum 90°C conductors, direct buried and for duct banks underground, in Appendix D, Tables D8A to D16B and Appendix B, Configurations ketches B4-1 to B4-4.
Each of the Table D8A to D16B refer to one of the wiring configurations and dimensions given in sketches B4-1 to B4-4 for multiple and single conductors underground. These ampacities have been pre-calculated using IEEE Standard, Standard Power Cable Ampacity Tables, IEEE 835.
As you ponder this wealth of information, you will find that the underground conductor ampacity tables in Appendix D fall into two separate groups as explained in the notes following each of the tables.
The A group of tables, Tables D8A to D16A provide maximum 90°C underground conductor ampacities for:
- Either continuous or noncontinuous loads when connected to electrical equipment other than a service box, fusible switch, circuit-breaker or panelboard (any piece of electrical equipment which doesn’t contain fuses or breakers); or
- Noncontinuous loads connected to a service box, fusible switch, circuit-breaker or panelboard.
The B group of tables, Tables D8B to D16B provide maximum underground conductor ampacities for continuous loads connected to a service box, fusible switch, circuit-breaker or panelboard (any electrical equipment containing overcurrent protection).
Continuous loads are defined in Rule 8-104. A load is considered continuous when it persists for more than one hour in a two-hour period for loads up to 225 amperes.
At first glance, the arrangement of the tables and configuration sketches in the code appears quite complicated. Everything including all of the notes should be read carefully in order to apply this information correctly. You will also discover that some of the detail columns in these tables contain two sets of conductor ampacities, for continuously loaded equipment operating at 100% and 90% capacity.
I have drawn up the following tables in the hope that they will help to eliminate some of the complexities encountered in applying this part of Rule 4-004.
Table 1–Underground 90°C Conductor Ampacities
Conductor Ampacity Table:
* For continuous or noncontinuous loads other than connected to a service box, fusible switch, circuit-breaker or panel board; or
* For noncontinuous loads connected to service box, fusible switch, circuit breaker or panelboard.
|D8A||B4-1 Direct Buried||Single, Copper|
|D9A||B4-2 Duck Bank||Single, Copper|
|D10A||B4-1 Direct Buried||Single, Aluminum|
|D11A||B4-2 Duck Bank||Single, Aluminum|
|D13A||B4-3 Direct Buried||*Multiple, Cooper|
|D14A||B4-3 Direct Buried||*Multiple, Aluminum|
|D15A||B4-4 Duck Bank||*Multiple, Cooper|
|D16A||B4-4 Duck Bank||*Multiple, Aluminum|
Table 2–Underground 90°C Conductor Ampacities
Conductor Ampacity Table:
* For continuous loads connected to a service box, fusible switch, circuit-breaker or panel board
|D8B||B4-1 Direct Buried||Single, Copper|
|D9B||B4-2 Duck Bank||Single, Copper|
|D10B||B4-1 Direct Buried||Single, Aluminum|
|D11B||B4-2 Duck Bank||Single, Aluminum|
|D13B||B4-3 Direct Buried||*Multiple, Cooper|
|D14B||B4-3 Direct Buried||*Multiple, Aluminum|
|D15B||B4-4 Duck Bank||*Multiple, Cooper|
|D16B||B4-4 Duck Bank||*Multiple, Aluminum|
*Note–”Multiple” includes up to four conductors in a cable, single conductors in contact and single conductors multiplexed underground.
As in past articles, an exact interpretation of any of the above information should be obtained from the electrical inspection authority in each province or territory as applicable.
Read more by Leslie Stoch
Posted By Philip Cox,
Wednesday, September 01, 1999
Updated: Wednesday, August 29, 2012
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It’s Code proposal time again! The closing date to submit proposed Code changes for the 2002 National Electrical Code® cycle is drawing near and many people are talking of changes they want to see made. November 5, 1999, at 5:00 p.m. Eastern Standard Time, is the deadline for proposals to be received by the National Fire Protection Association. More than 4,200 proposals were submitted for the 1999 Code. It is likely that the number submitted for the 2002 NEC will equal or exceed that amount. Submitters of proposals can significantly help code panel members and the process in general by closely following NFPA guidelines, specifically the NEC Style Manual.
The 1999 Code includes many changes designed to make the NEC easier to use. AnNECTechnical Correlating Committee task group that was established to review the Code and make recommendations on how to make it more usable worked hard on that project. Within the scope of that task group, several subtask groups were established to address specific concerns or parts of the Code. Much of what was done by the task group and its sub-task groups was presented to the NEC code making panels and was included as part of the 1999 edition of theCode. The end result of that work was that many good steps were taken to keep theCodein step with the needs of the industry and to make it more user friendly.
Provisions of former Article 710, entitled "Over 600 Volts, Nominal, General,” were relocated to other articles. Article 250 which covers grounding was restructured. Many exceptions were revised and relocated as positive text. It takes time and effort to become familiar with these and other changes that were made. In order for one to be as familiar with the 1999 NEC as he or she is with the 1996 version, the new location of some material, new section numbers, and new material must be learned. A significant step was taken to make the transition from Article 250 as found in the 1996 NEC to the restructured Article 250 in the 1999. It is a cross-reference located in Appendix E of the NEC. Appendix E includes cross references from the 1996 NEC to the 1999, and from the 1999 NEC to the 1996.
Sometimes when adjustments are made in a long-standing, well established format, there is opposition to those changes. It is somewhat true in this case where changes have been made to make theNECmore user friendly. Where one has become familiar with the location and identification of a particular Code rule, there is a natural resistance to changing its location or identification. However, it is expected that within a short period of time, not only willCodeusers become familiar with the changes, those revisions will be welcomed.
Users of theCodehave had approximately a year to become familiar with changes in the 1999 NEC. This has given them a chance to gain first-hand knowledge of how the new or revised rules really work in the field. This knowledge and experience is an important part in learning whether or not any modification in Code rules needs to be made and can now be used to help make the 2002 NEC even better.
Submitting good proposals requires considerable thought and preparation. The important step of submitting good proposals is sometimes a difficult one. One of the critical parts of the process is the composition of wording or recommended text. It is not always an easy task to write language for a document that contains technical provisions but must also be suitable for adoption as a law or ordinance. Not only should the wording be clear, it should also effectively convey its intended purpose. Recommended changes should be well thought out as to how they apply and how they may affect other provisions in the Code. It is not appropriate for a submitter to recommend that the code making panel develop wording to address his or her concerns.
Substantiation submitted to support a proposal needs to include technically sound reasons as to why the change should be made. Code panel members look for accurate and valid information upon which they can evaluate the merits of proposals, and they try hard to consider all aspects of each proposals, so that it will do exactly what it is intended to do. Ideally, the substantiation should include adequate information in a form that panel members can readily extract needed data. Overwhelming panel members with large volumes of material that include minimal amounts of relevant information puts an unnecessary burden on them.
