Mixing voltages within a force guided relay

kku · Jun 25, 2015

Is it against code to mix voltages within one force guided relay? I have a force guided relay with a 24 VDC coil. This relay has five normally open contacts and one normally closed contacts. Two of the five normally open contacts are used: one to switch 208 VAC and one to switch 24 VDC. Is this allowed or just poor practice? My gut tells me the machine builder is just cheap and didn't bother separating the two circuits.

The coil for this force guided relay is switched using a safety PLC.

jparale · Jun 25, 2015

Im not sure with Canadian Rule, didn't find it in the code, but in general practice in building a control panel, is not to mix the power circuits with control circuits. First is safety, for your self and for the control circuits alike. There are some controllers in which input cannot be contaminated with EMF coming from the wires with high voltage

JesperMP · Jun 25, 2015

At least in the EU, yes you can mix voltages on adjacant relay contacts or aux contacts.
One should separate power and control circuits as best as one can, but you cannot achieve 100% separation.

boneless · Jun 25, 2015

Interesting.

If this would not be appropriate, how would you get feed back from a contactor?

If I have one contact from a pressure switch, controlling a 230 VAC valve, providing 24 VDC feed back to PLC and providing a pot free contact for another system, would I need to use 3 separate relays?

Would a 24 VDC be susceptible for EMF from 230 VAC (hardly high voltage?)

Just thinking out loud

kku · Jun 25, 2015

boneless said:
Interesting.

If this would not be appropriate, how would you get feed back from a contactor?

If I have one contact from a pressure switch, controlling a 230 VAC valve, providing 24 VDC feed back to PLC and providing a pot free contact for another system, would I need to use 3 separate relays?

Would a 24 VDC be susceptible for EMF from 230 VAC (hardly high voltage?)

Just thinking out loud

I think normally we will assume the normally closed contact will switch 24 VDC for feedback to the safety PLC. Simiarly we will know the coil will be switched using 24 VDC. The problem I have is with the machine builder using the normally open contacts for different voltages. If I probe one set of contacts and measure 24 VDC then I wouldn't expect another set of normally open contacts to carry 208 VAC.

kku · Jun 25, 2015

JesperMP said:
At least in the EU, yes you can mix voltages on adjacant relay contacts or aux contacts.
One should separate power and control circuits as best as one can, but you cannot achieve 100% separation.

This machine actually came from Germany so maybe it's OK. I am just wondering if there is anything in the US or Canadian electrical code to say this isn't allowed.

JesperMP · Jun 25, 2015

We do export some to both the US and Canada, and I know of a few things to be aware of, but have not heard this one.

boneless · Jun 25, 2015

kku said:
If I probe one set of contacts and measure 24 VDC then I wouldn't expect another set of normally open contacts to carry 208 VAC.

Why not? Isn't that why we use different colors of wire?

just the cowboy · Jun 25, 2015

Motor starter

Just think of a motor starter, aux contacts are rarely the same voltage.

kku · Jun 25, 2015

boneless said:
Why not? Isn't that why we use different colors of wire?

Good point. I just checked and noticed the machine builder had used all red wiring. Although the 24 VDC circuit was a smaller gauge.

Mixing voltages within a force guided relay

kku

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