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IAEI News provides educational forums, updates on electrical codes and reports of innovative research to facilitate the development and enforcement of practices designed to drive efficiency and compliance with the highest standards of product development and safety—for the public as well as for electrical personnel. The magazine reaches authorities with power of product specification, approval and acceptance. Published six times a year by the International Association of Electrical Inspectors.

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Radio equipment that uses a screw-type fastener to bond or attach the grounding conductor to the antenna assembly

Posted By Tom Moore, Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012

Question

We have some radio equipment that uses a screw-type fastener to bond or attach the grounding conductor to the antenna assembly. Can this screw be any ordinary screw, or does it need to be a "listed” screw meeting the requirements of 250.70 or, perhaps, 250.8? Does the screw need to be "green” in finish? Do the grounding and bonding rules in Article 250 apply to radio and television equipment? Or is Article 810 the only applicable rules that would apply? DB

Answer

Thank you for your correspondence. The question presented is frequently discussed by many with diverse results. Radio equipment as referenced in the question falls under the scope of Article 810 of the NEC.

We first need to point out that the term grounding conductor as referenced in the question and used in previous editions of the NEC has been revised to three more appropriate terms: grounding electrode conductor, bonding jumper, or bonding conductor. CMP-16 accepted these revisions throughout Article 770 and all Chapter 8 articles to provide consistency and correlation with defined grounding and bonding terms in Article 100, and to avoid the use of an undefined term in the communications articles.

Let’s begin with the first part of your question. Section 250.70 does apply for the connection to grounding electrodes as referenced in 810.21(K). If I understand the question correctly, we are referring to the grounding electrode conductor or bonding conductor termination to the equipment. Section 810.21 is silent as to the requirements for connection of the grounding electrode or bonding conductor to the equipment; therefore, the listing requirements of the radio equipment need to be followed.

As for the screw terminal, Chapter 8 has no requirements that would require the screw to be identified by a green color.

In the third part of the question dealing with the application of grounding and bonding rules of Article 250, we need to review the code arrangement in 90.3. The second paragraph of 90.3 points out that Chapter 8 is basically a stand-alone chapter, and that Chapters 1 through 7 only apply where specifically referenced in Chapter 8. One example is the reference to 250.70 in 800.21(K) and discussed above.

Tom Moore
IAEI Representative, Chairman-CMP-16

Tags:  Focus on the Code  September-October 2012 

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Has UL Certified any small wind turbines?

Posted By Underwriters Laboratories , Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012

Question

Has UL Certified any small wind turbines?

Answer

Yes, UL has certified small wind turbines under the product category Small Wind Turbine Generating Systems (ZGEN), located on page 466 in the 2012 UL White Book or you can check online at UL’s Online Certification Directory at www.ul.com/database by entering ZGEN at the category code search field. This category covers small wind turbine generating systems (WTGS) investigated for risk of fire and shock, including safety-related control system electrical performance and utility (grid) interconnection performance for Utility Interactive models.

Small wind turbines are considered to be wind turbines where a user or service person cannot or is not intended to enter the turbine to operate it or perform maintenance.

Safety-related control system performance is defined as the electrical hardware and software operation of the controls and protection functions up to the electromechanical interface of the associated power and control circuits.

Wind turbines provided with an inverter or converter are classed as Utility Interactive, Stand-alone or Multimode. Utility Interactive devices operate in parallel with the utility grid. Stand-alone devices are intended to operate independent of the utility grid. Multimode devices can operate as both or either Stand-alone (utility independent) or Utility Interactive (grid-tie).

Units marked "Utility Interactive” have been investigated for electric utility grid interconnection performance in accordance with the requirements in UL 1741 Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources and IEEE 1547, "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems.” The Standard for Small Wind Turbine Systems, UL 6142 also allows for other grid interconnection protection evaluation options to meet specific grid interconnection protection needs that may be allowed or required by some local utilities. These other grid interconnection options are clearly defined on the companies UL Listing page on UL’s Online Certification Directory, product markings and in the product manual.

Mounting means, support structures, wind turbine blades and/or rotors are investigated only to the extent that they include the necessary electrical components to comply with the applicable electrical safety standards, so additional installation codes may apply.

These devices are intended for installation in accordance with Article 694 and 705 of ANSI/NFPA 70, National Electrical Code.

Some devices in this category are intended to be installed and operated with an external transformer. Such devices are provided with markings and instructions to indicate the type of transformer required.

These devices may require external output overcurrent protection, which is specified in product markings and installation instructions.

The Certification Mark for these products includes the UL symbol, the word "CLASSIFIED” above the UL symbol and the following additional information:

SMALL WIND TURBINE GENERATING SYSTEM
IN ACCORDANCE WITH UL 6142
Control No.


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Tags:  September-October 2012  UL Question Corner 

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Is there a way to interact with UL’s Regulatory Services Department Staff on social media?

Posted By Underwriters Laboratories, Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012

Question

Is there a way to interact with UL’s Regulatory Services Department Staff on social media?

Answer

Yes, UL’s Regulatory Services Department has a discussion group on Linkedin.com, entitled "UL Codes and Technical Services”. To join, go to Linkedin.com, become a member, then select "groups” from the search pull down menu at the top right of the screen and then enter "UL Codes” in the search field. Once you are a member, you can enter the conversation or post a new topic to discuss.

What is Linkedin? LinkedIn is a business-oriented social networking site and is the world’s largest professional network with over 120 million members and growing rapidly. LinkedIn connects you to your trusted contacts and helps you exchange knowledge, ideas, and opportunities with a broader network of professionals.

If social media is not your thing, you can always visit us online atwww.ul.com/codeauthorities.


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Tags:  September-October 2012  UL Question Corner 

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Fact or Fiction

Posted By David Clements, Saturday, September 01, 2012
Updated: Tuesday, September 18, 2012

Summer has drawn to a close and families have sent their children back to school. Fall is upon us and IAEI members are attending the Annual Section Meetings. Organizing committees for each section worked hard for the past year planning logistics, educational offerings and networking opportunities, all part of providing a great educational adventure. If you haven’t yet signed up to attend one of the section meetings, now’s the time. Go to http://www.iaei.org/member/section-meetings/ to register. Despite what some may believe, we welcome both members and non-members to our section meetings.

Recently IAEI placed a booth at one of the large electrical industry trade shows and I had the opportunity to participate and to meet a number of individuals, most of whom were not IAEI members.Many were somewhat familiar with our organization so having the opportunity to talk one-on-one provided me insight on what they knew, or didn’t know, about IAEI. To some degree I felt as if I were in the TV show Undercover Boss as many didn’t know who I was, which was different from attending one of our section meetings. In other words, I didn’t need to wear a hairpiece!

So what did I discover? What I found out was not overly surprising but it confirmed what the ad-hoc membership committee told me as they were tasked with developing a new membership recruitment and retention plan.

So let’s start with identifying fact from fiction from some of the conversations I had.

  • In order to join IAEI, I need to be an inspector. Fiction.
    Fact. IAEI has an open membership policy and anyone who has a vested interest in electrical safety can join. Our membership consists of electrical inspectors, building officials, electricians, contractors, certification agencies, manufacturers, electrical engineers, electrical consultants, union and non-union, private or governmental agencies. Everyone is welcomed.

  • I can only attend a section meeting if I’m an IAEI member. Fiction.
    Fact. Registration is open to anyone who wishes to attend a section meeting. However, being a member has its benefit, as there is a member vs. a non-member registration fee.

  • My membership is for the local IAEI chapter only, and I’m not permitted to attend other chapter or division meetings. Fiction.
    Fact. When you become a member, you are a member of IAEI; and membership provides access to any IAEI section, chapter or division meetings or functions. Belonging to a host chapter gives you the benefits of attending local meetings, educational opportunities and networking with others with common interests within a geographical area, normally where you reside or work. We encourage members travelling outside of their chapter or division, either on business or vacation, to check to see when the local chapter or division is holding its regular meeting; any member is welcome to attend those meetings. A listing as to when chapters and divisions meet can be found at www.IAEI.org/calendar or in the IAEI magazine under "Dates Ahead.”

  • As an associate member, I do not have a say or a vote at my local chapter or division meeting. Fiction.
    Fact. IAEI membership is broken into several different types; the two main types are inspector members and associate members. Inspector members have different voting rights from associate members. These differences are spelled out in the local chapter by-laws.

  • As an associate member, I’m not permitted to hold office at the local chapter or division level. Fiction.
    Fact. Chapter by-laws have provisions for associate members to be on the board of directors. There is a ratio that must be maintained between inspector members and associate members in order to maintain the principles on which the association was founded.

So as you can see from the above conversations I had during the trade show, my answers to these questions can help dismiss the difference between fact and fiction and encourage individuals to see the value of joining IAEI.