Occasionally one hears individuals expressing their opinions on how frequently the Code should be changed. Some want the Code cycle longer and some want it shorter. In reality, the three-year cycle presently used seems to work very well. When the need for a Code change is recognized, the time needed to get it to the point where it can be adopted into law is considerable. In order for all affected participants in the Code process, including the public, to get involved, enough time must be provided to allow them to do so. Rushing the process would not allow participants to evaluate proposed changes wisely and to respond to them effectively. Committee members responsible for acting on proposed changes need sufficient time to review and discuss them in order for good decisions to be made. The existing NEC process does provide the needed time for review and action on proposed changes. It appears that an acceptable balance has been reached between the need to implement new provisions to stay abreast of change and the need to have a set of rules in place over a reasonable length of time.
A note of interest is that information is included as part of the 2002 NEC proposal form that should help those involved in the Code change process. The space on the form where the proposed new or revised wording is to be placed now includes a note that states, "Proposed text should be in legislative format: i.e., use underscore to denote wording to be inserted (inserted wording) and strike-through to denote wording to be deleted (deleted wording).” This change will help panel members more clearly identify what the submitter intends without having to do a word-for-word comparison between the proposed wording and the existing Code text. It will also be better for those who use the Report on Proposal (ROP) and Report on Comments (ROC) as the proposed change may be readily identified.
Each time the NEC is revised, it provides an opportunity for Code users to contribute to the improvement of that document. Let’s all do our part to make the 2002 NEC the best yet.
Read more by Philip Cox
Posted By Daniel Langlois,
Thursday, July 01, 1999
Updated: Wednesday, August 29, 2012
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On August 1, 1998, at approximately 8 p.m., a fire occurred at a single storey, 2,400 sq. ft. log home in a rural area of Stouffville, Ontario. The log home was of main floor open concept design with three bedrooms and three bathrooms. It was originally built in 1964 making the home 34 years old at the time of the fire. The fire originated at the front area of the home directly above the front porch at the roof level. The fire traveled upwards along the frame of the roof line resulting in significant fire damage to half the roof and subsequent heat, smoke, and water damage throughout the home. The damages were estimated at $160,000 to the building and $40,000 damage to the contents.
For purposes of this article, we will refer to the original owner of the subject log home as Ms. A and the owner of the home at the time of the fire as Mr. B.
In late 1992, Ms. A faced some major roof repairs resulting from ice damage. The poorly insulated ceiling caused considerable snow melt on the roof and ice buildup at roof edges, resulting in damage to the roof overhangs. The damage was most severe over the large overhanging front porch of the home.
In October, 1992, Ms. A hired a contractor to replace the shingles over the damaged roof section at the front of the house, and a second contractor to install a 200 foot long, 120 VAC de-icing heating cable on the roof to prevent a recurrence. The heating cable was installed along the overhang and the eaves trough of the roof line at the front of the home. The front of the home faced southwest. No heating cable was installed on either sides or back of the house. A duplex receptacle was installed in the porch area nearest to the eaves trough, and a separate outdoor type switch was installed near the front door to control the receptacle. An additional duplex receptacle, also controlled by the switch, was installed near the front door. While the switch was not identified as to its function, surface mounted conduit used to route the wiring to both receptacles made it quite obvious that the switch controlled these receptacles. The power supply cord for the heating cable was plugged into the receptacle nearest the eaves trough. This particular circuit, protected by a 15 Amp circuit breaker, was shared with some auxiliary lighting exterior to the house.
In June, 1997, Mr. B, the owner of the home at the time of the fire, took possession. Mr. B bought the property and the home for its land potential and recognized that the log home needed some work.
Although the solid log home was well built 34 years ago, it had seen nothing more than obligatory repairs since then. Mr. B and his wife were aware of the outside circuit since it was used to control some external lights which were plugged into the receptacle near the door, but which they had chosen not to use. Therefore, the outdoor switch was usually left in the "off” position. Both Mr. B and his wife were unaware of the heating cable installed on their roof, even though the plug for the heating cable was clearly visible in the receptacle near the eaves trough. The manufacturer’s identification label was also attached to the cable power cord at the plug.
In late July, 1998, Mr. B hosted a children’s theater camp at his home. In preparation for the event, he needed power outside of the home to operate a portable hot tub (approximately 12 amp load). On Friday July 24th, 1998, Mr. B. plugged the hot tub into the receptacle near the door and turned it on, energizing the circuit. Since the heating cable was still connected to the second receptacle controlled by the switch, the total load on the circuit then exceeded 15 Amps. This caused the circuit breaker to trip about 20 minutes later. Mr. B. then removed the hot tub’s power supply cord from the receptacle and inserted it into another inside circuit via an electrical extension cord. Mr. B. also then reset the tripped circuit breaker.
The hot tub began to operate again without difficulty. However, Mr. B. did not return the switch to the "off” position, resulting in the heating cable remaining energized. The days following July 24, 1998, were hot and dry.
On August 1, 1998, the day after the conclusion of the children’s theater camp hosted by Mr. B., the fire occurred. At approximately 8:15 p.m., Mr. B was outside, approximately 150 feet southwest of the house. He became alarmed by a nauseous smell that became more intense with time. It was described as "a smell similar to burning tires.” Mr. B investigated to see if the odour was emanating from his home. At that time, Mr. B’s daughter came out of the front door of the house. When Mr. B turned to talk to his daughter, he noticed smoke coming from the outside of the roof above the front porch eaves trough and down spout. He ran back toward the house and with a closer view, noticed a small flame above the eaves trough and smoke from a radius of approximately two feet coming from leaves on the roof. Within a couple of minutes, Mr. B brought a bucket of water and his garden hose to the front porch. The roof was too high for him to reach and suppress the fire. He threw the running hose onto the roof and proceeded to call 911, advising the fire department of a small fire on his roof that he was trying to bring under control.
During an interview with Mr. B, he explained the fire continued to spread in spite of the running water from the garden hose (There wasn’t much water pressure). Mr. B climbed up the T.V. tower at the northeast side of the home, and proceeded to the area of the fire. He noticed burnt and smoldering leaves one to two feet from the readily apparent flame. The leaves did not appear to be burning. Mr. B directed the garden hose to the base of the flame which appeared to have originated from inside the eaves trough, however the flame and the risk of falling off the roof, kept him at a distance.
Mr. B called 911 a second time when it was apparent the fire was out of control. The flames extended 4-6 inches above the eaves trough. Minutes later, the flame burned through the fascia board of the roof and traveled into the roof and attic space. Mr. B could hear the draft of air being drawn by the fire into the roof and attic and observed smoke coming out of a roof vent nearby. He quickly alerted his family to evacuate the building. Mrs. B shut off the main power switch of the electrical breaker panel on her way out. The master bedroom, immediately adjacent to where the fire started, also caught fire. The newly installed security system, including smoke and rate of rise heat detectors, began to alarm shortly after the bedroom caught fire. By the time the fire department arrived, the roof was fully engulfed in flames. It took more than 30 minutes to put the fire out. There were no injuries.
Investigation based on fire damage and burn patterns revealed the area of fire origin to be at the roof level, adjacent to the eaves trough, directly above the front porch.
The eaves trough was made of metal with a plastic leaf screen. The top of the roof over the eaves trough, where the heating cable was installed, was blanketed with a few inches of dry leaves and small broken twigs and branches. The fire traveled upwards along the frame of the roof line. However, in addition to the fire damage to the roof, the master bedroom and adjacent bedrooms sustained severe heat and fire damage as a result of drop burning.
