Posted By Steve Foran,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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Can you think of an accomplishment that is a source of great pride?
My foray into selective coordination served as my introduction to the electric utility industry. A coordination study was my very first job as an engineer. I had to review and update the protection along a number of 12-kV and 25-kV rural distribution feeders that had expanded as a result of many cottage developments. The loads had increased significantly as did the length of the feeders as well as the number of branch circuits. Quite honestly, when I started, I had no idea of what I was being asked to do.
Looking back almost 25 years to this project, several things really stand out in my memory. Namely, I was eager to do an excellent job and I learned a lot in the process of doing it. I developed a very good relationship with a senior engineer named Dave who became a mentor to me. He shared his expertise unselfishly and helped me successfully complete the project. In fact, it was from him that I learned the importance of practicality in engineering work. Dave once stoically told me, "Steve, never forget that "V” equals "I” times "R”.” Having just spent five years studying electrical engineering, I thought V=I*R was an odd piece of advice, but later realized the wisdom in its simplicity when faced with very challenging troubleshooting problems. Lastly, as I drive around that cottage region today, I can still see devices covered with my fingerprints from that coordination study back in 1987. As you might imagine, this continues to serve as a great source of pride although my wife is probably sick of hearing me drone on about the protection coordination stories I have told her dozens of times.
There are two lessons that strike me about this story. The first is that my success was almost totally dependent on others. True, I was keen and anxious to do a great job and I worked hard, but without Dave, the senior engineer who also became a good friend, I would have been lost. I have found this to be a universal truth in all aspects of life. So much of our success is dependent upon the work and contribution of others. This truth is not always acknowledged because it is sometimes hard to see. But consider that even Bill Gates had to depend on someone else to design and manufacture computers to run the software that Microsoft produces.
You likely have many successes that you can easily attribute to others. But what about that success you think you achieved on your own? For this success, I challenge you to dig a bit deeper and identify who helped you and what they did. If you have the courage to do this, here is the guarantee — you will be profoundly grateful for them and their contribution.
The second point that amazes me is how the impact of that simple coordination study has lasted for such a long time. I have not worked at the utility for more than ten years and yet the result of my contribution continues to affect how the power system operates in that area. There probably have been some changes to the protection, but these changes would have been built upon the work I did in my very first engineering role. Regardless, this remains a source of pride. The critical lesson you must remember is that everything you do makes a difference. Whether it is a coordination study in a processing plant, an inspection at a renovated home, a performance appraisal for a member of your team, how you greet your customers, or the way in which you say, "Thank-you” to the server at a restaurant —everything you do makes a difference. Remember this important caveat though: you get to decide if your actions are going to make a positive difference or a negative difference.
Thinking back to that accomplishment of yours that makes you proud, I bet you can see the positive impact that it has made in the world and I bet you had a mentor, a Dave, who helped you succeed.
Now go thank your Dave and then be a Dave for someone else.
Read more by Steve Foran
Posted By Marcus Sampson,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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The carnival or traveling show will only be in town for the duration of the local festival or county fair before it’s torn down, packed up and moved to the next location. While the "jump” is something the carnie crew and roustabouts do each week, this may be the only inspection of a transient enterprise you are called on to do the entire year. Making sure your neighbors, friends and family are not exposed to electrical hazards means having an understanding of the National Electrical Code rules for transient electrical distribution systems.
Carnivals are amusement parks on wheels and a look at the wiring can reveal the negative effects of frequent teardowns, arduous road trips and rushed re-assembly. Many times, the various parts of the electrical distribution are composed and mended from random parts and pieces and inevitably show the consequences of life on the road. Equipment is subject to the moisture, dirt and vibration during hundreds or even thousands of miles traveled each year. Cords are often damaged by exposure to oils, gasoline, direct sunlight, foot and vehicular traffic and temperature extremes, arriving on-site worse for the wear. Disconnects, distribution boxes and cords are unloaded at each stop in various shades of disrepair.
Carnival work is grueling and repetitive. Workers are not always welcoming to code officials, who frequently add to their already long list of duties. Often, trucks arrive barely in time to set up before their first performance and taking time for safety inspections is not a priority. With pressure from event organizers, joint owners and ride operators, time for completing the inspections, identifying violations, and making the necessary re-inspections is limited.