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Tags:  Editorial  September-October 2012 

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Energy Alternatives and NEC Implications

Posted By Michael Johnston, Monday, July 02, 2012
Updated: Monday, September 10, 2012

Energy alternatives and NEC implicationsThe world is changing rapidly and becoming more dependent than ever on expanded use of alternative energy systems and reducing and preserving grid-produced energy. The electrical infrastructure in the United States is one of the best in the world, but is aging and, to some degree, becoming more vulnerable to the effects of aging and loading. Society is typically not removing loads from the electrical grid, but is adding load.

Alternative energy technologies such as electric vehicles, solar and wind power sources are providing society with the means to reduce load on the electrical grid and take advantage of natural resources for power production. These technologies are fast to evolve and become attractive and affordable to consumers and even mandatory to a degree, in some cases, to address energy code requirements. As more of these technologies are included in premises electrical systems, necessary safety requirements must be applied.

Whatever technology, new or old, is employed it must be a safe. There is significant responsibility to society to provide safe and dependable electrical power. Technology that is dynamic and evolving is exciting but also means there are necessary implications for the National Electrical Code®.

Reactive or Proactive Change

NEC® changes have historically been reactive to industry progress and incorporated revisions based on identified needs, statistics, and other justification. In recent years, NFPA’s NEC® technical committees have become much more proactive relative to incorporating revisions and new rules in the Code, and for good reason. Technology is advancing in many areas often faster than codes and standards can be developed.

The 2014 NEC® development process is underway with approximately 3745 proposals submitted. The stage is set for the technical subcommittees to act on these proposed changes in 2012, several of which are proactive and progressive and address new and evolving technology.

It is imperative that any applicable codes and standards for the built environment not be viewed as obstructions or roadblocks to progress, but rather as an essential component of our electrical safety system. The electrical codes must advance and evolve so they can be applied to all the great technologies. Some areas in the electrical industry that are evolving are solar photovoltaic systems, electric vehicles, wind generated power, energy storage, and DC wiring and systems. Another activity that is driving change in the NEC® is energy management. Let’s take a closer look some of the areas where the NEC® is experiencing changes and expansion to address alternative energy technology.

Energy Management

As aggressive initiatives to address energy use and loading of the current electrical grid, the codes and standards development communities work to align their documents to address identified safety concerns. This is apparent for product safety standards and electrical installation codes, and specifically theNEC®. More often energy codes are incorporating performance requirements that address reduction of energy use in buildings and premises wiring systems. It is important that these energy codes continue to provide the performance requirements that drive the installation requirements that are included in the NEC®. As the smart electric grid initiatives and timelines become clear to utilities and consumers, the related work and installations will be necessary. This work will happen on both sides of the service point, meaning it will include work on utility grids and premises wiring systems. Energy management systems are becoming more common as smart metering is employed and consumers become aware of their energy use and how they can reduce it.

The NEC® technical committees have been active, through the work of another specifically assigned Smart Grid task group, which included representatives from IAEI, NFPA, NIST, EEI, BICSI, NESC, NEMA* and others. The work of this group resulted in the development of a new proposed NEC® Article 750 titled Energy Management Systems. See NFPA 70 2012 ROP Proposal 13-180 for complete information. The Code must address which loads on the premises can be controlled and those which cannot be controlled by energy management systems.
It is also important as energy codes include specific energy reduction performance for buildings that they be able to refer to the installation rules in the NEC®, rather than developing installation rules in those other codes. Once again, the NEC® technical committees responded proactively to be sure the NEC® remains as an integral part of smart grid initiatives on the customer side of the service point. Interoperability will be necessary to implement an effective and "smart” grid system. It is becoming more a reality that one method of demand response will be through expanded use of energy storage systems. This is anticipated in large utility scale battery systems and battery systems installed in buildings. The NEC® must include rules that can be applied to these energy storage systems.

AC or DC Power

Photo 1. Increasing quantities of electric vehicles will require a capable charging infrastructure.

Photo 1. Increasing quantities of electric vehicles will require a capable charging infrastructure.

The Tesla and Edison debate and questions continue. It seems apparent that the use of more direct current (DC) systems and wiring is in store for the built environment. DC systems are starting to become more attractive in building wiring systems for other than just data centers and backup power systems. Currently, energy is necessary to convert AC power to DC for electronic and other types of utilization equipment. Part of reducing energy could include eliminating DC power supplies where possible. Many electrical appliances and electronic equipment such as home computers, flat screen TVs, game boxes, audio equipment and so forth operate on DC power and have a built-in DC power supply to convert the AC power to DC. This takes energy. Imagine if the DC were supplied directly to this utilization equipment.

This concept is real and technology is driving reality. The next edition of the NEC® is being revised and expanded to include appropriate rules that can be applied to DC wiring. DC power will become more popular for normal lighting and power systems in buildings and will be an important part of expanded demand response capabilities when connected to power storage (batteries) systems installed on the premises.

New technologies have been manufactured that drive the need for not only NEC® rules but development of applicable product safety standards. Light-emitting diode (LED) technology has advanced to an attractive energy alternative that provides energy savings and quality light. This edition of theCodeincludes proposed requirements for listed retrofit kits for luminaires and signs. A new definition of the term retrofit kit has also been proposed in Article 100. A new Article 710 titled Direct Current Microgrids was proposed; however, it was rejected in the proposal stages of the process.

A new Article 393 (proposed as 302) titled Low-Voltage Suspended Ceiling Power Distribution Systems provides requirements for a ceiling grid system that would provide power for lighting and other loads. See NFPA 70 2012 ROP Proposal 18-10a for complete information.

Electric Vehicles

One of the more popular alternative energy initiatives has been the resurgence of electric vehicles as an alternative to combustion engine vehicles. The vehicles are here, and they will need charging stations, both at the home and in the commercial setting. Nearly every vehicle manufacturer has joined in this long-range effort. Even though many of the new EVs are smaller and compact, the technology will no doubt evolve to a point where the larger vehicles many are accustomed to will be available in electric versions. This industry is progressing at a fast pace. Saving the environment from vehicle exhaust is a huge effort and one the current administration has placed significant emphasis on and, consequently, has set aggressive goals.

The growth of the electric vehicle market brings a need for building a sufficient and safe charging infrastructure, a great opportunity for the electrical industry. With these opportunities come the challenges of adding the vehicle charging loads to existing wiring services and the utility grid. At the same time, managing the loads on the existing electric grid and the smart grid activity grows. No easy task and this will require deliberate communication and coordination between the utilities and consumers.

Recognizing this fast approaching need, the NEC® development community responded by assigning a specific task group with the responsibility of addressing identified needs inNEC® Article 625. This article was added to theCodein the mid-1990s and was a great example of a successful proactive approach taken by NFPA in ensuring that the Code was ready for this technology. At that time, the NEC® was ready for electric vehicle market growth, but the automobile industry and the technology still had to evolve to where it is today.

This cycle, an assigned task group has once again readied NEC® Article 625 by proactively addressing requirements for individual branch circuits, clarifying provisions for cord-and-plug connected electric vehicle supply equipment (EVSE), and including the option of automatic load management systems where existing service or source capacity is insufficient for the added charging load. Article 625 was also reorganized to provide a more logical sequence and to address usability issues. Once again, the NEC® technical committees have responded proactively to ensure that the NEC® is equipped and addresses safety requirements necessary for the safe and successful growth of the electric vehicle charging infrastructure.

The Importance of Code Adoption

Adopting the latest edition of the NEC® is just plain smart, especially in this time of fast-paced technological development. Many jurisdictions have long recognized that the electrical industry advances each year at a pace that is faster than required codes can be written.

Delaying adoption of the latest NEC® requirements for even one cycle creates challenges for jurisdictions that are required to approve technologies that may not be fully addressed in previous Code editions. Not applying the requirements in the latest edition of the NEC® also means that consumers may not have the latest required protection in their electrical systems. The cost of doing business has to include safety for persons and property. Those jurisdictions adopting the latest edition can rest assured that the NEC® will adequately address the electrical technologies that authorities having jurisdiction must routinely inspect and approve.

Summary

While this article did not address all of the proposed Code revisions for this cycle, some of the significant implications related to alternative energy technologies have been provided. The NEC® development process is dynamic and has evolved over the years to become progressive and proactive whenever possible, as compared to being reactive. It is evident that the NEC® development process is serving the electrical industry well and in timely manner.

The work of the NEC® technical committees is extensive this cycle and involves many organizations sharing common safety objectives. There have been vast achievements in the electrical industry in the last couple of decades and moving ahead it is obvious that the pace of change will increase.

The electrical safety system depends heavily on codes and standards that adequately address all safety concerns for persons and property. It is a tremendous responsibility shared by many industry stakeholders. TheNEC® development process is evolving to not only react but also anticipate and respond accordingly.

For complete information on the changes proposed for the next NEC® edition refer to the NFPA 70 2012 Report on Proposals available from the National Fire Protection Association.

