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  1. #1
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    Area Classification Above A Trench

    I have a situation involving a trench below a Class I Div 2 area. Per NFPA 497 it must be rated Class I Div 1. However the trench also extends beyond the boundaries of the Div 2 area above it. My understanding is that the entire trench needs the Div 1 rating. Unfortunately NFPA 497 does not address this situation. Is it possible for the area directly above a section of a Class I Div 1 trench to be nonhazardous?

  2. #2
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    Quote Originally Posted by circuitrider View Post
    I have a situation involving a trench below a Class I Div 2 area. Per NFPA 497 it must be rated Class I Div 1. However the trench also extends beyond the boundaries of the Div 2 area above it. My understanding is that the entire trench needs the Div 1 rating. Unfortunately NFPA 497 does not address this situation. Is it possible for the area directly above a section of a Class I Div 1 trench to be nonhazardous?
    No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.

    One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.

    This is one of the the more subtle differences between NEC Divisions (and Zones) and IEC Zones. IEC does not necessarily require a physical barrier between unclassified and Zone 1. For most practical purposes, IEC Zone 1 is a moderately upgraded NEC Division 2.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  3. #3
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    Quote Originally Posted by rbalex View Post
    No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.

    One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.

    This is one of the the more subtle differences between NEC Divisions (and Zones) and IEC Zones. IEC does not necessarily require a physical barrier between unclassified and Zone 1. For most practical purposes, IEC Zone 1 is a moderately upgraded NEC Division 2.
    Don't know about the incident you mentioned, but unfortunately a gas pipe in a trench doesn't trigger hazardous location if under normal operation the gas is expected to remain inside the pipe. Gas leak in an underground pipe can follow trench for a long distance though especially if soil that got filled into the trench isn't as compacted as other soil around the trench.
    I live for today, I'm just a day behind.

  4. #4
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    A closed piping system in a trench (or anywhere else) does not require classification.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  5. #5
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    Quote Originally Posted by rbalex View Post
    A closed piping system in a trench (or anywhere else) does not require classification.
    Not sure if you consider 'closed piping system' but some AGA standards and gas utility meter/reg station standards define 15' from flanged connections as Cl I Div 2. Although this doesn't sound like what the OP has.

  6. #6
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    Quote Originally Posted by nhee2 View Post
    Not sure if you consider 'closed piping system' but some AGA standards and gas utility meter/reg station standards define 15' from flanged connections as Cl I Div 2. Although this doesn't sound like what the OP has.
    This isn’t the OP’s case. In the scenario discussed in the OP, the pressure differential between the Division 1 and unclassified locations is essentially zero and 18” is the common hazard radius.

    Closed piping has no designed route to atmosphere, such as a relief valve. Other (non-catastrophic) potential routes may be evaluated based on pressure differential, flange pressure rating, anticipated flow rates, etc. Typically, a 300psi rated flange on a 100 psi rated piping system would still be considered “closed”.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  7. #7
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    Quote Originally Posted by rbalex View Post
    No. See Section 500.5(B)(2)(3). Philosophically, without an impervious barrier between them, you cannot have an unclassified location adjacent to a Division 1 location. However, the Division 2 hazard radius may be as little as 18”. It’s a judgement call.
    I believe you are right that Section 500.5(B)(2)(3) requires a Div 2 area separating Div 1 from nonhazardous. If you could justify 18" though, why not, say, 4"? Or is that what you mean by judgment call?

  8. #8
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    Quote Originally Posted by circuitrider View Post
    I believe you are right that Section 500.5(B)(2)(3) requires a Div 2 area separating Div 1 from nonhazardous. If you could justify 18" though, why not, say, 4"? Or is that what you mean by judgment call?
    18" is pretty much a historic minimum. "Judgment" could put it as much as 10' although it isn't likely.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  9. #9
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    Quote Originally Posted by rbalex View Post
    18" is pretty much a historic minimum. "Judgment" could put it as much as 10' although it isn't likely.
    OK, I agree. I really would like to see NFPA 497 address this though.

  10. #10
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    Quote Originally Posted by circuitrider View Post
    OK, I agree. I really would like to see NFPA 497 address this though.
    NFPA 497 goes through a revision cycle just as the NEC does. Make a Public Input.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

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