With the safety of the carnival workers and the public at stake, it is important to check the entire distribution system for properly sized overcurrent devices, grounding and bonding continuity and GFCI functionality.
It’s easy to think that the portable wiring for traveling shows is permitted to rest a bit below the usual standards, but all temporary wiring must comply with the provisions for permanently installed wiring found in chapters 1 through 4. While some accommodations are found in Articles 525 or 590 for the conditions, additional provisions such as GFCI protection of branch circuits and equipotential bonding of equipment also apply.
Photo 1. Inspectors check the lighting on a kiddie ride for grounding continuity.
When you first arrive on the carnival or festival site, be sure to note the location of any overhead power distribution. Portable structures, including carnival rides, games, concessions and other units cannot be located within the area beneath a 4.5 m (15 ft) horizontal distance from conductors operating at more than 600-volts. Where the overhead conductors operate at 600-volts or less, no part of a portable structure can be located within a 4.5 m (15 ft) radius of the conductors.
Look around for rides or attractions that use large volumes of water and verify that the overhead conductor clearances of Table 680.8 are also met.
There are multiple options for power sources for carnival, fairs and other outdoor venues. Power can be obtained from permanently installed distribution in the park or fairgrounds or can be supplied directly from the local electric utility. Some shows will use a "hot truck,” a trailer with a high-voltage transformer meant to be connected to a utility primary distribution system, but often, one or more large generators will be positioned at deliberate locations.
The service equipment is generally located away from the action because it is not permitted to be accessible to unqualified persons unless it is lockable. From the service, feeder conductors and cables are laid out to the weatherproof power distribution units which, in turn, supply the game trailers, rides, joints, concession stands, ticket booths, etc.
Photo 2. Conductors used for portable power distribution should be examined for physical damage.
Grounding Electrode System
Keep in mind that both portable transformers and generators will be separately derived systems. Be sure to verify that the system bonding jumper is installed and sized per NEC 250.28. Next, explore the available grounding electrode options:
- Permanently installed rods or concrete-encased electrodes may be available if the site is host to recurring events.
- A plate electrode may also be used, if it meets the requirements of 250.52(A)(7).
- Water or other underground metal piping in the area may meet the provisions of 250.52.
- If pipe or rods are used to create the grounding electrode system, two must be installed at least 1.8 m (6 ft) apart and they must be fully driven. Alternatively, 3 or more partially driven rods spaced at least 1.8 m (6 ft) apart may be an acceptable means of achieving the required low-impedance earth contact. To assure a connection to earth with no more than 25 ohms of resistance, on-site soil conditions must also be considered.
NOTE: Before an 2.44 m (8 ft), copper-plated, pointed rod is fully driven, location services should be consulted to avoid unintentional contact with underground utilities.
Check the size of the unspliced grounding electrode conductor against Table 250.66, remembering that a conductor to a rod electrode need not be larger than 6 AWG copper, and must be securely fastened in place or protected by a raceway or armor.
Feeders and Branch Circuits
Look at the generator and at all the distribution boxes to be sure that the conductor terminations are made with suitable terminals or lugs used for no other purpose, such as securing devices within the equipment or mounting the equipment. Make sure that terminals and lugs contain only one conductor, unless specifically approved for more.
Photo 3. Cord caps and other equipment can be damaged during the road trip.
Whether the electrical distribution is extended from a transformer, generator or existing power source, it must be installed in approved wiring methods. Be sure that feeder and branch-circuit cables are sized per Article 210 and provided with overcurrent protection per Table 310.15(B)(1) or with the tap rules of Section 240.21.
Although you may see open wiring in walls of units that were built prior to today’s wiring standards, open wiring is not acceptable within the walls of attractions or concession trailers. In addition, you may find some older carnival rides that have the original open conductors installed along framework and covered with layers of paint. If these do not show signs of damage or disrepair, they may still be accepted.
That said, all damaged wiring needs to be repaired with approved methods or replaced. Cord and cable sheaths must be fully inserted and secured into cord caps, plugs or cam-lock devices.