References *

NFPA, National Fire Protection Association
NIST, National Institute of Standards and Technology
EEI, Edison Electrical Institute
BICSI, a professional association supporting the informationtechnology systems (ITS) industry
NESC, National Electrical Safety Code
NEMA, National Electrical Manufacturing Association


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Tags:  Featured  July-August 2012 

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Transitioning Between Raceways

Posted By Erik Senseney, Monday, July 02, 2012
Updated: Monday, September 10, 2012

Transitioning Between RacewaysFor most electricians, the proper use of fittings is elementary. Fittings themselves are pretty straightforward in their use. ReferencingDefinitionsin theNational Electrical Codeunder the entry:Fittings, we are informed that a fitting is "an accessory such as a locknut, bushing, or other part of a wiring system that it is intended primarily to perform a mechanical rather than an electrical function.” From experience, we know that fittings are straps, hangers, couplings and connectors which connect (or couple) a raceway to itself, a box, device, enclosure, or similar electrical apparatus — pretty simple. This simplicity becomes a matter of thought and caution when coupling different types of raceways together, especially, when compounded with the issues of wet locations.

In dry locations, transitioning between raceways has been historically accomplished by a box, conduit body, or through the use of a steel coupling. The steel coupling transition is probably the most common. In this application, two connectors are used in conjunction with a steel coupling to form a "from–to” (from this to that). While this type of installation is accepted by most AHJs, it has several inherent weaknesses that should be understood and addressed. The most obvious weakness of this fitting assembly lies in its lack of a listing. Without a listing, the fitting violates the NEC and must rely on AHJ acceptance.

The "from–to” fitting assembly is a violation of the NEC because of section 300.15 which states: a fitting must be used only in the application it is listed: "fittings … shall be used only with the specific wiring methods for which they are designed and listed.” The MC connector and the EMT connector are both listed for use with MC and EMT, but the steel coupling is only intended for use as a coupling for rigid metal conduit, and not as a coupling for two different connectors. Therefore, the "from–to” in photo 1 is not a listed assembly. Many jurisdictions allow this type of installation, relying on AHJ acceptance, but should only do so with an understanding of the inherent weaknesses of the "from–to” assembly.

In photo 1, we see a dry locations conversion. This "from–to” assembly uses a ¾″ EMT set screw connector, a steel coupling; a ¾″ to ½″ reducing bushing, and a duplex mc connector to assemble a transition between ¾″ EMT and two 3/8″ metal-clad cables (MC). When referring back to the definition of a fitting, we understand that a fitting must meet two criteria. First, it must perform its mechanical duties in terms of connecting the raceways or cables together. Second, a fitting must also provide a low-impedance path for the continuity of the raceway, or its electrical function. Any fitting, used with metallic raceways, must not offer impediment to the flow of current. When constructing or evaluating any transition between raceways, these two factors can provide the guidelines for their acceptance. Does the transition offer good mechanical strength? Will the fitting or assembly impede the flow of current?

While the dry location transition in photo 1 likely offers good mechanical strength, it might not meet the second criteria of a fitting: its electrical function. This is where a "from–to” becomes problematic as the connection relies on the tightening of the duplex MC connector against the steel coupling. In this case, the shoulders of the MC connector are in contact with the steel coupling in order to make the assembly tight. These shoulders and the threads (which also include the addition of a reducing bushing) must work together to create the system bonding path and offer little or no impedance to the flow of current. If the fitting is slightly corroded, the two small points of contact on the MC connector will provide the primary equipment bonding path and this "from–to’s” electrical function will likely be less than adequate. If installed loosely or if corroded, the fitting is in violation of the National Electrical Code as it does not perform both of its intended functions. While the AHJ may still find their use acceptable, this acceptance should be dependent upon a tight and corrosion-free installation until an acceptable listed alternative becomes readily available.

When transitioning between conduit systems, it is possible to use code-compliant products that are listed expressly for coupling different raceways.The combination coupling or the transition fitting is available for most of the applications in the field. The conversion of EMT to Rigid, EMT to FMC, Rigid to Liquidtight or any combination thereof can be accomplished using UL listed products. As an example of these products: a Bridgeport Fittings 4361-DC (photo 2) is a listed fitting, specifically designed for transitions between EMT and Liquidtight. The 4361-DC is also suitable for use in wet locations.

Photo 1. A dry locations conversion which likely offers good mechanical strength; however, it might not provide good electrical function.

Photo 1. A dry locations conversion which likely offers good mechanical strength; however, it might not provide good electrical function.

Unfortunately, listed transition fittings are seldom offered above the 1 inch trade size. As a solution for larger trade sizes, the electrician has a few options. The first, and most obvious, option would be to install a "from–to” using the steel coupling transition between two connectors. This should not be considered an option for wet locations as the internal threads of the steel coupling are prone to corrosion. A more electrically viable option is for the electrician to use threaded fittings, joined together by a junction box or enclosure. This option can be costly and dependent on the space available. The third option is to use two connectors installed in a conduit body; in particular, a "C” style conduit body. By threading two different raceway connectors into either end, the installer is afforded an opportunity to transition, as well as access to the conductors. Using a conduit body to transition between two different raceways requires the consideration of several variables. First, a proper fit must be insured, so that that the mechanical and electrical functionality is maintained. Second, this transition method requires AHJ acceptance as this fitting assembly has risks related to listing requirements which are similar to the "from–to.”

The primary issue with the conduit body transition is thread compatibility. It is important to select connectors based on the thread type of the conduit body. There are several important variables which should be understood when installing a conduit body transition. These variables are also pertinent when installing any fitting into a hub, be it a conduit body, motor attachment point, or service-entrance equipment.

There are two common types of threads used in the manufacture of fittings:

NPT or National Pipe Standard Tapered Thread. In this configuration the threads gradually taper at a very slight angle. This is the thread type which is applied to rigid conduit during field treading and can be found on many types of liquidtight fittings (Figure 1).

Straight Threads or NPSM (Pipe Straight Mechanical Fit): Straight thread type is manufactured to one dimension only. NPSM is essentially straight style threading with a very slight modification for ease of fit. Commonly referred to as a modified straight thread, this is the most common type of threading found on fittings (Figure 1).

Figure 1. A comparison of NPT (tapered) and NPSM (straight) threads

Figure 1. A comparison of NPT (tapered) and NPSM (straight) threads

The two main types of threads are also manufactured with a wide degree of variability and tolerance. It is this variability that will affect how the fittings fit. These variables also add another layer of complication when installing fittings into hubs and it also illustrates the issue at hand: a lack of standardization.

There are general guidelines which manufacturers follow: Straight or NPSM threads are usually utilized on fittings designed for dry location use with a locknut. Tapered threads are primarily utilized on hubs which are designed for use with rigid conduit. They can be found on EMT fittings for use in wet locations (commonly referred to as "raintight”) and metallic liquidtight connectors. This guideline isn’t always the rule, especially, when fittings are used in both wet and dry locations.

By examining one of the most common wet location raceway conversions, several important installation and inspection issues become evident. The assembly in photo 3 is made up of an EMT connector, a liquidtight connector and a steel coupling. In this instance, we know that the steel coupling has straight threads. This will allow for a reasonably good mechanical connection, but this "from–to” might prove problematic over time as the fittings age and corrode. Steel couplings offer very little corrosion protection, especially on the internal threads. Either way, we will have a good mechanical connection, but our electrical connection could be impeded by corrosion. To what degree this ability is hindered, we would have no way of truly knowing, but it is fair to assume that the threads inside of the steel coupling would experience oxidation rather quickly, inhibiting the continuity of the raceway, and, again, we have a violation of the NEC with an increase in raceway impedance.

Photo 2. Bridgeport fittings 4361-DC

Photo 2. Bridgeport fittings 4361-DC

Had the installer used a wet location rated "C” style conduit body (see photo 4), the connection would have much greater wet location integrity. Unfortunately, using a conduit body as a transition between raceways offers several hurdles that must be overcome in order to insure that a mechanically strong, low-impedance connection is made. Dry location fittings are often designed with straight or NPSM threads and are considered to be listed for use with locknuts only. On the other hand, so are FMC connectors which have an expectation of use on motor termination housings which are usually constructed with hubs. The same logic applies to wet location fittings. There is also an expectation that liquidtight and EMT connectors can be used with conduit bodies and have been for many years. The issue of fit has created some confusion. The problem with conduit bodies occurs when we mix an NPSM fitting with an NPT conduit body. The two will not thread together correctly. This is the crux of the problem: straight threads into tapered bodies.

Photo 3. this assembly is made up of an emt connector, a liquidtight connector and a steel coupling with straight threads, which allows for a reasonably good mechanical connection. over time, however, this

Photo 3. This assembly is made up of an EMT connector, a liquidtight connector and a steel coupling with straight threads, which allows for a reasonably good mechanical connection. Over time, however, this "from–to” might prove problematic as the fittings age and corrode.

The transition in photo 4 offers a good mechanical connection and offers good corrosion protection. The conduit body and the fittings are all NPT. While this connection is more than adequate for the application, it may not be listed and would likely have to rely on AHJ acceptance. Installing an EMT or liquidtight connector into a conduit body, motor hub, or hub of service-entrance equipment may not be a listed application. Do we violate the listing of the product and 300.15 when using this practice? Let’s start with UL’s interpretation.