The heating cable was installed along a 38 foot length of the front roof line, southwest side of the home. There was no heating cable installed on either sides or back of the home. Approximately 29 feet of the front, southwest roof line was occupied by heating cable which was intact. This 29 foot length of roof occupied approximately 83 feet of heating cable which appeared to be properly installed. However, not including the length of heating cable running through the down spout, the remaining 9 foot length of roof (38-29) in the area of fire origin, occupied the remaining 78 feet of heating cable. The entire length of heating cable measured 200 feet long. There was no evidence to suggest that the remaining 78 feet of heating cable on the roof, in the area of fire origin, was correctly installed. Examination of and sifting through fire debris revealed no evidence of mounting clips used for securing the heating cable to the roof shingles. Furthermore, there was only 9 feet of roof line remaining to install 78 feet of heating cable. Based on the roof construction, it appeared the entire remaining length of heating cable was applied to the 9 foot section of roof directly above the front porch, an amount of cable in excess of what should have been applied to this area.
Electrical continuity testing confirmed that the duplex receptacle the heating cable was plugged into was operated by the remote switch mounted on the exterior wall. There was no identification on the switch indicating it operated heating cable installed on the roof. In an interview with Mr. B, he confirmed the exterior wall mounted switch was on and the heating cable was plugged into the duplex receptacle at the time of the fire.
CONCLUSION / RECOMMENDATIONS
The installation of the heating cable was not in compliance with the manufacturer’s instructions, however it was used on a metal eaves trough as recommended.
Information provided by the owner revealed the heating cable installation was unknown by him as he was not notified of the heating cable by the previous owner. Not knowing what the problem was at the time the circuit breaker tripped while operating the portable hot tub, Mr. B re-set the circuit breaker and plugged in his hot tub to another receptacle. The switch he turned on, remained on for eight days, effectively energizing the heating cable for the eight days prior to the fire. If the owner had known what the switch was operating, he likely wouldn’t have left it on. The combination of a dry warm summer, and dry combustible leaves and branches blanketed over the electrical heating cable, coupled with the poor installation and the accidental energizing of the heating cable switch, are the most likely cause of this fire.
As a result of the subject fire, the CSA International, Audits and Investigations Department at the Canadian Standards Association reviewed its computer data base and confirmed this type of incident was the first of its kind. However, a recommendation was forwarded to the sub-committee responsible for CSA International Standard CAN/CSA-C22.2 No. 130.2-93 – "Heat Cable Systems for Use in Other Than Industrial Establishments” to review this information and consider implementing amendments to the referenced standard to prevent a fire recurrence.
The recommendation is currently being reviewed by the sub-committee for action as deemed necessary.
Use and Care of Heating Cables
At least part of the reason for this fire was the improper use and care of the heating cable installation. The build-up of leaves and twigs on the heating cable thermally insulated it and may have led to overheating. Heating cable manufacturers recommend that cables be checked annually to ensure they are in proper working order and capable of performing as expected. Certainly, leaves covering a roof de-icing cable will impede the proper operation of the cables, as heat from the cables will be insulated by the debris, thereby allowing ice to form above the leaves.
Similarly, pipe freeze protection cables must be inspected annually for proper operation and installation. If insulation surrounding pipe heating cables has deteriorated, or if the cable has been damaged in any way, the installation should be disconnected.
If a heating cable, or any other electrical device, is connected to a ground fault protected circuit/receptacle, and the ground fault device trips when the heating cable is connected, the heating cable likely has been damaged and must be removed from service.
It is also important for new homeowners to fully familiarize themselves, not only with any heating cables, but with all electrical devices which may be installed on or in the home. If a new homeowner is unsure about any such electrical devices, a qualified electrician should be consulted for appropriate advice. Similarly, the previous homeowner, or the homeowner’s agent/seller, should also ensure the operation of any non-standard electrical items, such as heating cables and saunas, is appropriately communicated to the new homeowner.
Ground Fault Protection for Heating Cables
The 1998 version of the Canadian Electrical Code Part 1 (CEC), Rule 62-300, provides requirements for ground fault protection for fixed heating cable installations, including those heating systems for melting snow and ice on roofs. In addition, most manufacturers strongly recommend the use of such a device with their products. Most outdoor receptacles on houses constructed since about 1980 contain ground fault devices, which will help to prevent damaged heating cables from being energized. Note, however, that overheating caused by improper heating cable maintenance will not necessarily be prevented by ground fault protection devices, as likely evidenced by this fire. Furthermore, homes built prior to about 1980 may not contain any ground fault protection devices. For these situations, some heating cable companies offer a convenient, economical plug-in ground fault protection device, which can easily be connected by the homeowner (just plugs in to existing receptacle).
Electrical Fire Reporting System
The forwarding of information to CSA International regarding fires involving CSA certified electrical products aids in the facilitation of improving Canadian Standards. Even though fires are reported to a fire department or fire marshal, and the most likely cause of the fire identified and documented, these reports do not always get forwarded to CSA International. This was the case with the fire described in this article, where CSA International were only notified of this fire by a letter from the homeowner written directly to CSA International concerned about how this fire could have happened. It is important that all fires involving electrical products be reported to CSA International’s Audits and Investigations Department. This will ensure that incidents such as this one are properly investigated and documented in the Audits and Investigations computer data base.
Read more by Daniel Langlois
Posted By J. Philip Simmons,
Thursday, July 01, 1999
Updated: Wednesday, August 29, 2012
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Reviewing and understanding the information on equipment nameplates is important in properly evaluating an air-conditioner or heat pump installation. It also simplifies the selection of the correct branch-circuit wire, overcurrent protection and disconnect sizes. Most all the information needed by the installer can be found by reference to the units data name-plate. The following is an example of the type of information found on the nameplate for a residential central air-conditioning unit.
|Model No. RAKA-036JAZ MFD. 03/94Serial No. 5340 F0994 8995 Outdoor Use |
Volts 208/230 Phase 1 Hertz 60
Compressor R.L.A. 18.0/18.0 L.R.A. 96
Outdoor Fan Motor F.L.A. 1.3 HP. 1/5
Min. supply circuit ampacity 24/24 amp
Max. fuse or ckt. brk. size* 40/40 amp
Min. fuse or ckt. brk. size* 30/30 amp
Design pressure high 300 PSIG
Design pressure low 150 PSIG
Outdoor Units Factory Charge 77 OZ. R22
Total System Charge OZ. R22
See instructions inside access panel
*HACR type breaker for U.S.A.
Air-conditioning and heat pump equipment have a hermetic motor-compressor and are not treated the same as a conventional electric motor. The hermetic motor-compressor operates in the refrigerant environment and does not have a horsepower or full-load current rating such as a standard motor. Special terms are used to provide the necessary information to properly install wiring for this equipment.
One of the terms noted on the above nameplate is "Compressor R.L.A.” This is the Rated-Load Amperes or Rated-Load Current for the motor-compressor and is established by the manufacturer of the equipment. The rated-load current is the current resulting when the motor-compressor is operated at the rated load, rated voltage and rated frequency of the equipment it serves. This value is used in calculating the minimum circuit ampacity and overcurrent protective device ratings specified on the nameplate for this equipment. The RLA for our example is 18.0 amperes.
