NFPA496 Purge & Pressure System Control Power, C1D2


New member
Beaver, WV, USA
I am working on a P&P system and I am struggling with the interpretation of NFPA496 5.4.1

It states "Protected equipment shall not be energized until at least four enclosure volumes of the protective gas(ten volumes for motors, generators, and other rotating electric machinery) have passed thru the enclosure while maintaining an internal pressure of at least 25Pa(0.1 in of water).

I have several items in this enclosure but the main item is a VFD drive which feeds a motor(fan) downstream of this P&P enclosure. The item I am concerned about is a small PT that supplies 120V control power. The control power is used to run the P&P unit. I cant perform the purge(passing 4 volumes of air) thru the enclosure w/o this control power b/c the customer does not have any upstream 120vac power available.

Am I in compliance of NFPA496 or by having the PT energized at all times in this enclosure, am I failing to comply?



Staff member
No idea about strict code compliance, but I have two questions:
1. Is the CT considered to be "protected equipment"? Does it have to be in the same enclosure with protected equipment?
2. Could you use a purely manual purge (e.g. valve and nitrogen tank) for the initial startup?

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Staff member
Since Part 5.4 applies to both Type Z and Y systems, it would be helpful to know which you are using; however, in absence of that knowledge, Subpart 5.4.2 is an alternative to Subpart 5.4.1 and relatively simple to achieve. If Subpart 5.4.2 is not achievable then we need to know the system Type.