In June 2010, Mark Ode, a staff engineer at UL, commented on the matter ("Fittings into Hubs: Good for grounding or not?” Electrical Contractor Magazine, p 119). Mark Ode’s interpretation of the issue is that all hubs are intended for use with rigid conduit only. His article focused on the use of EMT connectors and the hub, or boss in a conduit body. Mr. Ode also discussed the specific hazards associated with the practice and identified the thread style as the main obstacle to the practice. His article concluded that EMT fittings should not be used with conduit bodies and that mixing of thread types leads to improper fit, and added raceway impedance. He is absolutely correct that mixing threads can lead to a compromised connection. Mr. Ode’s assessment is that rigid conduit is the listed application for most conduit bodies and that connectors for EMT are not typically listed for use with hubs or bosses, but there are two other important factors that must also be addressed.

Flexible metallic conduit connectors and liquidtight connectors offer the same hurdles as EMT connectors and there is an expectation that they are for use with hubs.

Conduit bodies are just one of many applications where we encounter hubs. Motors, service entrances, weatherproof boxes and exterior lighting packs offer the same hurdles as conduit bodies.

Mark Ode’ "Fittings into Hubs” article addresses these issues, but his assertion that EMT fittings into hubs of conduit bodies should never be done, should be tempered with the qualifier: unless a qualified electrician and AHJ deem it acceptable. Electricians can use their knowledge as craftsmen to determine if an application will provide both: good mechanical strength, and a low impedance path to the flow of current. Using fittings in conduit bodies or hubs can be permitted provided the thread types are not to be mixed unless a good fit is the result. Many conduit bodies offer a high tolerance NPSM thread that will engage properly with both an NPSM (straight) and NPT (tapered) thread.

As an installation requirement to this listing issue with both the conduit body and the steel coupling, we can make some assumptions based on what we know:

In dry locations, corrosion is not usually a factor, so the steel coupling shouldn’t corrode, but installers should insure that the fitting is installed correctly and that it is more than hand tight. Looseness should not be accepted. A fitting should not be able to be unscrewed by hand.

Photo 4. a wet location rated

Photo 4. A wet location rated "C” style conduit body offers much greater wet-location integrity, but using a conduit body as a transition between raceways also offers hurdles to a strong, low-impedance connection.

In wet locations, rigid conduit is manufactured with a tapered thread and a tapered thread is also applied during field threading. This thread configuration provides adequate continuity for a safe raceway in terms of mechanical strength and electrical continuity when installed into a straight thread steel coupling. The same can be said for conduit bodies using NPSM (straight) and NPT (tapered) hubs that are connected to a threaded connector whatever the configuration is, as long as a proper fit is ensured. Raintight or liquidtight connectors must be installed with a sealing washer as it is a part of the listed assembly, so the sealing washer will have to remain on any connection.

The easiest way to see whether a fitting engages a hub properly, or not, is to dry fit the assembly first. This will ensure that during the installation, the final product will offer both a low-impedance connection and good mechanical strength.

In the end, installers and the AHJ must determine what will and will not be accepted. It should be noted that the "from–to” is an accepted practice and refusing to accept such assemblies overnight is unfair to the installers in any jurisdiction. Using steel couplings as conversions can be a stopgap measure, as can the use of conduit bodies, but the advantages and disadvantages of each should be understood. For now, the compatibility of a fitting into a hub is something that electricians will have to understand and account for. Installers should not install any fittings into hubs without pre-fitting the connection prior to assembly. Dry fitting the assembly ahead of an installation and inspecting the threads for proper engagement are a must. Using this method of pre-screening fittings will also insure that any raceway has the conductivity needed for safe grounding and bonding. It is also important to note that the inclusion of the connector’s wet sealing washer must be ensured even on a conduit body as it is part of the listing.

Electricians should use care when evaluating the construction methods for transitioning between raceways. The use of a junction box should be considered the first choice, or in smaller trade sizes, the use of listed fittings is an inexpensive alternative. When a junction box is impractical, and listed transition fittings aren’t available, the installer should evaluate the use of the conduit body and the steel coupling as a means of transitioning. The conduit body transition is a viable option as long as there has been a proper selection of fittings based on the thread type and configuration. This logic applies to any fittings installed in the hubs of conduit bodies, weatherproof boxes, motor connection points, or service-entrance equipment. They should be pre-fit and evaluated by the electrician prior to the installation. The fittings and the conduit body should be evaluated for proper engagement; they should thread completely into the boss (hub) and the fitting should remain tight. The "from–to” or steel coupling transition should not be utilized in damp or wet locations due to the likelihood of corrosion developing and impeding the fitting’s electrical continuity function. Any "from–to” transition fitting should be inspected for tightness. Using these practices, electricians can continue to make transitions with good mechanical strength and low impedance and inspectors can be assured that transitions between raceways will maintain their functionality for years.


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Evaluation of Onset to Second-Degree Burn Energy in Arc-Flash Hazard Analysis

Posted By Michael Furtak and Lew Silecky, Monday, July 02, 2012
Updated: Monday, September 10, 2012
Evaluation of Onset to Second-Degree Burn Energy in Arc-Flash Hazard AnalysisOur interest in determining accurate onset to second-degree burn energy and its significance in computing the arc-flash boundary is focused on the prevention of injury to the skin of a human who might be exposed to an arc flash. During the last two decades different formulas have been proposed to calculate incident energy at an assumed working distance, and the arc-flash boundary in order to determine arc-rated personal protective equipment for qualified electrical workers. Among others, the IEEE Standard P 1584 Guide for Performing Arc-Flash Hazard Calculations1 and formulas provided in Annex D of NFPA 70E2 and CSA Z462 Workplace Electrical Safety Standard are the most often utilized in the industry to perform arc-flash hazard analysis. The formulas are based on incident energy testing performed and calculations conducted for a selected range of prospective fault currents, system voltages, physical configurations, etc.

 

Use of Incident Energy as a Measure of Burn Severity in Arc-Flash Boundary Calculations

The IEEE P 1584 was developed by having incident energy testing performed based on methodology described in the ASTM F1959-99 standard. The incident energy to which the worker’s face and chest could be exposed at working distance during an electrical arc event was selected as a measure for determining hazard risk category and calculating arc-flash protection boundary. The incident energy of1.2 cal/cm2(5.0 J/cm2) forbare skinwas selected in solving equation for the arc-flash boundary in IEEE P 1584.1Also, NFPA 70E2states that "a second-degree burn is possible by an exposure of unprotected skin to an electric arc flash above the incident energy level of 1.2 cal/cm2( 5.0 J/cm2)” and assumes 1.2 cal/cm2as a threshold incident energy level for a second-degree burn for systems 50 volts and greater.2IEEE 1584 Guide states that "the incident energy that will cause a just curable burn or a second-degree burn is1.2 cal/cm2(5.0 J/cm2).”1To better understand these units, IEEE P 1584 refers to an example of a butane lighter: "If a butane lighter is held 1 cm away from a person’s finger for one second and the finger is in the blue flame, a square centimeter area of the finger will be exposed to about5.0 J/cm2or1.2 cal/cm2.” However, IEEE P 1584 equations 5.8 and 5.9 for determining the arc-flash boundary can also be solved with other incident energy levels as well, such as the rating of proposed personal protective equipment (PPE). The important point to note here is that threshold incident energy level for a second-degree burn or onset to second-degree burn energy on a bare skin is considered constant value equal to1.2 cal/cm2(5.0 J/cm2) in IEEE P 1584 standard.

Flash Fire Burn Experimentations and Observations

Much of the research which led to equations to predict skin burns was started during or immediately after World War II. In order to protect people from fires, atomic bomb blasts and other thermal threats, it was first necessary to understand the effects of thermal trauma on the skin. To name the few, are the works done by Alice M. Stoll, J. B. Perkins, H. E. Pease, H. D. Kingsley and Wordie H. Parr. Tests were performed on a large number of anaesthetized pigs and rats exposed directly to fire. Some tests were also performed on human volunteers on the fronts of the thorax and forearms. A variety of studies on thermal effects have been performed and thermal thresholds identified for different degree burns. We will focus on second-degree burn as this is the kind of burn used to determine the arc-flash boundary in engineering arc-flash analysis studies.

Alice Stoll pursued the basic concept that burn injury is ultimately related to skin tissue temperature elevation for a sufficient time. Stoll and associates performed experimental research to determine the time it takes for second-degree burn damage to occur for a given heat flux exposure. Stoll showed that regardless of the mode of application of heat, the temperature rise and, therefore, the tolerance time are related to heat absorbed by the skin.3Results of this study are represented in figure 1 line (A) along with other studies discussed below.

Figure 1. Stoll criterion time to second-degree burn for various incident heat fluxes on bare human skin

Figure 1. Stoll criterion time to second-degree burn for various incident heat fluxes on bare human skin

A. Stoll found that the results from her experiments could be predicted using Henrique’s burn integral.4Henrique and Moritz were the first to describe skin damage as a chemical rate process and to show that first order Arrhenius rate equation could be used to determine the rate of tissue damage.

