Hazardous Locations

[tgr58]

We have a room that is rated for Class 1/Div 2/Group D. I understand the regulations pretty well from section 501 of the NEC but there is some disagreement about an elevation rating. Is there any heigth in the room, such as 48", where the rating changes such that arcing devices such as switch contacts not hermetically sealed in a housing are okay as lonmg as they are installed inside a type 4 nema enclosure?

Thanks

 

[rbalex]

... Is there any heigth in the room, such as 48", where the rating changes such that arcing devices such as switch contacts not hermetically sealed in a housing are okay as lonmg as they are installed inside a type 4 nema enclosure?...

It’s not likely. In fact, depending on the ventilation, you may actually have a Division 1 location. Even a well ventilated location will commonly have a hazard radius substantially greater than 48” from the source.

We just haven’t been given enough information about the processes involved.

 

[tgr58]

Thanks. A little more information. The room is ventilated with air being pulled from the bottom through a vent and fresh air supplied to the room from up above. We have type 4 enclosures in another area but the enclosures are purged. IMy concern is that I can not find anything in the NEC that references a heigth exception. These rooms and the equipment in them wer installed around 1996. Could it be that the code was different back then?
Thanks

 

[rbalex]

We still don't have enough information.

• Still don?t know the ventilation rates.
• Still don?t know the Group D material involved; they can either be lighter than air (methane) or heavier than air (just about everything else).
  [*]Don?t know if the application involved is covered by Articles 510 through 516
  

With respect to the open questions, the 1996 NEC wasn?t that different. Unless the ceiling is high, the room is HUGE and well ventilated throughout (almost like outdoors) or the installation is covered specifically by Articles 510 through 516 there is no NEC basis for a ?height exception?. In any case, there would be no basis for a 48? envelope around a source.

Standards NFPA 497 and API RP500 are the two most general electrical area classification documents. They are both referenced in Section 500.4(B) along with other location specific practices.

I strongly recommend you contact a person qualified to do electrical area classifications. A good starting point would be the facility?s insurance provider.

 

[tgr58]

Thanks. Good information.

 

[jdsmith]

Do you have any idea where the concept of a room that is partially general purpose and partially C1D2 have come from? I heard a similar story yesterday about an installation using C1D2 practices within 36" of the floor and general purpose equipment > 36" above the floor - I'm curious where someone may have gotten this idea from.

 

[rbalex]

Do you have any idea where the concept of a room that is partially general purpose and partially C1D2 have come from? I heard a similar story yesterday about an installation using C1D2 practices within 36" of the floor and general purpose equipment > 36" above the floor - I'm curious where someone may have gotten this idea from.

If you have access to NFPA 497, review Section 5.9.1 and its associated diagrams. As I said previously:

Unless the ceiling is high, the room is HUGE and well ventilated throughout (almost like outdoors) or the installation is covered specifically by Articles 510 through 516 there is no NEC basis for a ?height exception?. In any case, there would be no basis for a 48? envelope around a source.

While it isn't a specific Article 501 requirement it is still good to review "wiring above classified locations" such as Section 511.7 and similar Article 510 - 516 concepts.