Flexible Cords and Cables
Overcurrent protection for feeders and branch circuits extended using flexible cords and cables cannot exceed the maximum allowable ampacities in Table 400.5(A) (1) and (2) which has specifications for both single and multi-conductor cable types. Note that the adjustment factors found in Table 400.5(A)(3) for cables with more than 3 current-carrying conductors are the same as those in 310.15(B)(3)(a) for more than 3 current-carrying conductors in a raceway.
You may discover both single and multi-conductor cords with no type designation. These are not permitted, as cords are required to be type recognized in Article 400. Single conductor cables such as Type PPO or Type W are permitted in sizes 2 AWG or larger, but welding cables and locomotive cables are not allowed.
Photo 4. Conductors, cords and cables that cannot be properly repaired must be replaced.
Cords must be sunlight-, oil- and water-resistant and approved for extra-hard usage, although hard-usage cords and those with a "J” in the designation are permitted to be used within a portable unit, where not subject to physical damage. For example, SJO cords may be used to supply lighting inside a tent, where the cords are routed up and secured along the support poles.
Special precautions apply when cam-lock type connectors are used. These are quick-connect single conductor splicing and terminating devices, and while it’s easy to look at them as such, they are not attachment plugs or receptacles. They are meant to be installed and used by qualified persons and are to be guarded from accidental disconnection. When inspecting single-pole separable connectors look closely at the rules of 530.22 which apply, per 525.22(D).
As always, conductors of a feeder or branch circuit, including the equipment grounding conductor shall be part of the same cable assembly or must be grouped. Look to see that the individual conductors within cords are properly identified and that white conductors are only used as grounded — not equipment grounding — conductors.
You can easily use a simple lamp or audible-type tester on de-energized multi-conductor cords to verify continuity while verifying that conductors are properly terminated on cord caps and plugs.
Look at the overall distribution to assure that all the cords and cables are generally protected from physical damage. Splices are not permitted in cords; however, the outer covering of a cord may be repaired with listed shrink tubing products.
A good site plan will route all cables away from vehicular and pedestrian traffic, but some locations will be a challenge. In an open field, where a carnival or circus may set up for a few days, temporary shallow burial of the electrical distribution cords, permitted by NEC 525.20(G) may be appropriate.
Photo 5. Distribution boxes located in areas accessible to the public must be lockable.
If these solutions are impracticable, cable protection can be provided by mats or ramps, but be sure that they do not create a greater tripping hazard than the exposed cords alone. Extremely durable, commercial-grade ramps are available, but adequate protection can be achieved by other approved methods.
Junction Boxes and Panelboards
Check the separation of grounded circuit conductors from equipment grounding conductors and look for adequate wiring space within the enclosures, as well.
When looking at the large distribution boxes and panelboards make sure they are designed to be weatherproof and are situated so that the bottom of the enclosure is no less than 6 inches above the ground.
And when inspecting switch, circuit breaker and fuse enclosures be sure that they are dead-front, so operators and the public are never exposed to live parts.
Remember that all electrical equipment must be supplied by feeders and branch circuits that contain properly sized equipment grounding (bonding) conductors and that the continuity of the equipment grounding conductor system must be verified each time that portable electrical equipment is connected.
Not only receptacle devices, but all the equipment and enclosures must have the equipment grounding conductor connected to an approved grounding terminal. The EGCs are permitted to be bare; but if they are covered or insulated, single conductor or part of a multi-conductor cable, they must be green or identified with green paint or tape.
As with permanently installed wiring systems, the equipment grounding (bonding) conductors must be installed as a complete point-to-point system, so verify that enclosures are not used for grounding continuity.
On larger sites, you may see mobile units (rides, concessions, games, tents, etc.) supplied from different power sources and positioned with less than 3.6 m (12 ft) between them. Be sure that those units are bonded together to eliminate the possibility of potential difference, as required by 525.11. The bonding conductor can be covered, insulated or bare and is not required to be installed with other circuit conductors or in a raceway. Use the rating of the largest overcurrent device supplying the units and Table 250.122 to size the bonding conductor, keeping in mind that it cannot be smaller than 6 AWG.
Photo 6. Single-pole separable connectors (cam-locks) used for portable wiring on festival sites must be installed per 525.53(K).
