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garbo said:
Just wondering if it would be safer if taper threads were on heavy wall couplings in explosion proof areas. A lot of sparkies do not know that you are not suppose to thread in a straight thread chase nipple into a heavy wall coupling. Have done it and always tightened them up and never came loose.
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The areas where an explosion are most likely to occur (items with arcing components inside) are required to have a seal fitting nearby (IIRC 18" max in the conduit) and generally must be the first fitting other than explosion proof reducing fittings are permitted, explosion proof union or capped ell I believe are also permitted.
 
garbo said:
Just wondering if it would be safer if taper threads were on heavy wall couplings in explosion proof areas. A lot of sparkies do not know that you are not suppose to thread in a straight thread chase nipple into a heavy wall coupling. Have done it and always tightened them up and never came loose.
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I thought that and tried to get the term "coupling" from the last part of 501.15(A) in the 2020 code cycle. I just don't see the sloppy straight threads in a standard conduit coupling being able to cool the escaping hot gasses enough to prevent external ignition of any flammable gasses. One CMP 14's members is from UL and he works in the testing of explosionproof equipment and he said they have never seen any external ignition when they test explosionproof enclosures with a standard coupling between the enclosure and the seal fitting.
From PI 1381 for the 2020 code:
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An enclosure, identified for the location, and marked “Leads Factory Sealed”, or “Factory Sealed,” or “Seal not Required,” or equivalent shall not be considered to serve as a seal for another adjacent enclosure that is required to have a conduit seal.
Conduit seals shall be installed within 450 mm (18 in.) from the enclosure or as required by the enclosure marking. Only explosionproof unions, couplings, reducers, elbows, and capped elbows that are not larger than the trade size of the conduit shall be permitted between the sealing fitting and the explosionproof enclosure.
Statement of Problem and Substantiation for Public Input
There is no such thing as an explosion proof conduit coupling, other than a flexible coupling. The common application of the current code text permits a standard conduit coupling between the enclosure and the sealing fitting. All of the other items that are listed have tapered female treads and will provide the required cooling as the hot gasses are released in the event of an explosion within the enclosure. A standard conduit coupling has straight threads and is unlikely to provide sufficient cooling of the hot gasses as they escape in the event of an explosion. A standard conduit coupling should not be permitted in this application.
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From FR 7623:
An enclosure, identified for the location, and marked “Leads Factory Sealed”, or “Factory Sealed,” or “Seal not Required,” or equivalent shall not be considered to serve as a seal for another adjacent enclosure that is required to have a conduit seal.
Conduit seals shall be installed within 450 mm (18 in.) from the enclosure or as required by the enclosure marking. Only threaded couplings, or explosionproof fittings such as unions, couplings, reducers, elbows, and capped elbows that are not larger than the trade size of the conduit, shall be permitted between the sealing fitting and the explosionproof enclosure.
Committee Statement
Committee Statement: The panel affirms that a conduit coupling is not identified as explosionproof. However, couplings are permitted by the Code to be connected to explosionproof equipment. The sentence has been amended to reflect this and clarify the requirement. The Panel affirms that unions, elbows, capped elbows and reducers are required to be explosionproof.
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don_resqcapt19 said:
I thought that and tried to get the term "coupling" from the last part of 501.15(A) in the 2020 code cycle. I just don't see the sloppy straight threads in a standard conduit coupling being able to cool the escaping hot gasses enough to prevent external ignition of any flammable gasses. One CMP 14's members is from UL and he works in the testing of explosionproof equipment and he said they have never seen any external ignition when they test explosionproof enclosures with a standard coupling between the enclosure and the seal fitting.
From PI 1381 for the 2020 code:

From FR 7623:
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How often is a coupling ever installed within that 18" to the seal fitting? I've seen many explosion proof reducers, unions, and capped ells but possibly never a regular coupling.
 
Isn’t this 2023 that allows a coupling between the 18” section between seal and an explosion proof enclosure?

I’ve always known reducers and unions to be.

In Class I, Division 1 locations, conduit seals shall be located in accordance with 501.15(A)(1) through (A)(4).


Conduit seals shall be installed within 450 mm (18 in.) from the enclosure or as required by the enclosure marking. Only threaded couplings, or explosionproof fittingssuch as unions, reducers, elbows, and capped elbows that are not larger than the trade size of the conduit, shall be permitted between the sealing fitting and the explosionproof enclosure.
 
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