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Thread: 501.15(B) (2) exception questions

  1. #1
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    501.15(B) (2) exception questions

    Folks need help understanding these two exceptions.
    NEC 2014, 501.15(B) (2) Can you explain exceptions 2 and 4. I am having a hard time understanding why you don't seal conduit routes in and out of a C1D2 boundary. I got brought into the project at its end and conduit seals came up to me. My understanding is not to allow gas to be transported to adjacent equipment that's outside the boundary. Boundary is defined by the AE on their drawings. Exception 4 seems to block exception 2 based on splices, terminals or taps. Can bolted motor connections or ring lugged connections on a TB be an ignition source.


    Examples I have
    Motor termination box ,conduit goes out to tray outside of the boundary. (splice connection in motor box) Motor is outside in C1D2
    Termination box same scenario as above. (terminal block connections).

    Any examples of E 2 & E 4 would be helpful.

    Thanks
    Bob

  2. #2
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    I have not used either exception and am not creative enough at the moment to come up with an example, however I can answer that bolted motor connections or ring lugged connections on a TB are not ignition sources, unless they operate at a temperature within 80% of the AIT of the source of release. Which makes me curious if a 90 degC terminal can ever present an issue, even in theory. 90/.8 = 113 degC. Browsing through NFPA 497 Table 4.4.2 ... aha, I found one result, Hydrazine! If you have Hydrazine present, you may have problems, since Hydrazine can have an AIT as low as 24 degC.

    Do you have Hydrazine? If not then terminal blocks would not be an ignition source.

    Either of your examples would qualify under Exception No. 2.

    Example 4 is in my interpretation meant to deal with locations that do not make their hazardous area boundaries simply follow the perimeter of their plant. If you have small gaps of unclassified areas in the plant, this exception is to prevent you from having to install seals as the conduit enters and leaves those areas. Perhaps can be applied on long runs going in between plants (e.g. over roadways).

  3. #3
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    Begin by assuming there is no vaportight barrier between the Division 2 and the unclassified locations at the boundary. If such a barrier does exist, only Exception No. 2 can apply; however, in either case, an explosionproof seal would still not be required at the boundary per the last sentence of Section 501.15(B)(2), main text. Both Exceptions assume the Division 2 location and the unclassified location are at the same air pressure at the boundary where the transition occurs and thus gases or vapors are no more likely to cross the boundary through the raceway than in the open air surrounding it. Also Since we are talking Division 2 we might even ask, "What gases and vapors are we talking about?", in the first place.

    Exception No. 2 generally applies where there is a ‚Äčtransition between wiring methods that occurs in "open air".

    Exception No. 4 primarily applies to installations that have long continuous raceway runs in the open that may pass through multiple boundaries. Raceways in a Division 2 terminus (on either end, if it applies) must be sealed. Such seal may or may not be required to be explosionproof, depending on the whether the terminus enclosure is required to be explosionproof or not. The method of terminating the conductors in the Division 2 location, splice or terminals, is irrelevant in either case.

    Exceptions
    only apply to the main text and not to each other.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  4. #4
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    Thanks all

  5. #5
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    Bob & MRKN missed your question Natural gas is the medium.

  6. #6
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    Quote Originally Posted by Citrusflight View Post
    Bob & MRKN missed your question Natural gas is the medium.
    I don't believe that would change either of our responses.

    Code Making Panel 14(CMP14) continually seeks ways to make the wiring method transition between Division 2 and unclassified locations easier to "rewire" if necessary.

    Personally, I don't believe many boundary seals are necessary at all since gases and vapors aren't supposed to actually be in Division 2 except in unusual conditions in the first place OR when those unusual conditions do exist there is no more likelihood that the gases and vapors would cross the boundary through the raceway than in the open air. There are some exceptions to that where the Division 2 location has a continuous or often positive differential pressure with respect to the unclassified location. Typically, this would occur where there is a physical boundary created by grade or vaportight wall. This is only my opinion, Not CMP14's.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

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