In 1952, J. B. Perkins, H. E. Pease and H. D. Kingsley of the University of Rochester investigated the relation of intensity of applied thermal energy to the severity of flash fire burns.5 Comparing results of this study with those of Alice Stoll shows that a larger amount of energy is required to induce second-degree burn. Results of this study are represented in figure 1 line (B).

Figure 1 line (C) shows second-degree burn threshold as reported by Wordie H. Parr.6 The results were obtained by exposing skin to laser radiation and determining dose-response relationship for producing different grades of burns. Figure 1 shows that the Wordie H. Parr curve lies between those proposed by Alice Stoll and those proposed by the University of Rochester study. The explanation for these second-degree burn threshold differences could be interpreted by the fact that thermal injury depends on energy absorbed per unit volume or mass to produce a critical temperature elevation. Skin reflectance and penetration greatly influence this absorption. Also, heat conduction in tissue is far more efficient for small than for larger irradiated areas and exposure to higher levels of irradiance would be possible before injury occurred. Indeed, with extensive irradiation, injury would occur at far lower level of irradiance.7

After reviewing these three studies, it was concluded that the curve presented by Stoll is most suitable to evaluating the type of burn hazard expected with arc flash. Stoll’s study is a good choice because it is more conservative than the other two studies and, therefore, minimizes cases where the burn severity for a specific thermal flux exceeds the associated degree of burn, and is less open to criticism.

We have also included on figure 1 an arrangement of onset to corneal injury thresholds from CO2 laser radiation (see square markers on figure 1).7The data follows the trend similar to that observed by Stoll and others. The range of scatter in the data is thought to be mainly due to the use of different corneal image sizes.

Stoll’s results can be theoretically extended to include heat flux rates over 40 cal/cm2/sec experimentally observed, and they are represented by line (D) on figure 1. The observed and extrapolated data lines A and D can be expressed analytically as:

t = 1.3 * H-1.43, ( Equation 1)

where t is time to second-degree burn in seconds, H is heat flux in cal/cm2/sec.

As an example of using equation 1, the projected time to second-degree burn at a heat flux rate of 2 cal/cm2/sec is approx 0.5 sec. During this time interval the skin would be exposed to a total of 1 cal/cm2 incident energy (2 cal/cm2/sec x 0.5 sec = 1 cal/cm2), whereas at 30 cal/cm2/sec flux the time to second-degree burn is equal to 0.01 sec resulting in only 0.3 cal/cm2incident energy exposure but inducing, nevertheless, the same burn severity as the former less intense and more lasting exposure.

Discussion and Conclusion

Our understanding of the burn mechanism is not perfect or complete, but it is sufficient for the practical purposes concerned here. The important point to notice from figure 1 and equation 1 is that the degree of burn injury dependsnot only, and in fact not as much, on the total dose of energy received by the skin but also on therateat which the energy is received.

The concept of destructiveness of rapid liberation of heat is not new and is widely used in many industrial and military applications. Apart from total amount of heat released during an arc-flash event, it is the high heat flux rate that causes the gaseous products of arc flash to expand and potentially generate high pressures similar to most explosive reactions. This rapid generation of high pressures of the released gas constitutes the explosion. The liberation of heat with insufficient rapidity will not cause an explosion. For example, although a kilogram of coal yields five times as much heat as a kilogram of nitroglycerin, the coal cannot be used as an explosive because the rate at which it yields this heat is much slower.

Figure 2 shows onset to second-degree burn energy threshold adjusted for heat flux rate as a function of exposure time. The onset to second-degree burn energy threshold was calculated as a product of heat flux rate and time to second-degree burn as per the Stoll’s data from figure 1 lines A and D.

Figure 2. Threshold incident energy for a second-degree burn vs. exposure time

Figure 2. Threshold incident energy for a second-degree burn vs. exposure time

Figure 2 demonstrates thatthe threshold energy for a second-degree burn injury is not a constant but rather a variable. Note that the 1.2 cal/cm2 onset to second-degree burn energy for bare skin used in IEEE P 1584, NFPA 70E and CSA Z462 (dashed line on figure 2) intersects with the curve produced using the Stoll’s data at one (1) second point on figure 2. This observation supports the choice of Stoll’s curve we made for evaluating the type of burn hazard expected with an arc flash. For exposures lasting less than 1 second, the irradiance required for an injury would significantly increase as the duration of exposure decreased;however, the amount of incident energy required to cause second-degree burn would decrease. Equation 2 is an analytical expression for the threshold line represented by figure 2.

Eb= 1.2 * t0.3, (Equation 2)

where t is exposure time in seconds.Ebis threshold incident energy incal/cm2that needs to be released during the exposure timetto cause second-degree burn.

As an example of using equation 2, consider 1, 10 and 100 kA faults in 600-volt grounded switchgear with one (1) inch gap between conductors. Table 1 summarizes arcing current, incident energy and the arc-flash boundary (AFB) predicted using IEEE P 1584 empirical model. We deliberately assigned arc duration to 1, 0.1, and 0.01 seconds for the 1, 10 and 100 kA faults respectively, which is consistent with inverse nature of typical protective device time-current characteristics. Column F lists AFB values calculated using 1.2 cal/cm2 onset to second-degree burn incident energy recommended by IEEE P 1584 Guide. Column I lists AFB values calculated using onset to second-degree burn energy evaluated from equation 2 and published in column H.

Note that the amount of incident energy the person would be exposed to remains the same and equal to 2.1 cal/cm2in all three instances (Column D). The arc-flash boundary also remains the same when incident energy at AFB is assigned 1.2 cal/cm2value onset to second-degree burn energy as recommended in IEEE P 1584. Therefore, applying the same onset to second-degree burn energy for the above fault scenarios would make them appear to be of same severity. However, the arc-flash boundary drastically changes when incident energy at AFB is being evaluated using equation 2. AFB will now increase with an increase of the available fault current, predicted arcing current and heat flux released by an arc.

Table 1. This table summarizes arcing current, incident energy and the arc-flash boundary predicted using IEEE P 1584 empirical model.

Table 1. This table summarizes arcing current, incident energy and the arc-flash boundary predicted using IEEE P 1584 empirical model.

Therefore, using onset to second-degree burn energy for bare skin exposure fixed to1.2 cal/cm2in calculating the arc-flash boundary for arc durations other than one (1) second is, as far as we are concerned, open to dispute and, in our strong opinion, heat flux rate should be factored-in when estimating skin damage imposed by an arc flash. Using the1.2 cal/cm2energy for exposure times less than one second will result in undervalued arc-flash boundaries while resulting in conservative but safe arc-flash boundaries for exposure times more than one (1) second. As the IEEE 1584Guidestates, theGuide’sequations (5.8) and (5.9)1can be used to calculate the arc-flash boundaries with boundary energy other than1.2 cal/cm2; and we believe the equations should be, in fact, solved for boundary energy computed using the equation 2 especially for cases when arc duration is less than one (1) second.

References

11584 IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE Industry Applications Society, September 2002.

2NFPA 70E Standard for Electrical Safety in the Workplace, 2012.

3Stoll, A.M., Chianta M.A, Heat Transfer through Fabrics. Naval Air Development Center, September 1970.

4Torvi D.A., A Finite Model of Heat Transfer in Skin Subjected to a Flash Fire. University of Alberta, Spring 1992.

5J. B. Perkins, H. E. Pearse, and H. D. Kingsley, Studies on Flash Burns: The Relation of the Time and Intensity of Applied Thermal Energy to the Severity of Burns, University of Rochester Atomic Energy Project, Rochester, NY, UR-217, December 1952.

6Wordie, H. Parr, Skill Lesion Threshold Values for Laser Radiation as Compared with Safety Standards. US Army Medical Research Laboratory. February 1969.

7IPCS. Lasers and Optical Radiation. World Health Organization, Geneva, 1982.


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Article 220, Continuation

Posted By Randy Hunter, Monday, July 02, 2012
Updated: Monday, September 10, 2012

Starting with Part III of Article 220, which is titled Feeder and Service Load Calculation, we will finally start with the actual math work. First, we will discuss demand factors as referenced in 220.42, and listed in Table 220.42. These demand factors take into consideration that in buildings we don’t normally have every electrical device operating at the same time. For instance, in a residence you will see in the table that the first 3000 VA is taken at 100%, then from there on we step down to a factor of 35% up to 120,000 VA, then down to 25% beyond that. If we refer to Table 220.12, we find that in dwelling units we use 3 VA per square foot. When doing the math, we have to take the first 1000 square feet at 100%.

