Explosion Proof Housing vs Explosion Proof Assemblies

[dnunez43]

I've been told that if an explosion proof housing is used, an entire switch can be considered explosion proof, i.e. the same explosion proof certifications can be offered on an instrument that the housing carries. This seems suspect since I know that it is required that companies send complete units to labs such as Baseefa to be tested before you can receive a certification. I've asked the question over and over and I am continually told that it is acceptable to sell an assembly as explosion proof when only the housing has been certified; more specifically, I'm told "it's an industry standard practice to use the certification of the housing to deem an assembly explosion proof".

Can someone please shed some light on this

 

[xnuke]

An explosion-proof enclosure must contain any explosion that occurs inside of it and prevent the external hazardous atmosphere from igniting or exploding. Thus, it is perfectly acceptable to install devices that could cause an internal explosion inside the enclosure and still rate the entire assembly as explosion-proof.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ScottyUK]

Not sure if this is an IEC or North American application. Under IEC rules for an Ex 'd' enclosure, any device which penetrates the enclosure would need to be tested as part of the overall assembly. As soon as the enclosure is modified from its tested and certified form then it must be re-tested. 'Modified' includes, for example, drilling an additional gland entry or adding a new indicator lamp.

 

[FreddyNurk]

I'm guessing your question is somewhere between the answers both xnuke and ScottyUK provided as you mentioned 'switch'.
I'd expect, as per Scotty's assertion, that anything that penetrates the enclosure would require certification, if you're asking about a pushbutton or contact or similar (like ScottyUK's indicator lamp example) to be provided for the user on the outside, then I'd expect that both the switch and the enclosure would need to be certified, as its then providing a possible explosion vent point that wouldn't be covered by the original enclosure certification.

If all the equipment is entirely housed within the enclosure, with no additional penetrations required, then xnuke is correct, the enclosure suffices. If the enclosure is designed to manage an internal explosion without propagating an ignition path, there's little to be gained by additional certified equipment.