hazardous area valve limit/position switches


Thread Starter


I have an application where the area is class 1 div 2 group C and D. I will be using type Z purge on the main controller. I have to wire from the main contoller out to valve position/limit switches and then back to the PLC input in the main controller. The switches on the valves are rated for the area.

Is it better/safer to use 24Vdc or 24 vac or 120 Vac or doesn't it matter? Thanks.

Bob Peterson

Better and safer are pretty subjective terms.

There are several ways to skin this cat. What kind of switches do you have?


Reply: Standard paractice is to use 24 VDC with Active isolators (MTL or P&F). However, if the distance between controller and Limit switches is greater than 300 metres, then 120 Vac may be used through interposing Relay provided the Limit switch housing is Ex-proof/Flameproof type suitable for the Hazardous Area. 24 vac is a non-standard voltage level and generally not used.
The switch housing is rated for the area. The switches will small microswitches SPDT mechanical style. Thanks.

Bob Peterson

What possible reason could you have for putting an inperosing relay in the circuit?

BTW, 24VAC is not all that uncommon.