So let’s look at this in a different way. Pretend there is a bubble around you and you will only need to have the electrical devices within your bubble turned on. As you move around, you normally turn off devices as you go from one area to another, so with this picture in mind, imagine this bubble as being about a 1000 square feet. This will help to explain why we take a small portion of a dwelling at 100% demand factor instead of the entire home. So if you think of it this way, the concept is to include the area immediately around you, where you will be using electrical devices calculated as if you have everything on near you and the areas where you are not present and hopefully have the lights, etc., turned off are not counted. Now of course, this is only conceptual, as you may have the washing machine or oven operating while you are doing something in another part of the house, or you may have multiple occupants in different part of the home. Demand factors still work out in practical experience because most homes never use more than a fraction of their service capacity except in very rare instances (big gatherings like Thanksgiving come to mind). Of course, I always get this comment, "You should come to my house where nothing gets turned off!”

Photo 1. Pictured is a high rise condominium project. The load calculations for this project were able to take into account large reductions by using the demand factors provided in Article 220. During construction, the installed load values continued to change and many of the feeders installed for several units had to be abandoned and re-run due to the increased loads as the owners selected different appliances. The inset photo shows one of the two penthouse units which are each three-story units.

Photo 1. Pictured is a high rise condominium project. The load calculations for this project were able to take into account large reductions by using the demand factors provided in Article 220. During construction, the installed load values continued to change and many of the feeders installed for several units had to be abandoned and re-run due to the increased loads as the owners selected different appliances. The inset photo shows one of the two penthouse units which are each three-story units.

Taking this concept forward, we understand that load diversity applies to every type of building. Generally speaking, the larger the facility the more we are allowed to use demand factors, as we can see in Table 220.42. Examples would include warehouses where we take the first 12,500 square feet at 100% and then anything over at just 50%. Hotels and hospitals even start out without any portion being calculated at 100%, starting at 40% or 50%. However, there is a footnote regarding these two types of occupancies, and if there are any areas in which all the lighting is likely to be used at one time then we are not allowed to use a demand factor for that portion of the facility. These areas are likely to be operating rooms, ballrooms or dining rooms. When you sit back and think about it, it makes pretty good sense. Please take the time to review Table 220.42 so you are familiar with typical demand factors for different types of occupancies.

Section 220.43 covers the demand factors for both show windows and track lighting. This is very important to remember as show windows are present in nearly all retail stores that have window fronts. Here we don’t have a reduction for larger installations, just a flat figure per linear foot of the units.

In other than dwelling units, we consider loads associated with the receptacles. To do this, remember we calculate these at 180 VA per receptacle yoke [Section 220.14(I)]. Starting with this number, take the number of receptacles to get a total VA for the receptacle load and then we take the first 10,000 VA at 100%; then any receptacle loads over 10,000, we are allowed to use a 50% demand reduction. Remember that in the last article, we discussed in 220.14(K) (which deals with banks or office buildings) that we have to use the larger of two methods to get our total receptacle loads. Once we have our total receptacle load we can apply the demand allowances.

Figure 1. This is a residential load calculation, courtesy of Wright Engineers. Please note the code references to the right.

Figure 1. This is a residential load calculation, courtesy of Wright Engineers. Please note the code references to the right.

Motor loads will be added into our calculations as mentioned in 220.50, using Articles 430 and 440. In 220.51 we learn how to add in fixed electric space heating; in a nutshell, we just add these in at 100 percent.

We move on to 220.52 through 220.56, which specifically deal with dwelling units only. These items are to be taken on top of the general lighting loads calculated from the square footage and reduced for demand factors. We start out addressing the small-appliance circuit load, for which we know from 210.11(C)(1) that we have to have a minimum of two small-appliance circuits; these are to be added in at 1500 VA per circuit. So if we have the minimum of 2 circuits that would be 3000 VA, 3 would be 4500 VA and so on. Some of the larger custom homes have had up to 4 or 5 small-appliance circuits. Next, we address the laundry circuit, and again it is taken at 1500 VA per circuit. Remember this is the general laundry circuit. If you have an electric dryer, we add that on top of this figure. In 220.54 we have the requirements for electric dryers; we generally just use 5000 VA for the dryer in single-family dwellings.

The next item in dwellings to consider is appliances, which include such items as dishwashers, trash compactors, water heaters, garbage disposals, built-in appliance centers, built-in microwaves, instant hot water dispensers, warming drawers, and wine coolers. The key issue with these is that according to 220.53, if these are fastened in place then we take each of these at the load rating of the factory label and add them to our calculations. There is one bit of help, which is if you have 4 or more of these, then you can apply a 75% demand factor to the sum of the devices. The thinking is that you aren’t going to be running more than three at one time. During the plan submittal stage of a dwelling unit, the exact label information is hard to come by as the appliances have not usually been selected, at least not the exact model and options, so we often have to use some approximate numbers here based on cut sheets of what is expected to be utilized. One thing to keep in mind is if any of the equipment is gas instead of electric, then we simply note "gas” next to those items during a load calculation.

The next items for dwelling units are the calculations for electric clothes dryers and ranges or other cooking appliances. The most common practice here is an automatic 5000 VA figure for the electric dryer, and the base figure of 8000 VA for the electric oven/range. Again, this is the standard starting point for electric cooking units; however, if you know the exact loads and they fit into the allowances of Note 3 in Table 220.55, you may also use this method of calculation. For single-family dwellings, we seldom see anything other than the standard 8000 VA figure. However, when we get out of the single-family world and start to look at multi-family, then the 75% appliance factor and Tables 220.54 and 220.55 provide demand factors which will reduce the size of the main service, again based on the concept that not every tenant will be using every device at the same time as other tenants. This becomes a huge advantage when dealing with high-rise condominiums, for example, where you may have 100 occupancies or more. For dryers, if you have 43 or more in the facility, then we only use 25% of the value for sizing the service; electric ranges between 3.5 and 8.75 KW would use a 16% figure for more than 60 units.

Enough on dwellings for now, let’s move on to other occupancies. Section 220.56 addresses kitchen equipment in other than dwelling unit(s). This is directed toward commercial kitchens that utilize commercial electric cooking equipment. Commercial food service facilities have a high number of different devices. To name a few: deep fryers, ovens, cooktops, warming drawers, heat lamps, warming heaters, dishwashers, flash heaters for some dishwashers, microwaves, soup-pots, steam tables, refrigerators, freezers, and so on. Performing inspections in kitchens is always a time consuming and tedious procedure, you have to look at each piece of equipment separately, review the load requirements, the disconnecting means, proper access to the electrical panels, etc. However, here we are only concerned with the load calculations for those units, and we are referred to Table 220.56 which has demand figures depending on the number of pieces of electrical equipment to be used. As you can see from my list, it isn’t too hard to get to the chart maximum of 6 or more units, and at that number a 65% demand factor can be applied. Often times equipment is not always delivered according to the approved plans or load calculations; for instance, a piece of equipment that was originally shown as a gas unit gets changed to electric, or equipment gets added without the knowledge of the design professional. I’ve even connected equipment that was shown as gas and the bottom cooking surface was gas; however, the upper portion of the clamshell style grill was electric. My main point here is that commercial kitchens are electrical pigs and we have to be very thorough during the plans check, load calculations and inspections process.

Figure 2. Commercial load calculation, courtesy of Wright Engineers.

Figure 2. Commercial load calculation, courtesy of Wright Engineers.

The next section 220.60 deals with non-coincident loads. The language states that where it is unlikely that two or more non-coincidental loads will be in use simultaneously, then it will be permitted to only use the larger of items for calculating the load. To me, this concept is pretty simple but let’s explore some examples. First, if you have an air-conditioning unit which only provides cooling and you use electric baseboard heat for your heating source, then you only need to add the one with the larger electric demand for the purpose of load calculation. Along the same line, when I first started doing electrical work in Las Vegas, several of the older houses had air-conditioning units which were not heat pumps units. They were straight cooling and had electric resistance heat strips for heating, so when we did the load calculations you had to know the size of the strips which are rated in kW, and then compare that load to that of the AC compressor and the outdoor fan. With that information, we would only use whatever the larger one was for sizing the service. I have to add in a note here regarding heat-pump units; these are air conditioners which use the compressor and coolant gas for both heating and cooling. They do this by reversing the direction of the refrigerant. Often these heat-pumps will also have electric resistance heat strips for emergency heating assistance in extremely cold conditions. From my experience, some of these utilize both the heat strip and compressor so you have to take the total load into account. These units often have labels for the minimum circuit ampacity with several options which the installer is to check when the resistance heater strip is added in. Once this is checked, it now gives you the new information to use for loads, wire size and overcurrent protection.

The sizing of the neutral for the service or feeder is addressed in 220.61. If you read this, it will probably just confuse you like it does the rest of us. So let’s think about the neutral. If you have any piece of equipment for which the main consumption of power is has very little or no phase-to-neutral load, then why do we need to have the neutral sized equal to the ungrounded conductors? The equipment which typically falls into this area are ovens, ranges, and dryers. So the code states we can do a 70% factor for sizing the neutral for the loads associated with these types of equipment. That seems like a lot of calculation, so I will share what typically happens instead. Usually the neutral is two wire sizes smaller than the ungrounded conductors; so as an example, a 200-amp residential service could have 4/0 AWG aluminum ungrounded conductors and a 2/0 AWG neutral. If no 240-volt loads are in a dwelling, then the code tells us in 220.61(A) the neutral has to be sized for the maximum net calculated load between the neutral and any one of the ungrounded conductors, which means it must be equal to the ungrounded conductors.