Another term noted on the air-conditioner nameplate is "Compressor L.R.A.” This term represents Locked-Rotor Amperes and is the maximum current flowing to the motor when it is in a locked, or not turning, condition. This value is necessary to ensure that the air-conditioner disconnecting means and controller have proper interrupting capacities. The LRA for our example is 96 amperes.
The term "Min. supply circuit ampacity,” or similar wording, is the minimum circuit ampacity required to select conductor size and switch rating for the unit. This value is determined by the following formula:
Minimum Circuit Ampacity = (RLA x 1.25) + Other Loads
The term "Max fuse or ckt brk size” indicates the maximum fuse or circuit breaker size permitted. The manufacturer has selected a rating that will permit the motor to start as well as provide overcurrent protection. The term "Min fuse or ckt brk fuse” rating indicates the smallest rating that will allow the motor to start.
The Rated Load Amperes (RLA) is used in calculating the maximum overcurrent protection permitted for the unit. Later in this chapter we will include a calculation for the unit covered by the above nameplate.
In some cases, the equipment is also marked with a "Branch-Circuit Selection Current” (BCSC). This is a value in amperes that must be used instead of the rated-load current for determining ratings of motor branch-circuit conductors, disconnecting means, controllers, and branch-circuit short-circuit and ground-fault protective devices whenever the running overload protective device permits a sustained current greater than the rated-load current. The value of branch-circuit selection current will always be equal to or greater than the marked rated-load current. Since the air-conditioning nameplate included in this chapter does not include a branch-circuit selection current, the rated-load current, or in this case RLA, is used for the calculations. SeeNECSections 440-2 and 440-4.
Note that the nameplate also references "HACR type breaker.” The marking "HACR” means Heating, Air Conditioning and Refrigeration. This marking means the circuit breaker has been tested and found suitable for use on this equipment including providing adequate protection for the smaller motor of the group, which is the outdoor fan. Circuit breakers used to supply this unit must be marked "HACR.”
Most circuit breakers manufactured today are HACR rated. This marking is common on most circuit breakers manufactured today but may not be present on older breakers located in existing equipment.
Remember thatNECSection 110-3(b) requires compliance with nameplate markings. If the nameplate specifies fuses only, substitution of even a HACR rated breaker marked only for fuses or if the breakers must be of the HACR type, failure to comply with the marking is a violation of this section. Generally the manufacturer offers a choice of either fuses or HACR rated circuit breakers. Check the nameplate information to make sure.
The required ampacity of branch-circuit conductors and rating of equipment for a hermetic refrigerant motor-compressor are based upon marking found on the unit nameplate giving the rated-load current. This information is necessary to determine the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection. SeeNECSection 440-6(b).
Branch-Circuit Conductor Size
For a typical air-conditioning unit or heat pump outdoor unit which has a motor-compressor and additional load(s), such as a fan motor, the conductors supplying this equipment must have an ampacity not less than 125 percent of either the rated-load or branch-circuit selection current, whichever is larger, plus the full-load current of the fan motor. For the example nameplate, this value is:
(RLA x 1.25) + Fan Motor FLA
18.0 x 1.25 + 1.3 = 23.8 amperes
This value rounded up to 24 is the "MIN SUPPLY CIRCUIT AMPACITY” stated on the nameplate and is the required minimum ampacity of the branch-circuit conductors selected fromNECTable 310-16. Since the equipment nameplate includes this value, it is not necessary for the installer or inspector to perform this calculation.
NEC Table 310-16 shows that a No. 12 Type TW, THW or THWN copper conductor can safely carry 25 amperes continuously where operated in an ambient temperature not exceeding 86°F. Where the temperature in an attic or on the roof reaches temperatures in excess of this figure, ampacity correction factors listed inNECTable 310-16 must be applied, which will make it necessary to increase the size of the conductors to compensate for the rise in temperature.
NEC Section 240-3(d) requires that the "overcurrent protection” must not exceed 20 amperes for a No. 12 conductor unless otherwise specifically permitted in Sections 240-3(e) through (g). Section 240-3(g) permits air conditioning circuit conductors to be protected in accord with Parts C and F of Article 440. Section 440-21 specifically states that the provisions of Part C are "in addition to or amendatory of the provisions of Article 240.” For example, the air conditioner nameplate marking indicates a "Maximum Fuse or Circuit Breaker Size” of 40 amperes. The minimum supply circuit ampacity is 24-amperes. In this case the nameplate on the air conditioner is marked for a Maximum Fuse or Circuit Breaker Size of 40 amperes, which will satisfy the requirements. Consequently, if no derating for ambient temperature is required, No. 12 conductors having an ampacity of 25 amperes are acceptable to supply this unit.
The concept for protection where a No. 12 copper wire with an ampacity of 25 amperes is permitted to have overcurrent protection of 40 amperes is as follows:
1. The 40-ampere fuse or circuit breaker at the origination of the circuit will protect the conductors from short circuit [ungrounded (hot) conductors which fault together, line-to-line] and ground fault [ungrounded (hot) conductor(s) which fault to the equipment grounding conductor or grounded equipment].
2. The conductor is protected from overload by the running overcurrent device usually contained in the motor controller.
The combination of the two protection elements provides the overcurrent protection for safety.
Branch-Circuit Maximum Overcurrent Protection Rating
Paragraph 36.15 of the UL Standard 1995Heating and Cooling Equipment requires the maximum current rating of the branch-circuit overcurrent protection be based on:
(RLA x 2.25) + Other Loads (Fan Motor FLA)
18.0 x 2.25 + 1.3 = 41.8 amperes
If this calculated value is not a standard current rating of an overcurrent protective device, paragraph 36.16 of UL 1995 requires use of the next lower standard rating. In this example, the next lower standard device is 40 amperes as stated on the nameplate. Note here again it is not necessary to actually perform this calculation. It has been done, and the information is included on the units nameplate.
If the nameplate includes a "MINIMUM FUSE OR CIRCUIT BREAKER SIZE,” thenNECSection 110-3(b) would also require compliance with that marking in addition to the marking of the "MAXIMUM FUSE OR CIRCUIT BREAKER SIZE.”
Disconnecting Means Rating
Where the air-conditioning or heat pump compressor unit consists of a hermetic refrigerant motor-compressor(s) in combination with other loads, such as the fan motor, the horsepower rating of the disconnecting means is based on the summation of all currents at rated-load condition and also at locked-rotor condition.
In the example used in this chapter, the 18.0 ampere.0-ampere RLA of the compressor motor is added to the 1.3 ampere FLA of the fan motor. The total of 19.3 amperes is then considered to be the equivalent full-load current for the combined load. According toNECTable 430-148, the full-load current rating of a 230-volt, single-phase, 3 horsepower motor is 17 amperes, while the full-load current rating of a 230-volt, single-phase, 5-horsepower motor is 28 amperes. Consequently, since the equivalent full-load current of the example A/C unit is 19.3 amperes, we must use the next higher rating, and the disconnect switch must have a minimum of a 5-horsepower, 230-volt, single-phase rating.
