Grounded (neutral) conductor disconnect

U

Uze

Member
Location
France
Occupation
Engineer
My question might seem a bit silly but since I'm based in Europe, I'm not fully familiar with American standards. Is it allowed to disconnect the grounded (neutral) conductor in a UL508A control panel that will be used to operate an industrial machine (NFPA 79)? I'm working on a 480Y/277V network.
In the UL508A standard, I found the two following points, which require disconnection of the ungrounded conductors but, in my opinion, do not explicitly prohibit disconnection of the neutral:
  • 31.2.1 A branch circuit protective device shall be installed in each ungrounded conductor to the load(s) involved.
  • 30.3.2 The disconnecting means shall open each ungrounded conductor of the supply circuit.

I would like use 4P or 3P+N circuit breaker or disconnecting means.

Thank you very much for your replies!
 
The general rule is you must simultaneously disconnect all associated ungrounded conductors. A common trip breaker is acceptable but you can not have a situation with a fuse in a grounded conductor.
 
Just to add: common practice in North American building wiring (NEC standards from NFPA) is to disconnect the ungrounded conductor(s) and leave the grounded conductor connected. NEC 404.2(b) makes things explicit, you may disconnect the grounded conductor, but only if you ensure the ungrounded conductors get disconnected:
404.2(B) Grounded Conductors. Switches or circuit breakers shall not disconnect the grounded conductor of a circuit.
Exception: A switch or circuit breaker shall be permitted to disconnect a grounded circuit conductor where all circuit conductors are disconnected simultaneously, or where the device is arranged so that the grounded conductor cannot be disconnected until all the ungrounded conductors of the circuit have been disconnected.
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