Photo 2.

Photo 2. "This is a good example of one of the new modern high rise projects now being constructed”


Part IV of Article 220 deals with optional feeder and service load calculations. Because these articles are based on general information for combination inspectors, I won’t be going into any details on these methods as they are pretty specialized according to the type of occupancy and how to handle existing loads. Please take a moment to review these, since in some parts of the country they are used. For decades they were not allowed in Clark County, Nevada, due to the heavy use of cooling equipment; however, they have started to allow them with a modification requiring a 100% factor for A/C loads.

The last part of Article 220 is Part V which deals with farm loads — buildings and other loads. Here we address how to size the services or feeders for a farm dwelling, which is the same as we did for other dwellings. However, if we share the service or feeder with other farm buildings, then we have demand factors that may be applied. Again, this is not in the common realm of work performed by a combination inspector, so if this may apply to you, please review.

I also recommend reviewing the example of load calculations pictured here, as it provides an actual view of what we have described within this article. In the next article, we will cover Article 225, Outside Branch Circuits and Feeders.


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SCADA and Homeland Security

Posted By Jonathan Cadd, Monday, July 02, 2012
Updated: Monday, September 10, 2012

Not so long ago when an industrial plant operator needed to redirect flows or even start and stop a procedure, it was often times a long and arduous process of turning valves, painstaking sequences, and long trips from area to area within a plant or facility with a handheld radio. While there are still facilities that might require this type of manual operation, the landscape has significantly changed, with the introduction of supervisory control and data acquisition (SCADA).

While all of this control and operational stability has benefitted the global industrial process as a whole, what happens when the control and data are compromised or even lost? Will we have the ability and the sheer manpower to physically achieve and complete the many steps and processes within industrial facilities or plants that have now been taken over and controlled by the massive computer systems designed to control and acquire data through a system such as SCADA? Therein lays the question or perhaps the difficulty.

Photo 1. The QATAR National Control Center is a highly sophisticated example of how SCADA can be used for control of systems and the data acquisition from those systems as well.

Photo 1. The QATAR National Control Center is a highly sophisticated example of how SCADA can be used for control of systems and the data acquisition from those systems as well.

Our Journey

In this article we will look at how this relatively new control technology has changed the industry forever, and has placed the control and data of many complex processes at the operator’s fingertips. We will visit the NEC and see what guidance it has for us and the requirements we need to adhere to when considering and installing SCADA systems. And we will look at what’s on the horizon with regard to Homeland Security, Article 708 – Critical Operations Power Systems (COPS), the overall security of SCADA systems, and how it is all integrated into the mix.

A Little History

Supervisory control and data acquisition (SCADA) systems have become a critical part of the landscape of late, silently and methodically marching forward, providing the vital infrastructure for a new industrial revolution. Not only have these systems been proven to save time and to provide economic benefits, the new operational flexibility that is now gained by SCADA can have enormous benefits when it comes to safe control of systems and data that can be viewed about any process instantaneously.

Long gone are the days when industrial plant operators jumped in golf carts and were aided by maps of industrial pipeline drawings to figure out which valve to open or close and at what stage of the process. Not only was this dangerous, it was very inefficient and costly, requiring many men and a lot of time.

While the process world grew and perfected these systems, additional elements were born, digital control systems (DCS) and programmable logic controllers (PLC), to further enhance this supervisory control and data acquisition. With this trifecta, if you will, we now have all the control we need to speed up, and safely conduct these processes and increase production to supply the demand of goods and products for a hungry nation.

Can we use these requirements?

The requirements for SCADA systems reside in the informative annexes of the NEC, in Annex G.

Photo 2. SCADA today is a complex system of computers, PLCs, and high-speed communication to accomplish the many tasks that are necessary for safety and efficiency.

Photo 2. SCADA today is a complex system of computers, PLCs, and high-speed communication to accomplish the many tasks that are necessary for safety and efficiency.

While it is not part of the requirements of the NEC, it has been included for informational purposes, as well as adoption. Conversely, if an informative annex is referenced in the body of the NEC, such as Annex C – Tables, then it becomes part of the NEC by reference.

SCADA, the NEC, and Critical Operations Power Systems (COPS)

The requirements for SCADA systems have incorporated Homeland Security requirements by the integration of Article 708 – Critical Operations Power Systems (COPS) into Informative Annex G.

This provision among others is to ensure that a SCADA system for the COPS loads is completely separate from the building management SCADA system.

Informative Annex G – (A)(2) General – requires that when a SCADA system is employed "no single point failure should be able to disable the SCADA system.”

We also see in Informative Annex G – (A)(3) General – what the SCADA system is permitted to control. This section indicates that the SCADA system cannot only control and monitor mission critical electrical and mechanical systems, but other systems as well. These systems include but are not limited to (a) the fire alarm system, (b) the security system, (c) power distribution, (d) HVAC and ventilation [damper position, airflow speed and direction], (f) load shedding, and (g) fuel levels or hours of operation.

Planning and the NEC

Before the installation or deployment of a SCADA system, proper planning needs to be done. Informative Annex G – (A)(4) General – requires that an operation and maintenance analysis, as well as a risk assessment be completed to provide maintenance parameter data, prior to installation.

One should also notice that if a redundant system is provided for backup, there are also requirements.

Informative Annex G – (A)(5) General – indicates that if a redundant system is employed, it shall be in either warm or hot standby mode at all times.

Open Protocol

When a SCADA system is employed not only does the NEC tell us in Informative Annex G – (A)(6)that the controller must be a programmable logic control (PLC) , Informative Annex G – (A)(7) requires that the SCADA system must utilize open, not proprietary, protocols.

Damage Assessment and Graphical User Interface

In the unfortunate event of a system failure or problem, Informative Annex G – (A)(8) General – informs us that the SCADA system is required to not only assess the damage to the facility, but to also determine system integrity after an event.

The actual monitor display itself is required to have a graphical user interface that will allow the user to readily recognize all major components that are being monitored as well as controlled by the SCADA system, with easily discerned color schemes as identified by Informative Annex G – (A)(9) General.

Storage of Critical System Parameters

Photo 3. SCADA real time automation controller (RTAC)

Photo 3. SCADA real time automation controller (RTAC)

To round out the general requirements for SCADA it is indicated in Informative Annex G – (A)(10) General – that the system shall have the capability to provide storage of critical system parameters for at least 15 minutes or more, if an out-of-limit condition exists anywhere within the system. This data is also to be stored off site at a separate secure data storage facility located off site.

SCADA systems and the Power Supply

Informative Annex G – (B) Power Supply – lays out the requirements in the NEC to ensure that the power that is being provided to the SCADA system itself is well-protected, not subject to failure. In addition to the regular power supply, a direct-current station battery system, rated between 24 and 125 volts dc, and capable of 72 hours of sustainment is the very minimum allowed.

All of the batteries supplying the SCADA system will be required to be completely separated from batteries of other electrical systems.

All SCADA system power supplies are required to have a properly installed surge protection device (SPD) at its terminals with a direct low-impedance path to earth, as well-protected and unprotected circuits are required to employ physical barriers to prevent coupling.

Security and SCADA

Informative Annex G – (C) Security against Hazards has six criteria that are required to be adhered to in the effort to prevent the SCADA system from becoming adversely affected.

Controlled physical access by authorized personnel to only the system operational controls and software is to be provided.

The SCADA systems and its components are required to be protected against dust, dirt, water, and other contaminants by appropriate encloses for the environment served.

Conduit or EMT cannot violate the integrity of the SCADA system enclosure.

The SCADA systems are required to be located in the same secured locations as the systems that they are monitoring.

The SCADA system is required to be provided with a dry agent fire protection system or double interlocked pre action system using cross zoned detection, to minimize the threat of accidental water discharge into the unprotected equipment. The fire protection system is also required to be monitored in accordance with NFPA-72 The National Fire Alarm and Signaling Code

SCADA systems are not to be connected to other network communication systems outside the secure location without encryption or the use of fiber optics.

Are we safe yet?

While we hope that these new provisions in the NEC, born from the Homeland Security Act, are enough to at least fend off threats to our national infrastructure, there are those who work tirelessly to develop the means to cripple us with devices such as electromagnetic pulse (EMP) and high-powered microwave (HPM) that can adversely affect SCADA systems and create havoc in a moment’s notice.

Maintenance and Testing of SCADA systems

For any system to work properly maintenance is required. Informative Annex G – (D) Maintenance tells us that a documented preventative maintenance program is required to allow the testing, troubleshooting, repair, and/or replacement of a component or subsystem, while redundant components or subsystems are actually serving the load.

