Non-Incendive Circuit

[Stan Stephan]

A multi-pair cable (24 Vdc non-incendive ) has 1 of the pairs operating a solenoid. A current limiting resistor (needed for the non-incendive design) prevents the solenoid from operating. Designer wants to remove resistor and re-classify the solenoid loop. Can a single multi-pair cable contain both non-incendive and 1 pair that does not meet the non-incendive design?

 

[Smart $]

A multi-pair cable (24 Vdc non-incendive ) has 1 of the pairs operating a solenoid. A current limiting resistor (needed for the non-incendive design) prevents the solenoid from operating. Designer wants to remove resistor and re-classify the solenoid loop. Can a single multi-pair cable contain both non-incendive and 1 pair that does not meet the non-incendive design?

Need more details to provide any conclusive answer, but I believe once the cable contains an incendive circuit, the entire cable must comply with incendive field wiring... plus the equipment and/or enclosures at each end must meet the requirements for incendive operation.

I think the designer is asking for a bigger headache than he realizes. The best alternative is, if possible, find a solenoid coil with a lower operating current... that's why the resistor is there in the first place.

 

[rbalex]

From 501.10(B)(3):

Separate nonincendive field wiring circuits shall be installed in accordance with one of the following:
(1)In separate cables (2)In multiconductor cables where the conductors of each circuit are within a grounded metal shield (3)In multiconductor cables, where the conductors of each circuit have insulation with a minimum thickness of 0.25 mm (0.01 in.)

The minimum insulation thickness cited in 501.10(B)(3)(3) is equal to 10 mils. Except for some lacquered insulations, it virtually impossible to get a thermoset or thermoplastic insulation that wouldn?t meet that criterion.

The rules for nonincendive are not nearly as rigorous as those for intrinsically safe; however, one common requirement is either system must be installed per a ?control drawing.? The control drawing may prohibit the proposed installation. (Part of the ?details? Smart $ mentioned.

RBA Note: It would be understandable to challenge a requirement for a control drawing since it isn?t specifically mentioned in 501.10(B)(3). But it is mentioned, and its purpose and function outlined in Section 500.2.

 

[justdavemamm]

I'm curious. While I've designed and overseen the installation of many Class I Division 2 nonincendive systems, I've not heard of making a cable / circuit / solenoid nonincendive by putting a resistor in the wiring.

Who put this resistor in the circuit ?

Either a solenoid has been listed as nonincendive by its manufacture or not. If it is, then I wire it so, if it is not, then it would be (AFAIK) either I.S. or XP and installed & wired appropriately.

 

[Smart $]

I'm curious. While I've designed and overseen the installation of many Class I Division 2 nonincendive systems, I've not heard of making a cable / circuit / solenoid nonincendive by putting a resistor in the wiring.

Who put this resistor in the circuit ?

I am assuming the resistor is part of the controller, i.e. part of the manufacturer's design and not in the wiring.

Either a solenoid has been listed as nonincendive by its manufacture or not. If it is, then I wire it so, if it is not, then it would be (AFAIK) either I.S. or XP and installed & wired appropriately.

I'm of the impression the controller and the solenoid are incompatible for the purpose. Both may be listed as nonincendive for the purpose, but that in itself does not guarantee compatibility.