The ampere rating of the disconnecting means must also be a least 115 percent of the sum of all currents at rated-load condition. This minimum rating would then be 115 percent x
19.3 amperes = 22.3 amperes. If the disconnecting means includes or serves as the branch-circuit overcurrent protection for the unit, the rating required for the overcurrent device, rather than this minimum rating, would generally be the determining factor in sizing the disconnecting means. A fused disconnect switch containing either the maximum or minimum sizes of fuses listed on the nameplate would exceed this 115 percent minimum requirement. If an unfused disconnect switch is used as the disconnecting means, however, then this 115 percent rating and the horsepower rating would establish the minimum switch rating.
There is one other consideration in establishing the correct size of the disconnecting means serving the air- conditioning unit. The disconnecting means rating must also be based on currents at locked-rotor condition. Refer to NEC Table 430151A for the conversion of locked-rotor current to horsepower. In our example, the nameplate indicates that the motor-compressor LRA is 96 amperes. Since the nameplate does not give a LRA for the fan motor, we assume it to be six times the FLA or 6 x 1.3 amperes = 7.8 amperes. Adding this to the motor-compressor LRA of 96 amperes gives us an equivalent LRA for the combined load of 103.8 amperes. Again referring toNECTable 430-151, we find that for a single-phase, 230-volt motor with a 103.8 amperes motor locked-rotor current, the disconnect switch should be based on a 5-horsepower rating. SeeNECSection 440-12.
Disconnecting Means Location
NEC Section 440-14 covers the required location of a disconnecting means for an air-conditioner or heat pump compressor. This rule specifically points out that the disconnecting means is required to be located within sight from and readily accessible from the air-conditioning equipment. The following twoNECArticle 100 definitions must be clearly understood.
"In Sight From (Within Sight From, Within Sight)”: Where thisCodespecifies that one equipment shall be "in sight from,” "within sight from,” or "within sight,” etc., of another equipment, the specified equipment is to be visible and not more than 50 ft. distant from the other.”N
"Accessible, Readily: (Readily accessible.)”: Capable of being reached quickly for operation, renewal, or inspections, without requiring those to whom ready access is requisite to climb over or remove obstacles or to resort to portable ladders, chairs, etc.”N
This disconnecting means is permitted to be installed on or within the air-conditioning equipment. An air-conditioner or heat pump compressor is generally located on a concrete pad located at the outside of a one- or two-family dwelling. It may also be located under the house in a crawl space or on the roof. It is important to remember that the definitions ofwithin sight and readily accessible have a significant meaning in this application.
The disconnecting means is usually located next to the air-conditioning unit and is, therefore, considered as being readily accessible since the proper working space should be provided in accordance with NEC Section 110-26.
However, where the disconnecting means is located "behind or above” the air conditioning unit, accessibility to the disconnect will be hindered and will be in violation of the working space requirements of NEC Section 110-26(a). This type of installation is incorrect and still remains as a very common problem in the field.
The purpose of the required disconnecting means is to provide a "ready and visible means” of disconnect for the person who will service or repair the equipment. The requirements of NEC Article 430, where a "locking type” of disconnecting means may be permitted out-of-sight of the motor, are not applicable for this equipment.
Note: A disconnecting means as described above is not required for cord- and plug-connected equipment such as room air conditioners.
Working space about electrical equipment that is likely to "require examination, adjustment, servicing, or maintenance while energized”Nmust be provided in accordance with Table 110-26(a). The working clearance must be in the direction of access to the equipment, or the part of the equipment, that is likely to be worked on while there are live exposed parts. It is very common for service persons to examine or test this equipment while it is energized.
Generally, this working space is 30 inches wide and 36 inches deep. Compliance with this working space rule requires that consideration be given to providing safe access at the time the equipment is being installed. Clear working space is required in front of access panels on this equipment.
The closest a/c unit appears to be in compliance with the workable space requirements of Section 110-26 while the further a/c unit disconnects are clearly in violation of that section
Indoor Heat Pump Equipment
An indoor air-handling unit is installed for typical heat pump split systems. Refrigerant lines are run from the outdoor unit to the indoor unit that may be located in the crawl space, attic or indoors, such as in a utility closet or room. The indoor unit includes a fan for circulating air through the dwelling and controls, as well as the refrigeration coil. Where desired, resistance heating elements are added to the indoor unit to serve as backup heat in case of compressor failure and to provide extra heating capacity to provide faster heating recovery.
Many of the same rules apply to indoor units as apply to electric furnaces. A disconnecting means rated for the load to be served is required within sight of the indoor unit. In some cases, this disconnecting means consists of one or more circuit breakers that are located in the unit and are operational from outside the unit.
The minimum size of the branch circuit to the indoor unit is required to be not less that 125 percent of the resistance heat and motor load. SeeNECSection 424-3(b).
Room Air Conditioners
A room air conditioner is considered to be an alternating-current appliance of the air-cooled window, console or in-wall type that is installed in the conditioned room and incorporates a hermetic motor-compressor. The following requirements cover equipment rated not over 250 volts, single phase, and such equipment may be cord- and plug-connected.
In determining branch-circuit requirements for a room air conditioner, a cord- and attachment plug-connected unit is considered as a single motor unit if its rating is not more than 40 amperes, 250 volts, single phase; total rated-load current is shown on the air-conditioner nameplate; and the rating of the branch-circuit protective device does not exceed the ampacity of the branch-circuit conductors or rating of the receptacle, whichever is less.
The total marked rating of a cord- and attachment plug-connected room air conditioner shall not exceed 80 percent of the branch-circuit rating where no other loads are supplied. If the branch circuit supplies lighting units or other appliances, the rating of the unit cannot exceed 50 percent of the rating of the branch circuit. SeeNECSection 440-62.
An attachment plug and properly rated receptacle is permitted to serve as the required disconnecting means for a single-phase room air conditioner rated 250 volts or less if the following conditions are met:
1. The manual controls on the room air conditioner are readily accessible and located within 6 feet from the floor, or
2. An approved manually operable switch is installed in a readily accessible location that is within sight from the room air conditioner. SeeNECSection 440-63.
Where a flexible cord is used to supply a room air conditioner, the length of such cord cannot exceed:
1. 10 feet for a nominal, 120-volt rating, or 2. 6 feet for a nominal 208- or 240-volt rating. SeeNECSection 440-64.
A room air conditioner that is fastened in place or connected by permanent wiring methods (fixed) requires that any exposed noncurrent-carrying metal parts likely to become energized are to be properly grounded under any of the following conditions:
1. Where an air conditioner is located within 8 feet vertically or 5 feet horizontally of ground or grounded metal objects and subject to contact by persons.
2. Where located in a wet or damp location and not isolated.
3. Where in electrical contact with metal.
4. Where supplied by a metal-clad, metal-sheathed, metal-raceway, or other wiring method that provides an equipment ground. See NEC Sections 250-110, 250-112, and 250-114.
See also NEC Article 440, Part G.