SCADA systems require periodic testing under actual and / or simulated contingency conditions. The reader can find the testing requirements and intervals in NFPA 70B Recommended Practice for Electrical Equipment Maintenance.

What have we learned?

Not only have we visited the industrial facility process and have seen how SCADA systems have revolutionized the industry across the board, we have seen that the NEC has many very important requirements that need to be adhered to that will ensure the safe, efficient and long-term operation of a SCADA system. We have also looked at the added security and system processes that are now required to avoid the possibility of our vital national infrastructure from coming under attack from those who would promote chaos and civil unrest.

The accelerating penetration of SCADA systems, along with their electronic cousins, digital control systems (DCS) and programmable logic controllers (PLC), as critical elements in every aspect of every critical infrastructure in the Nation, is both inevitable and inexorable. While conferring economic benefit and enormous new operational agility, the growing dependence of our infrastructures on these omnipresent control systems represents a new vector of vulnerability in the evolving digital age of the 21st century, such as cyber security.


Read more by Jonathan Cadd

Tags:  Featured  July-August 2012 

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Standard Motor Control Circuits

Posted By Stephen J. Vidal , Monday, July 02, 2012
Updated: Monday, September 10, 2012

Single-phase and three-phase AC squirrel cage induction motors need some type of circuit to initiate a start or stop function. Usually single-phase motors and smaller horsepower three-phase motors can be started with full voltage across the line. However, larger horsepower three-phase motors require reduced voltage starting techniques.

Power circuits vs. Control circuits

Typically two type of circuits are used in motor control — the line voltagepower circuitand thecontrol circuit. The power circuit in full-voltage across-the-line starting consists of the overcurrent protective device (OCPD), whether fuses or circuit breaker; the line conductors that terminate on the L1, L2, and L3 terminals; the magnetic motor starter or solid-state device; and the load conductors that terminate on the T1, T2, and T3 terminals.

Thecontrol circuitconsists of components of the ladder diagram — such as start and stop pushbuttons, relay coils, pilot lights, and any other variety of contact closure devices, like limit switches, pressure switches, temperature controllers, proximity sensors, or float switches. The control circuit can be further classified as two-wire or three-wire depending on the application. It is also important to note that the power circuit is sized according to the voltage rating of the motor load: 115-V, 200-V, 230-V, 460-V or 575-V. The control circuit can operate at the same voltage as the power circuit, but it can also operate at lower voltages by using a machine tool transformer to step down the voltage to safer levels.

The diagram for a typical full-voltage across-the-line starting circuit is shown in figure 1. This diagram shows both thepower circuitand thecontrol circuit. Note the control circuit is a three-wire ladder diagram control circuit, which works well for smaller horsepower three-phase motors. The electric utility will have rules for how large a motor can be started across the line. Once the horsepower of a motor exceeds that rating, reduced-voltage starting techniques must be used. Motors are inductive loads; therefore, they have very high starting currents in the range of 2.5 to 10 times the full load running current of the motor. This excessive inrush current, also called locked-rotor current, causes voltage fluctuations on the lines. You probably have observed the effect of inrush current whenever the lights in a building dip as HVAC equipment comes online. When this excessive inrush current is drawn from the voltage source for a few seconds, it causes a voltage drop. This voltage drop means a lower voltage is available to equipment; and lighting fixtures, in particular, will flicker.

Figure 1. Full voltage three-wire control

Figure 1. Full voltage three-wire control

Reduced voltage starters

There are primarily six styles of reduced-voltage starters: primary resistor, reactor, autotransformer, part-winding, wye-delta, and solid state. Solid-state reduced-voltage starters are very common as they interface well with variable frequency drives (VFDs) and programmable logic controllers (PLCs).

Primary resistor startersuse resistors in series with the motor leads during the start function. Since this is now a series circuit, the applied voltage is dropped between the series resistor and the motor winding, causing a lower starting current. A timing relay operates a control relay whose contacts short the series resistors once startup is achieved.

Reactor startersoperate in the same manner, except reactors are used instead of resistors. Reactor starters are far less common than they were in the past.

More Motor Circuits

Autotransformer starters use tapped autotransformers, with taps typically set at 50%, 65% of 80% of the available line voltage. Relying on the concept of "turns ratio” in a transformer, this type of starter allows for smaller currents on the line side as seen by the electric utility and larger currents on the load side as seen by the motor during startup. An autotransformer is different from a two-winding transformer in that it does not provide electrical isolation between the primary and secondary windings. A step-up autotransformer is often called a "boosting” autotransformer, and a step-down autotransformer is called a "bucking” autotransformer.

Remember the "turns ratio” for a transformer? When looking at voltage, you rely on the following formula:

Vprimary/ Vsecondary= Nprimary/ Nsecondary

For current, you rely on this formula:

Iprimary/ Isecondary= Nsecondary/ Nprimary

Let’s take a simple example for illustration. A 1 kVA transformer has a 240-V primary and a 120-V secondary. The primary current is 4.17 A at 240 V, while the secondary current is 8.33 A at 120 V. The transformer has a 2:1 ratio. The voltage is stepped down by a factor of two, while the current is stepped up by a factor of two. This principle allows the autotransformer-type starter to operate.

Thepart-winding starter is designed to work with a part-winding motor that has two sets of identical windings. You can use 230/460V dual voltage motors, but you must exercise extreme caution. The concept is that the 230/460V motor operates at 230 V with the windings in parallel. Therefore, one half of the motor windings are in the circuit during startup; then a few seconds later, the other half of the motor windings are brought into the circuit. Serious problems can develop if the timing circuit does not connect the other half of the motor windings immediately after startup.

Awye-delta starter operates by allowing the motor to be started in a wye configuration and then run in a delta configuration. Utilizing this configuration allows the inrush current to be lower during the startup while still maintaining a starting torque of approximately 33%. Open transition is an important consideration to keep in mind with wye-delta starters because there will be a period of time between the wye configuration for start and the delta configuration for run when the motor windings will be disconnected. Closed transition starters overcome this disadvantage but at a much higher cost.

Solid-state starters are often called "soft start” starters because they use silicon-controlled rectifiers (SCRs) to accomplish the task. Gas-filled vacuum tubes called thyratrons were the early version of the solid-state thyristor family which includes SCRs Triacs, Diacs, and UJT (unijunction transistors). The SCR has three elements called the anode, cathode, and gate. By applying a signal to the gate element at precisely the right time, you can control how much current the SCR will either pass or block during a cycle; this is known as phase control. The ability of this device to allow either partial conduction or full conduction during a cycle offers much flexibility to the designer. This capability allows for precise control of current to a motor load during startup.

Ladder Control Circuits

The two types of ladder control circuits commonly used are the two-wire control and the three-wire control circuit. The two-wire control circuit uses maintained contact devices to control the magnetic motor starter. The three-wire control circuit uses momentary contact devices that control the magnetic motor starter.

The two-wire control circuit is shown in figure 2. It consists of a normally open maintained contact device that, when closed, energizes the coil of a magnetic motor starter, which, in turn, energizes the connected motor load. The two-wire control circuit provides what is known as "low-voltage release.” In the event of a power failure, the magnetic motor starter will drop out. Once power is restored, the magnetic motor starter will automatically re-energize, provided that none of the maintained contact devices have changed state. This can be very advantageous in applications such as refrigeration or air conditioning where you do not need someone to restart the equipment after a power failure. However, it can be extremely dangerous in applications where equipment will start automatically, placing the operator in danger.

Figure 2. Full voltage two-wire control

Figure 2. Full voltage two-wire control

The three-wire control circuit is shown in figure 1. It consists of a normally closed stop button (STOP), a normally open start button (START), a sealing contact (M), and the coil of a magnetic motor starter. When the normally open start button is pressed, the coil of magnetic motor starter (M) is energized. An auxiliary contact of (M) seals around the start button to provide a latched circuit. Pressing the normally closed stop button disrupts the circuit. The three-wire control circuit provides what is known as "low-voltage protection.” In the event of a power failure, the magnetic motor starter will drop out. However, in this case, once power is restored the magnetic motor starter will not automatically re-energize. The operator must press the start button to start the sequence of operations once again.

Compared to the two-wire control circuit, the three-wire control circuit provides much more safety to the operator because the machinery will not automatically start once power has been restored. Figure 3 illustrates a control circuit with multiple start and stop pushbuttons. In this circuit, multiple normally closed stop buttons are placed in series, and multiple normally open start buttons are placed in parallel to operate a magnetic motor starter. This is a common application of a three-wire control circuit in which you need to start and stop the same motor from multiple locations within the facility. The three-wire control circuit can be utilized in a variety of ways to meet specific circuit application.

Figure 3. Multiple stop/start control circuit

Figure 3. Multiple stop/start control circuit

AC motor control is a very interesting and specialized segment of our industry. Electromechanical magnetic motor starters have been the standard for many years. Solid-state devices have allowed for greater control of circuit parameters while allowing true integration with variable frequency drives and programmable logic controllers.


Read more by Stephen J. Vidal

Tags:  Featured  July-August 2012 

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