Mixing voltage on a relay

I have a two pole ice cube type relay. I want to pass 24 VDC through one pole and pass 120 VAC through the other pole. The 24 VDC coil is driven by a start and stop pushbutton combo. The first pole will latch around the N/O start pushbutton to latch in the coil. The second pole will recieve 120 VAC from a motor starter in another room and loop it back to the motor starter coil. Is this OK? Any reservations?

Go for it.

Obviously, if you have

a two pole ice cube type relay

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then it is a 2pole double throw. You have two sets of form C contacts with their own common. Just wire it up and go for it. Use one common for the 24VDC circuit and the other common from the second set of contacts for the 120VAC circuit. Not to be to picky, voltage does not go anywhere, current does.

Although it is physically possible, I would strongly urge you not to do this. Mixing voltages in a device becomes a safety factor, and I'm not sure that this situation is even allowed by code.

For safety issues alone, I would urge you to use two seperate relay's for this problem.

Pat

Afterthought

I knew I should have mentioned my personal preference in the earlier post. As vtfan has said, it is not good to mix voltages in the control circuit for safety reasons. However, you can do it, and you might take precautions such as labeling or otherwise notify people that you have done the mixing. Reservations are often personal if not defined by local or other codes.

"Although it is physically possible, I would strongly urge you not to do this."

I disagree. The function of a double pole or triple pole or four pole relay is to have separate contacts to switch different devices from a common signal. There is nothing wrong with putting 24 VDC and 120 VAC on different poles of the same relay as long as you use consistent wire color coding and use a relay with contacts rated for the highest voltage being used. I don't see how this is more unsafe than using two separate single pole relays to switch different voltages in the same panel.

After all, the auxiliary contacts on a starter are routinely used to switch 24 VDC or 120 VAC status indication even if the motor voltage is 480 VAC.

well put

Well put about the relay. As an integrator, things like this concern me. Nothing worse than cutting off 24V and getting zapped by 120. Of course, good practice is to always check it with a meter, but under the gun it may be overlooked. I say use two relays!

Thanks,

Pat

OK, Pat, if I don't turn off the 120 VAC, how does using two separate relays save me from getting zapped?

Mixing voltages on relay

It is okay to use the contacts for AC/DC voltages. As far as the troubleshooting or maintenance goes the ac and dc wires are color coded and just by looking at it one can make out what voltage it is carrying

As far as getting zapped, relay with 24 is isolated from relay with 120. Two seperate isolated ciruits.

I here you about the color coding, my answer to that is I just worked with my finance's brother doing some wiring at the house, he's an electrician and he's color blind.

It's preference, I do a lot of work for Dupont and they would never let me do this. I usually base my thoughts like this on their standards.

So, in a nutshell, yes you can do it, it may or may not be against code, but an IDEC relay will cost you about $20 with the base, I say use two relays.

I still don't understand how two relays are safer.

In both cases the same signal is tied to the coils. In both cases if you stick your finger on a hot wire you will get a shock. If the relays are mounted adjacent to each other on the same DIN rail the distance between them is almost identical. In both cases all of the wires are in the same panel even if the relays are on a different DIN rail.

Why is one safer than the other? (And don't just quote DuPont standards - we all know big companies can make really big mistakes! Even the NEC is corrected on occasion because somebody points out errors or omissions!)

One advantage to using one relay is you avoid a "relay race" where the difference in response time for two devices causes interlocks to operate incorrectly. There is one less point of failure in the panel when you use one relay. With one relay troubleshooting is expedited because you can see when the output trips the ONE relay that controls ALL the functions. And, on a minor note, the cost of the second relay is only part of the total cost - you have more panel space, and you have to wire two coils, you have to mount two relays. Fore one time, no big deal. For a panel with dozens, it makes a difference.

I agree with Tom. I think there are actually times when 1 DPDT relay is better than 2 SPDT. For example, you might want to use one set of contacts to energize a device such as a remote 120V receptacle outlet and use the other set as a feedback signal to a 24VDC PLC input to indicate that the relay is energized.

I also agree with Tom. Besides- even if you use 2 relays, you are still mixing 24VDC (coil) with 120Vac (contact) on the second relay.
On most ice-cubes the terminals are usually interspersed, too.

Re: well put

vtfan said:

. Nothing worse than cutting off 24V and getting zapped by 120.

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Why would you turn off 24V and not 120? Doesn't seem safe to me.

vtfan said:

I just worked with my finance's brother doing some wiring at the house, he's an electrician and he's color blind.

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You cannot be a colour blind electrician where I live........

You cannot be a colour blind electrician where I live........

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And I thought Canadians were supposed to be openminded.....