Non Incendive Components, Equipment, Circuits

[fifty60]

My understanding of 501.105(b)1 is that I can use a standard relay or instrument (pressure switch for example) if the circuit that it is breaking is nonincendive. Would I be able to apply an intrinsically safe isolator to a relay circuit, and then be able to call it nonincendive.

My concern with this approach is that I believe the intrinsically safe isolators are made specifically for intrinsically safe circuits, not for making a circuit nonincendive. The interesting thing to me is that adding the intrinsically safe isolator does make the circuit nonincendive.

So I agree that I can't just add an isolator to make a circuit intrinsically safe. My question is, can I add an intrinsically safe isolator to make a circuit nonincendive?

 

[rbalex]

It's a good question. Of course the NEC only describes where and how nonincendive or intrinsically safe systems may be used, the actual characteristics of such systems are defined in the various ANSI/ISA Standards referenced in associated Informational Notes.

I no longer have direct access to the ANSI/ISA Standards but my guess is, if the only requirement in a specific application to render it a nonincendive circuit were to use a nonincendive isolator/barrier, then the odds are an intrinsically safe isolator/barrier would work.

 

[fifty60]

Would I be able to consider a pressure switch a simple apparatus? I have a standard pressure switch in a general purpose enclosure that uses general purpose contacts. Relays and switches have some consideration for Class I Div 2 that make me think that I may not be able to use the nonincindive circuit approach.

However, if I take the Intrinsic Safety as the protection method, would I be able to consider the switch a simple apparatus and apply an isolator to it and consider the switch protected using Intrinsic Safety as the protection method.

To summarize the question, can I consider a switch or relay a simple apparatus and apply a barrier or isolator to it as an intrinsically safe circuit? I do not think nonincindive will allow me to use a general purpose enclosure on a switch...so I will need to use IS as the protection method. The question is can I consider the switch a simple apparatus?

 

[fifty60]

If the relay can be considered a simple apparatus, and the IS protection method is used, am I then able to use a general purpose enclosure and relay contacts? Article 501 does not give any exceptions that I think I can use for using a general purpose enclosure. However, if I use intrinsic safety as the protection method, does the non-general purpose enclosure/relay not apply?

 

[rbalex]

Intrinsic safety is purely Article 504. The basic application statements are in Sections 504.10(B) and 504.20. One must still be very careful about Sections 504.50, 504.60 and 504.70; they contain a few hidden traps for the unwary. Depending on the edition, many relevant definitions are in Section 504.2 [2014 and earlier] or Article 100 [2017].

That said, the ANSI/ISA 12.xx.xx reference documents mentioned earlier are still applicable and should be consulted. The odds are a simple switch in an otherwise properly designed nonincendive or intrinsically safe circuit is indeed likely to be simple apparatus, ​ a relay may be questionable. See the definitions.