Art. 504.50(c)

[crower]

The 2005 NEC says 'Where shielded conductors or cables are used, shields shall be grounded.'

The exception says 'Where a shield is part of an intrinsically safe circuit.'

My question is, should the shielded portion of this wiring be grounded before the intrinsically safe portion (a zener barrier, or associated apperatus 504.2)and ungrounded after the barrier?
The analog signal (4-20 mA) from a transducer is signalling a PLC in this application. It is through a zener barrier and I am wondering where am I supposed to ground the shield?

 

[rbalex]

Re: Art. 504.50(c)

It isn't always obvious, but this is a case where Code text should be taken at it's most literal.

Remember, the entire context of this Section is Intrinsically Safe (IS) installations.

For Section 504.50(C)the referenced "shield" in the basic rule is part of the cable , but it is NOT necessarily part of the IS circuit . An example of where the shield MAY be part of the circuit is a coaxial cable. The manufacturer's "control diagram" will then determine whether it should be grounded or not.

I suspect you may be discussing something like a "shielded, twisted-pair." In this case, the shield would be grounded on both sides of the barrier.

It should also be noted that the exception is a PERMISSIVE rather than a MANDATORY requirement; i.e.,. it is permitted to not ground the shield, it is not prohibited.

In all cases, the manufacturer's control diagram MUST (not just should) be consulted. See 504.10(A). This is currently one of the most overlooked requirements.