Safety Standard Information for Overcurrent Protection Requirements for Air-Conditioning Equipment
Section 110-3(b) of theNational Electrical Codestates: "Installation and Use. Listed or labeled equipment shall be installed, used, or both, in accordance with any instructions included in the listing or labeling.”N
Qualified laboratories typically list air-conditioning and heating equipment in accord with the following standards: This listing or labeling is performed by qualified electrical testing laboratories in accordance with the following UL Standards.
UL1995 —Heating and Cooling Equipment (This standard covers central heating, central air-conditioning, and heat pumps.) (UL1995 applies to central air-conditioning units and heat pumps.)
UL484 –Room Air Conditioners
These product safety standards detail the necessary safety tests and determine the required nameplate markings and instructions that are included by the manufacturer of the equipment. For example, paragraph 36.3(i) of UL 1995 specifies the equipment shall be marked with a "maximum overcurrent protective device size.” A typical nameplate will show the "MAXIMUM FUSE” and/or "MAXIMUM CIRCUIT BREAKER” size. If the nameplate specifies only fuses, then the unit is intended to be protected by fuses only. If the nameplate requires HACR (Heating, Air Conditioning and Refrigeration) circuit breakers, then the circuit breaker protecting the unit must be marked "HACR.”
If the nameplate includes both fuses and HACR circuit breakers, as is the case of our nameplate example in this chapter, then either is acceptable.
As noted above,NECSection 110-3(b) would require that the branch-circuit overcurrent protective device comply with the type and size specified on the air- conditioner nameplate.
For room air conditioners, the nameplate will also be marked to show the type and maximum size overcurrent protection permitted for the unit. The comments previously discussed relative to use of fuses or HACR circuit breakers are applicable to room or window units as well as to central air conditioners and heat pumps. Where a room air conditioner is added to an existing dwelling and supplied from an existing panelboard or load center, it is important to verify the branch-circuit overcurrent device for the circuit supplying the air conditioner complies with the type and size overcurrent protection stated on the units nameplate.
Product Safety Standard Requirements
It is recommended that the following electrical product safety guide card information and product safety standards be consulted for additional guidance on the proper installation, operation and use of electrical equipment covered in this chapter. The four letter code in parenthesis refers to the product category in the Underwriters Laboratories Directories.
- Equipment for Use in Ordinary Locations (AALZ)
- Air Conditioners, Central Cooling (ACAV)
- Air Conditioners, Packaged Terminal (ACKZ)
- Air Conditioners, Room (ACOT)
- Heat Pumps (AGUX)
NNational Electrical CodeandNECare registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02269. This reprinted material is not the official position of the National Fire Protection Association, which is represented only by the standard in its entirety.
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Posted By Philip Cox,
Thursday, July 01, 1999
Updated: Wednesday, August 29, 2012
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A question has been asked as to what the IAEI policy is on endorsing or promoting electrical products. It is a fair question. This inquiry was made because of an advertisement by a manufacturer of a product used in the electrical industry. The company product literature included a statement that implied the equipment was endorsed by the IAEI. I believe it is appropriate for a clarification of the general policy maintained by the IAEI on this question. The IAEI does not endorse any specific electrical product. It maintains a position of neutrality where electrical equipment and wiring materials are concerned. The very nature and purpose of the IAEI precludes the organization from taking a position to favor one electrical product over its competitor.
The IAEI Articles of Association states in Article II, Section 1(a) that one objective of the organization is "To cooperate in the formulation of standards for the safe installation and use of electrical materials, devices and appliances.” This objective emphasizes the fact that the IAEI purposely gets involved in the development of standards for electrical materials, devices and appliances where the need and opportunity arise. It should be noted that this action does not identify any specific manufacturer’s product but rather relates to the "safe installation and use” of those items.
The IAEI is specific in setting guidelines on this subject for its representatives on code-making panels. The IAEI Operating Rules states, "Each Code-Making Panel representative shall refrain, as a representative of IAEI, from the appearance or practice of endorsing any electrical product.” It should be noted that it not only prohibits the "practice of endorsing” an electrical product, but is even more severe in that aspect by prohibiting even the "appearance” of endorsing any electrical product. The very nature and sensitivity of the position of electrical inspectors dictates that they do not show favoritism toward or bias against any electrical product. It is likely that most every person in the electrical inspection field has personal preferences related to certain products, but the professional electrical inspector does not let personal opinions interfere with applying adopted electrical safety rules in a fair and uniform manner. There is an unwritten code of ethics that drives professional electrical inspectors to remove themselves from becoming personally involved in the interpretation and enforcement of electrical rules. There have been occasions where reports have been received of inspectors requiring contractors to "do it my way” where the adopted rules either do not require such application or where the suggested installation is in direct conflict with the code. Where this type of attitude exists, it is unfortunate and is in direct opposition to the objective of the IAEI to promote the uniform understanding and application of electrical codes.
The International Association of Electrical Inspectors must represent the electrical inspector and as an organization must refrain from showing any preference of one electrical product over another. Two objectives of the IAEI as stated in the Articles of Association are "To promote cooperation between inspectors, the electrical industry and the public,” and "To represent the Electrical Inspectors in all matters which are dealt with nationally and internationally by the electrical industry.” It would be counter productive to these objectives for the IAEI to endorse any specific electrical product. Members from the electrical manufacturing industry are vital for the existence and proper operation of the IAEI. Should preference be given to any manufacturer or manufacturer’s product over another, it would essentially mean that the IAEI takes sides. That would effectively compromise the position of neutrality and be detrimental to the organization and electrical inspectors in particular. The IAEI has no intention of becoming involved in the marketing strategy of manufacturers.
The IAEI does encourage the use of products recognized by the electrical code and emphasizes the importance of third party testing of products. In an effort to keep members and other readers informed, a section in this magazine is reserved for coverage of new electrical products. This portion of the magazine is devoted to a brief presentation of new products entering the market in hopes that readers of the magazine will be better informed on what they might see on the job site. This notice is intended to help inspectors and others stay current on what is going on in this aspect of the industry and to enable them to be better prepared. Inspectors who now a product is either entering or has just entered the market and makes the effort to obtain additional information is better prepared to evaluate the installation of that product on a construction site. One criteria for including the product in the new products section of theIAEI Newsis that the product be listed or certified by a third party testing laboratory where a standard exists for that product.
In regards to the incident that raised this issue, the organization referring to the IAEI in product literature as endorsing its product has been notified to cease making any such reference. Immediate action will be taken on known incidents where organizations improperly imply that the IAEI endorses or promotes their electrical product.
Read more by Philip Cox
Posted By Michael Johnston ,
Saturday, May 01, 1999
Updated: Tuesday, August 28, 2012
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Many businesses are concerned these days with the cost of electrical energy supplying power to their facility, but an increasing number of them are getting real concerned about the quality of the power being utilized at their facility. So what is the problem with the quality of power being delivered to a facility? Many businesses are extremely nervous about loss of data or data errors that can result from this "dirty power” or "electrical noise” on the system. Does this sound familiar? So what do they do? Many have extensive evaluations performed and end up with large surge protectors or filtering systems consisting of metal oxide varistors (MOVs) and capacitors to try and filter or stabilize their electrical supply system. Is that the answer? In some cases these types of devices can be effective in cleaning up some power quality issues.
