I have First Alert hardwired smoke alarms, and I'd like to add monitoring output to my home automation using their RM4 relay, a simple dry-contact relay that activates from the smoke alarm's data signal. It would be low-voltage on the relay contacts.

I'd like to put it into an Allied Moulded RSB-2 large 2-gang box which is the largest 2-gang box I found at 42 cu in.

With high voltage and low voltage in the same box, my understanding is that I need a divider plate.

The RM4 relay has pigtail wires with high and low voltage coming out opposite ends, so I assume I can put the relay on one side of the divider plate and feed the wires under the plate to the other side. Does it matter on which side (high/low) I put the relay? It's fully wrapped in heat shrink; no exposed points. (There is plenty of room for it on either side of the plate.)

Is this the right way to do this? Thanks

1 Answer 1


It doesn't look like that device does what you need, which is guarantee isolation between the input-signal side and the relay-contact side. From the documentation, it sounds like this is mostly for letting the smoke alarm signal (which is 9V DC at pretty low current, as it comes from the typical 9V backup-battery when the AC power is out) control 120V AC loads. This doesn't require guaranteed isolation (both sides are the same class) so it probably doesn't provide it.

Hard-wired devices that guarantee this isolation typically mount in a junction-box knockout so that the high-voltage and low-voltage sides can't mix.

You may be able to find a different single part that guarantees isolation, or you may be able to use this one to control a second device that guarantees isolation. I wouldn't go hooking up random relays to your smoke alarm interconnect - you want something designed and tested to safely interoperate, not something that might cause the signaling to fail in an actual alarm situation.

Junction box dividers are useful when you want to have different classes of wiring in one box that do not interconnect (like ethernet or telephone on one side, and AC power on the other). They can't help a non-isolating device actually provide isolation (if either set of wires cross the divider, some part of them will be on the "wrong" side).

  • Yeah, what one can do is use the RM4 to control one of the various RIBs that supplies isolation (look for the ones that have a barrier in the box) Dec 10, 2023 at 19:53
  • OK, sounds good. The RM4 install page says "1 Form C Dry Contact Relay", but now I get that in a j-box, some part of it would still be on the wrong side of the barrier. A RIB relay with the barrier in its box makes sense. Thanks!
    – Grunthos
    Dec 11, 2023 at 17:02
  • Like this one, right? functionaldevices.com/product/ribu1cw
    – Grunthos
    Dec 11, 2023 at 17:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.