Section 525.21 requires all rides, tents, and concessions to have a readily accessible switch that disconnects all power to the unit. The switch must disconnect the portable structure from all ungrounded conductors and be located within sight of and within 1.8 m (6 ft) of where the operator normally is stationed. This requirement is especially important when the ride is in operation; and for games and/or rides where the operator’s console is located away from the power disconnect switch, a shunt trip disconnecting device can be permitted. Be sure to check the functionality of the shunt-trip device.
Remember, too, that some larger rides may have a feeder for motor loads and a separate feeder for lighting, and if so, the switches must be positioned side-by-side or together, as this is intended as both a life safety switch as well as a maintenance disconnecting means. Check to see that these are clearly identified because in an emergency situation, it may not be the operator who needs to throw the switch to disconnect power.
When wiring for temporary lighting is installed inside tents, game trailers, concessions and other portable structures, it must be adequately secured with wire ties or other approved means. Festoon lighting or cord sets, both inside and outside, have to be installed at least 10 feet above ground where accessible to the public. The lamps themselves must be protected from accidental breakage by a suitable fixture guard.
While open conductors are generally not permitted for permanent or portable installations, they are allowed when used for festoon lighting or when part of a listed assembly. Section 225.6(B) requires a minimum of 12 AWG conductors for festoon lighting unless there is a messenger providing additional support. Be sure that neither the conductors nor the messengers are attached to a fire escape or secured to plumbing piping or downspouts.
Because of the electrical hazards associated with the use of electricity outdoors, all of the 125-volt, single-phase, 15- and 20-ampere non-locking-type receptacles on the festival grounds that are readily accessible to the general public or used for set-up are required to have GFCI protection.
But not only the receptacles, all equipment supplied from a 125-volt, single-phase, 15- or 20-ampere branch circuit that is readily accessible to the general public is required to have GFCI protection. This would include branch circuits used for free-standing perimeter lighting or lighting strings installed on fences as well as portable signs.
Both permanently installed GFCI receptacles and listed GFCI cord sets are permitted to be used to achieve the required level of life-safety protection. It is important for operators as well as inspectors to regularly check the functionality of the GFCI devices.
Remember the exception from the GFCI requirements for locking-type receptacles that are not accessible from grade. This provision would apply to the 120-volt interconnecting cables on removable portions of rides or concessions and that are not used to supply portable hand tools or equipment.
While 525.23 does require GFCI protection for almost all of the 15- and 20-amp, 120-volt distribution located outdoors, egress lighting that may be used in a tent or other portable structure is not permitted to be connected to a circuit or receptacle that is protected by a GFCI.
At the outset, it can seem like electrical inspections of Article 525 venues are significantly more difficult than inspections of other types of projects. However, the basic Code principals are the same whether the electricity is portable or permanent: a proper grounding electrode system; suitable overcurrent protection for services, feeders and branch circuits; guarding from weather and physical harm; intentional bonding of equipment and ground-fault circuit-interrupter protection for personnel where necessary.
Children of all ages attend the local festivals, fairs, carnivals and circuses without a thought toward their exposure to electrical hazards. Completing a thorough electrical inspection before the first customer enters the gate is the key to protecting your neighbors, friends and family.
Read more by Marcus Sampson
Posted By Steve Douglas,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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The Canadian Electrical Code consists of five parts. Part I covers the installation and maintenance of electrical equipment, Part II is the safety standards for electrical products, and Part III is for outside wiring. Part IV is the objective-based industrial electrical code, and Part VI, the electrical inspection code for existing residential occupancies. This article will focus on Part I amendments.
The Part I committee consists of Part I members, associate members, subcommittee chairs, subcommittee members. A list of these members, complete with affiliation, is located in the front of the Canadian Electrical Code (CE Code). The Part I committee consists of voting members and non-voting (associate members). The voting members consist of a maximum of 41 members, 16 of whom are regulatory authority representatives and the remainder, from the industry.
Appendix C of the CE Code sets out committee and subcommittee structures detailing responsibilities and expectations. Members include inspection authorities, manufacturers of electrical equipment, employers, employees, consultants, utilities, testing laboratories, underwriters, or fire marshals, primary and secondary industries, respective code-making panels of NEC and users. Presently we have 50 IAEI member positions covering each of the 43 subcommittees.