In large, or even smaller, facilities, if the owner or operator is considering an attempt to clean up the power being delivered to the facility, he should start with an extensive analysis of the serving utility’s power as delivered, the electrical service, the power distribution system, the panelboards, and finally the branch circuits and what is connected to the branch circuits. One of the most important aspects of this analysis is the inspection of the grounding electrode system, the equipment grounding conductors, the bonding system, and the grounded conductor of the system. If an electrical system is not grounded properly, the voltage could be flowing unstably to begin with before it’s filtered. Section 250-2(a) of the 1999 National Electrical Code® specifically states the reasons for grounding electrical systems. The electrical system shall be "connected to the earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines, and that will stabilize the voltage to earth during normal operation.”
The term "quality of power” is often described in different ways and by different elements of the electrical industry. Stabilized voltages and stable waveforms are two elements which are desirable in power systems when talking about the subject of "power quality.” Grounding affects voltage stability but, more importantly, is a critical element to personal safety. Harmonic currents are a mathematical model one often uses to analyze distorted waveforms flowing at higher frequencies than the common fundamental root frequency of 60 hertz.
The term "power harmonics” is probably familiar to most individuals involved in the electrical industry, but is probably more often misunderstood. Many technical articles and publications have been written about this subject, but these articles do not always address one’s basic problems and concerns in an understandable way. This article addresses the following questions.
- What are power system harmonics?
- What effect do they have on the power distribution system in a building?
- What are common symptoms or signs of harmonics or harmonic problems?
- How does one address these issues?
Harmonics is a mathematical model of the real world. Harmonics is simply a technique to analyze the current drawn by computers, electronic ballasts, variable frequency drives and other equipment which have modern "transformer-less” or electronic power supplies. These power supplies operate according to Ohm’s Law, which states that when a voltage is applied across a resistance, current will flow. This is how electrical equipment operates. Voltage applied across equipment is a sinewave which normally operates at 60 hertz (cycles per second) in the United States.
The utilities have the responsibility of generating this voltage at this 60-cycle sinewave. It has (relatively) constant amplitude and constant frequency.
Once this voltage is applied to any utilization equipment, Ohm’s Law is in effect. Ohm’s Law states that current equals voltage divided by resistance. The formula is simply:
I=E/R or Current (I) is equal to the Voltage (E)
divided by the Resistance (R)
Expressed graphically, the current ends up being another sinewave, since the resistance is a constant number. Ohm’s Law dictates that the frequency of the current wave is also 60 hertz. In the real world, this is true; although the two sinewaves may not align perfectly the current wave will indeed be a 60-cycle sinewave.
Fundamental Root Frequency 60 Hertz
Since an applied voltage sinewave will cause a sinusoidal current to be drawn, systems which exhibit this behavior are called linear systems. Incandescent lamps, heaters and, to a great extent, motors are linear systems.
Some modern equipment is taking on different characteristics. Computers, variable frequency drives (VFD’s), uninterruptable power supply systems and electronic ballasts are some types of non-linear electrical loads. In these systems, the resistance is not a constant and, in fact, varies during each sinewave. This occurs because the characteristics of the load drawn by the equipment are not a constant. The resistance, in fact, changes during each sinewave. The power supplies of these systems usually contain solid state devices such as power transistors, thyristors, or silicon-controlled rectifiers (SCRs). These devices draw current in pulses.
The major difference between an AC source and a DC switching power supply of most electronic equipment is simply that it draws current only within short periods of time or (cycles) within the normal sine wave. This is how harmonic currents are introduced into the return or neutral of an electrical power distribution system.
As voltage is applied to a solid state power supply, the current drawn is (approximately) zero until a critical "peak voltage” is reached on the sinewave. At this peak voltage, the transistor (or other device) gates or allows current to be conducted. This current typically increases over time until the peak of the sinewave and decreases until the critical peak voltage is reached on the "downward side” of the sinewave. The device then shuts off and current goes to zero. The same thing happens on the last 180° of the sinewave side with a second negative pulse of current being drawn. The resulting current is a series of positive and negative pulses, and not the true, smooth 60-cycle sinewave drawn by linear systems. Some systems have different shaped waveforms, such as square waves. These types of systems are often called non-linear systems. The power supplies which draw this type of current are called switched mode power supplies.
That is, one can create a series of sinewaves of varying frequencies and amplitudes to mathematically model this series of pulses. These are multiples of the fundamental frequency, 60 hertz. These multiple frequencies are called harmonics. The second harmonic would be two times 60 hertz,are called or 120 hertz. The third harmonic is 180 hertz and the fifth would be 300 hertz and on. In the typical three-phase power systems, the "even” harmonics (second, fourth, sixth, etc.) cancel. In these systems dealing with the "odd” harmonics is the challenge.
This figure shows the fundamental (60 Hz) and the third harmonic (180 Hz). As you can see, there are three cycles of the third harmonic for each single cycle of the fundamental. These two waveforms will be additive, as it flows it results in a non-sinusoidal waveform. Peaks will start forming that are indicative of the pulses drawn by switch mode power supplies. If one adds in other harmonics, one can model any distorted periodic waveform, such as square waves generated by UPS or VFD systems. It’s important to remember these harmonics are simply a mathematical model. The pulses or square waves, or other distorted waveforms are what one would actually see if one were to monitor an oscilloscope on the building’s wiring distribution systems. True RMS testing equipment is available and qualified testing facilities are available that can effectively measure these currents. These current pulses, because of Ohm’s Law, will also begin to distort the voltage waveforms in the building. This voltage distortion can cause premature failure of electronic devices.
On three-phase systems, the three phases of the power system are 120° out of phase. The current on phase B occurs 120° (1/3 cycle) after the current on A. The current on phase C occurs 120° after the current on phase B. Because of this, 60 hertz (fundamental) currents actually cancel on the neutral. If there are balanced 60 hertz currents on three-phase conductors, neutral current in theory and on the meters will be zero. It can be shown mathematically that the neutral current (assuming only 60 hertz is present) will never exceed the highest loaded phase conductor. Thus, overcurrent protection on phase conductors also protects the neutral conductor, even though there is no overcurrent protective device in the grounded or neutral conductor. This is in compliance with NEC® Section 240-22, which says one should not connect an overcurrent device in series with any grounded conductor of the system.
When harmonic currents are present, the third harmonic of each of the three-phase conductors is exactly in phase. When all of these harmonic currents return together on the neutral, rather than cancel, they actually add and can result in more current on the neutral conductor than on phase conductors. These neutral conductors, in effect, are no longer being protected against overcurrent by the overcurrent device (breaker or fuses) on the circuit. These harmonic currents will create heat, which, over a period of time, will raise the temperature of the neutral conductor. This temperature increase can overheat the associated conductors in the same enclosure and cause insulation failure. These harmonic currents also can cause sources (such as transformers or generators or converter windings) which supply the power system to overheat. This is the most obvious symptom of harmonics problems: overheating neutral conductors and transformers. This overheating is largely related to the "skin effect.” Simply stated: currents flowing at higher frequencies will not utilize the total conductor property or total circular mil area, but will flow on the skin of the conductor, creating heat. Some symptoms include:
- Heat in the conduit of wiring systems
- Computer malfunctions and data loss or errors
- Insulation degradation
- Nuisance tripping of circuit breakers
- Several solutions are available to address these symptoms:
Oversizing Neutral Conductors:
In three-phase circuits with shared neutrals, it is common to oversize the neutral conductor up to 200% when the load served consists of non-linear loads. For example, many panelboard manufacturers build a 200% neutral bus rated panelboard for these applications. Most manufacturers of office furniture systems provide a No.10 AWG conductor with 35 amp terminations for a neutral shared with the three No.12 AWG phase conductors. In feeders that have a large amount of non-linear load, the feeder neutral conductor and panelboard bus bar should also be oversized. Section 310-15(b)(4)(c) considers the neutral (that is carrying these types of currents as a majority of the load) current-carrying conductors.
