nonincendive circuit?

[dbuschor]

Can a general purpose limit switch (non sealed contacts) be used in a class I Div II Group D installation when:

Its supplied by a 24V DC source and connected to a DC Input.

The DC input would be AB 1794-IB16, has a 8.0 mA nominal on-state current (at 24V DC) and has an input impedance of 4.6K ohms.

I would assume under normal operation, this is a nonincendive circuit, but does the 24DC source need to be current limited such that a short to ground at the switch would not cause an arc capable of ignition?

 

[rbalex]

Re: nonincendive circuit?

Assuming you can obtain an appropriate "Control Drawing" (See 500.2) to "properly document" the installation (See 500.4(A)) - the limit switch may be used per 501.3(B)(1), Ex (c)

With regard to the power supply, the manufacture's control drawing will determine if it is suitable as the source of a nonincendive circuit.

I'm so glad I get to say "Yes" once in a while

 

[dbuschor]

Re: nonincendive circuit?

As for the power supply, I would suspect the answer from the Mfg would be it depends on how one uses it. In other words, the Sola power supply is rated for Class I, Div II, Group D operation, but it has 10 Amp capacity at 24V DC. If the limit switch circuit is normally current limited by the DC Input card, then I would suspect the circuit is nonincendive (provided its arc at 8 mAmps is less than what would cause ignition). However, my concern is that the 24V DC supply, which has a short circuit amp capacity of 10 amps, would definitely cause an arc if it were shorted in the field. Therefore, by code, do I have to limit the current feeding the limit switch (hot wire) in addition to the current limiting provided by the DC input?

 

[rbalex]

Re: nonincendive circuit?

Conceptually, you are correct but you will still need the Manufacture's control drawing to validate the circuit.

"Nonincendive" is now effectively Division 2's version of Division 1's "intrinsically safe." It's still less stringent with the equipment itself, but the documentation requirements are just as rigorous.

In their simplest conceptual form an "intrinsically safe" circuit can't provide sufficient ignition energy - even damaged; a "nonincendive" circuit doesn't provide sufficient ignition energy - unless damaged. Note the difference between "can't" and "doesn't." These are just the basics; there are additional concerns.

In either case, the rating must still be established by test. For a nonincendive circuit:

Nonincendive Circuit. A circuit, other than field wiring, in which any arc or thermal effect produced under intended operating conditions of the equipment is not capable, under specified test conditions , of igniting the flammable gas?air, vapor?air, or dust?air mixture.