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Thread: Area Classification Above A Trench

  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.

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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
    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?

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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.

  7. #7
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    Quote Originally Posted by rbalex View Post
    No. See Section 500.5(B)(2)(3).
    One of the worst fuel gas explosions occurred when a trench transported propane several miles from a source.
    A few questions just out of curiosity.

    Who has a trench that is several miles long without any breaks?

    Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
    Bob

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    Quote Originally Posted by petersonra View Post
    A few questions just out of curiosity.

    Who has a trench that is several miles long without any breaks?

    Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
    could be propane got into some other trench for whatever reason and followed it, like underground crossing
    I live for today, I'm just a day behind.

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    Quote Originally Posted by petersonra View Post
    A few questions just out of curiosity.

    Who has a trench that is several miles long without any breaks?

    Would an 18 inch break in the trench have done any good? Several miles of propane is a lot of propane. It seems to me that much propane would just spill over an 18 inch separation at grade.
    Propane is extremely heavy and can maintain an explosive mixture for a long time. As part of an API study, a slow leak with a relatively low wind a propane leak can remain explosive for at least 3 miles before it becomes dispersed.

    I won’t name the city. Basically, the trench was an above ground water drainage system that fed into a catch basin. The propane collected in the basin. How long the initial leak occurred is unknown.
    "Bob"
    Robert B. Alexander, P.E.
    Answers based on 2014 NEC unless otherwise noted.

  10. #10
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    Quote Originally Posted by kwired View Post
    could be propane got into some other trench for whatever reason and followed it, like underground crossing
    That would still be a long trench. Bob A. did not say a tunnel, or culvert, or pipe. He is very careful with his words.
    Bob

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