Using Separate Neutral Conductors:
On three-phase branch circuits, another method, instead of installing a multiwire branch circuit sharing a neutral conductor, is to run separate neutral conductors for each phase conductor. This increases the capacity and ability of the branch circuits to handle these harmonic loads. While this successfully eliminates the addition of the harmonic currents on the branch circuit neutrals, the panelboard neutral bus and feeder neutral conductor still must be considered
Oversizing Transformers and Generators:
The oversizing of equipment for increased thermal capacity should also be used for transformers and generators which serve harmonics-producing loads. The larger equipment can dissipate more heat effectively.
Special transformers (K-Factor Transformers) are being manufactured to dissipate the additional heat caused by these harmonic currents. These transformers that are specifically designed to handle the effects of circuits and feeders with harmonic currents are being installed for old and new computer rooms and information technology systems.
Surge and Dip Filtering Equipment:
While many filters do not work particularly well at this frequency range, special electronic tracking filters can work very well to eliminate harmonics. These filters are presently relatively expensive but should be considered for thorough harmonic elimination.
Special Testing Equipment:
Standard clamp-on ammeters are only sensitive to 60 hertz current, so they only tell part of the story. New "true RMS” meters will sense current up to the kilohertz range. These meters should be used to detect harmonic currents. The difference between a reading on an old style clamp-on ammeter and a true RMS ammeter should give you an indication of the amount of harmonic current present.
Grounding Conductors: Your Safety Insurance Factor
Often times when the quality of the power is considered dirty, recommendations to install "isolated” equipment grounding conductors, also referred to as "separate grounds” or "clean grounds,” are the course of action. The rules for installing a separate isolated grounding conductor are covered in detail in the NEC® in Section 250-96(b) and 250-146(d) (See Figures 1 and 2 below). The Fine Print Note following each of these sections explains that the use of this method for the reduction of electrical noise in the grounding circuit does not relieve the requirement for grounding the raceway system and outlet box. The use of supplemental grounding electrodes often is in the plan for each electronic piece of equipment. While the National Electrical Code® allows this "supplemental electrode” in Section 250-54, it does not in any way relieve the requirement of the proper connection of an equipment-grounding conductor. "The earth shall not be used as the sole equipment grounding conductor.” Grounding conductors are required by the National Electrical Code® in the United States and by most other major electrical codes in the world. No matter what they are called, these conductors serve the same purpose. Grounding conductors connect all of the non-current carrying parts of the electrical system or any metallic parts in the vicinity of the electrical system together. This part includes conduits, enclosures, supports and other metallic objects. [See Figure 1 and Figure 2]
Example – Figure 1
Example – Figure 2
This grounding system has two purposes:
1. Safety. The grounding conductor system provides a low impedance path for fault currents to flow. This path must have three important characteristics. The path must be permanent and continuous, have lowest impedance possible, and the capacity to conduct safely any current imposed on it. This allows the full current to be detected by overcurrent protective devices (fuses and circuit breakers), safely clearing the fault, quickly eliminating shock hazards to persons and protecting property. See NEC® Section 250-2(d).
2. Power quality. The grounding system allows all equipment to have the same reference voltage. The electronic power supply in electronic equipment uses the frame of the equipment for the reference point. This is where the ac equipment grounding conductor and the electronic equipment grounding circuit come together. (See figure below.) This helps the facility’s electronic equipment operation and helps prevent the flowing of objectionable currents on communication lines, conduits, shields, and other connections.
To examine the safety issue more closely, consider the following system: a power system consisting of a voltage source (transformer or generator) connected to a disconnect and a panelboard. An appliance is fed from this panelboard. When the circuit is formed current flows in the circuit allowing the appliance to operate. The grounding conductor connects the frame of the appliance to the panelboard enclosure and to the service enclosure. This enclosure is connected to the grounded conductor (often the neutral conductor) which, in turn, is connected to the grounded terminal of the transformer. If a ground-fault occurs, the grounding conductor connection allows current to flow. This current will be much greater than the normal load current and will cause the circuit breaker to open quickly. This safely clears the fault and minimizes any safety hazard to personnel. Suppose the grounding conductor is not connected properly or is interrupted. If a fault occurs, little or no current will flow in the grounding conductor since the circuit is interrupted. This opened grounding conductor could be caused by a grounding prong illegally cut off a cord cap, a loose connection, a conduit which is not connected properly or many other causes. This fault leaves the frame of the appliance energized. Should someone touch both the appliance and the building steel, another piping system, or possibly even a wet concrete floor, the circuit would then be completed in current flow through the body, injuring or killing the person. The National Electrical Code® recognizes the use of certain raceways and cables as equipment grounding conductors. Many designers today do not believe that using steel conduits is adequate for this use. Conduit has connections every ten feet and often low-grade, cast-metal couplings and connectors are used. The secondary benefit of this copper grounding conductor is it will provide an equipotential plane for all equipment connected to it. This often makes the so-called isolated grounding conductors specified by computer and other manufacturers unnecessary.
Article 250 of the National Electrical Code® sets up the minimum requirements for grounding and bonding of electrical power distribution systems. Article 250 took on a new look and feel for the 1999 Code cycle. First, it should be understood that all of the previous requirements in Article 250 are still in place as minimum requirements. Second, the article has been totally reorganized. The performance based requirements in 1996 Section 250-1 FPN 1 & 2 and also Section 250-51 have been written into the general requirements of Part A. The 1999 edition also continued its quest to migrate away from using the grounded conductor for grounding equipment downstream of a main bonding jumper at a service, or downstream from a bonding jumper at a separately derived system. This still is allowable in very few cases with several restrictions. The section on objectionable current flowing on the grounding system still exists in Article 250. It is worth a closer look. Section 250-6(d) deals with permissible alterations to stop objectionable current. This section explains that the provisions of this section should not be misunderstood as allowing electronic equipment from being operated on ac systems that are not grounded as required by Article 250.
Following the minimum requirements in the National Electrical Code® essentially provides for the safety aspect and purpose of chapter two (Wiring and Protection). By following the rules in Article 250 regarding properly sized grounding conductors, proper use of isolated grounding conductors, code compliant installation of supplemental grounding electrodes, and proper connections of both grounded conductors and equipment grounding conductors users can ultimately end up with one having the”best of both worlds” when it comes to power quality issues and most important safety concerns. These grounding and bonding topics are extensively covered in the IAEI Soares Book on Grounding (Seventh Edition).
Read more by Michael Johnston