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September 8, 2006 A Prism Business Media Publication Vol. IV No. 17



CONTENTS
250.4 General Requirements for Grounding and Bonding

What's Wrong Here?

Code Q&A

Code Quiz

Report on Proposals for 2008 NEC

EC&M E-Tradeshow Webinars


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    250.4 General Requirements for Grounding and Bonding

    By Mike Holt
    You basically perform work on two types of systems -- solidly grounded or ungrounded. The general requirements for grounding and bonding these systems can be summarized as follows.

    Solidly grounded systems
    You ground high-voltage system windings to the earth to help limit high voltage imposed on the system windings from lightning, unintentional contact with higher-voltage lines, or line surges. You must ground metal parts of electrical equipment to the earth by electrically connecting the building or structure disconnecting means [225.31 or 230.70] with a grounding electrode conductor [250.64(A)] to a grounding electrode [250.52, 250.24(A), and 250.32(A)].

    Metal parts of the electrical installation are grounded to the earth to reduce voltage imposed on them from lightning so as to prevent fires from a surface arc within the building or structure. Grounding electrical equipment to earth doesn't serve the purpose of providing a low-impedance fault-current path to clear ground faults. In fact, the Code prohibits the use of the earth as the effective ground-fault current path [250.4(A)(5) and 250.4(B)(4)].

    Grounding metal parts to earth is often necessary in areas where discharge (arcing) of the voltage buildup (static) could cause dangerous or undesirable conditions. Such an occurrence might be the failure of electronic equipment being assembled on a production line, or a fire and explosion in a hazardous (classified) area. See 500.4 FPN 3. However, grounding metal parts to earth doesn't protect electrical or electronic equipment from lightning voltage transients (high-frequency voltage impulses) on the circuit conductors. To protect electrical equipment from high-voltage transients, you must install transient voltage surge-protection devices in accordance with Article 280 at service equipment and Article 285 at panelboards and other locations.

    To remove dangerous voltage from ground faults, you must bond metal parts of electrical raceways, cables, enclosures, and equipment to an effective ground-fault current path with an equipment grounding (bonding) conductor of a type specified in 250.118. The fault-current path must have sufficiently low impedance to allow the fault current to quickly rise to a level that will open the branch-circuit overcurrent protection device. In addition, you must bond electrically conductive metal water piping systems, metal sprinkler piping, metal gas piping, and other metal-piping systems, as well as exposed structural steel members that are likely to become energized, to an effective ground-fault current path. A permanent, low-impedance fault-current path is one that facilitates the operation of the circuit overcurrent device. The earth is not considered an effective ground-fault current path.

    Click here to read the rest of this article.


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    Code Challenge
    What's Wrong Here?
    By Joe Tedesco
    Think you know how this installation violates the NEC? Visit EC&M's Web site to see the answer.

    Hint: Pay attention to the color red.


    Code Q&A
    By Mike Holt
    Q. I recently saw an advertisement for red conduit that is to be used for emergency circuits. Is this a Code requirement?

    Visit EC&M's Web site to see the answer.


    Code Quiz
    By Steven Owen
    Q. What is the maximum allowable ampacity of 12 AWG THHN insulated conductors installed in a strut-type channel raceway that is 1 1/2 inches by 1 1/2 inches? There are a total 13 conductors in the raceway -- 12 of which are considered current carrying. The terminals at the overcurrent protective devices for these conductors are rated at 75°C. It is unknown what the terminal ratings are at the load end of the conductors. The actual loads supplied by these conductors are not known.

    A) 30A
    B) 25A
    C) 20A
    D) 15A

    Visit EC&M's Web site for the answer and explanation.


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    Code News Update
    Report on Proposals for 2008 NEC
    The Code is the definition of a "living document." Constantly changing, it's the subject of thousands of proposals for revision throughout each cycle. Earlier this year, the Code Making Panels acted on 3,688 proposals associated with the 2008 NEC. Now the deadline for comments is fast approaching.

    If you're interested in submitting comments on any of the proposed changes, you better get moving. All comments must be submitted to the NFPA by 5 p.m. EST on October 20, 2006. You can submit your comments online or download a comment form.


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    www.exair.com/15/540_09a.htm


    Shows and Events
    EC&M E-Tradeshow Webinars
    The EC&M E-Tradeshow will feature three FREE webinars on September 20.
    • "Ten Trends That Will Shape the Electrical Market," presented by Jim Lucy, chief editor of Electrical Wholesaling magazine. Learn the latest trends in the distribution market and how they can affect your business and the bottom line.
    • "Power Cable Basics," presented by John DeDad, senior director, editorial and EC&M development. Learn why an MV power cable is a complex and sophisticated product requiring careful testing and handling. Find out how the current-carrying conductor, laminated dielectric, MV insulation, semiconductive shield, metallic shield, and outer jacket work together to control electrical stress and provide mechanical protection. Also, learn about hi-potential and dissipation factor testing.
    • "Hioki 3196 Quickstart Training," a hands-on webinar by PowerCET will show you how to use the VIEW screen (waveform, vector and DMM displays) to rapidly identify real-time power quality problems and the AUTO SETUP for quick monitor installation. You'll also learn how to create custom setups to capture more meaningful data, identify power quality problems using the 3196's simultaneous capture of power quality and energy data, and use Hioki PC-based software with a compact flash card reader to transfer stored data and perform additional analysis.
    Please e-mail your webinar attendance notice to John DeDad.



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