Six Steps to a Successful Code Change
Step 1. Fill out Annex B in Appendix C and send it to the standards administrator of the Canadian Electrical Code, Part I. Proposals need to include specific wording for a proposed new rule or rule change, the reasons for the request, and background information to support the change.
Step 2. The standards administrator fills out Annex A from Appendix C and sends a copy to the subcommittee chair.
Step 3. The subcommittee chair adds comments and returns the proposal, now referred to as a subject back to the standards administrator.
Step 4. The standards administrator sends the subject to the subcommittee members for their comments. Communication of the subcommittee uses the CSA Standards Development Online Workspace (SDOW)
Step 5. After the comments are received from the subcommittee members, the subject is sent back to the subcommittee chair.
The subcommittee chair then decides if the subject is ready to be sent to the Part I Committee for a ballot, or if there is a need for it to be resubmitted to the subcommittee with a reworded proposal or additional rationale. The original submitter may be consulted at this point to ensure the intent of the proposal remains as purposed.
Step 6. When the subcommittee has achieved consensus, the subject is forwarded via the standards administrator to the Part I Committee for a ballot. The Part I Committee meets yearly in June to discuss subjects that received negative ballots. Successful subjects are filed for inclusion in the next edition of the CE Code. Unsuccessful subjects may be returned to the subcommittee or closed. The most important part of the process is the original submission; the more detail and rationale provided the better the success rate.
This article is an update of an article published in March-April 2005 issue ofIAEI News.
Read more by Steve Douglas
Posted By Steve Douglas,
Sunday, September 02, 2012
Updated: Wednesday, September 19, 2012
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How may times have you said to yourself? "I don’t understand why that rule reads the way it does. It doesn’t take ________ into account. I think I could have done a better job.”
Every IAEI member has an opportunity to participate in development of the Canadian Electrical Code (CE Code). A very effective way is through the subcommittees. That is where the work is done and key decisions are made.
Participation is very easy — no time consuming meetings. All work is done by correspondence — telephone, letter, e-mail and on-line — with adequate time to respond. If you are a subcommittee member, you are in a special position to suggest changes at the subcommittee level. You have an opportunity to comment on every proposed revision. You are advised of the eventual subcommittee recommendation and CE Code Part I Committee decision.
How does one get involved? Easy — let me know the code section where you have an interest and a brief resume with emphasis on your experience with both the code and the section where you have an interest. I’ve included a form. You may wish to indicate several sections. I’ll do what I can within certain constraints:
The IAEI already has appointees on all of sections (see table 1). If you want to be on a particular section subcommittee, you may have to wait.
Appendix C Clause C 5.3.3 and C5.3.4 of the CE Code detail limitations on subcommittee composition and numbers.
Please send the application to me at the e-mail address below, or send me an e-mail and I will send you the application as a Word document.
In conclusion, the CE Code is unique in its complete national acceptance as a model installation code. One reason is CSA’s leadership in making it a national consensus standard. Another is the high quality input from hundreds of volunteers. I can’t think of better qualified people to make recommendations on improvements to the code than members of the International Association of Electrical Inspectors. I encourage you to volunteer your knowledge and experience.
IAEI CE Code, Part I Committee Representative
Tel:(416) 241-8857 ext. 237
Application for IAEI Rep on Canadian Electrical Code Part I Subcommittee
Read more by Steve Douglas
Posted By Michael Johnston,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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The 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.
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.
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.
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.
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.
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
Read more by Michael Johnston
Posted By Erik Senseney,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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For 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.
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
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
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 "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 "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.
Read more by Erik Senseney
Posted By Michael Furtak and Lew Silecky,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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Our 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 Calculations
1 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
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 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.
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.
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.
Read more by Michael Furtak
Read more by Lew Silecky
Posted By Randy Hunter,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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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.
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.
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.
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. "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.
Read more by Randy Hunter
Posted By Jonathan Cadd,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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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.
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.
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.
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)
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
Posted By Stephen J. Vidal ,
Monday, July 02, 2012
Updated: Monday, September 10, 2012
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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
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.
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
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
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