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We recently installed a whole house fan that has a max current of 6.5A at 120V. Due to the dangers of whole-house fans accelerating fires, we wanted to be safe and wire in a smoke alarm relay so that if the smoke alarms go off the fan is automatically shut off.

Our smoke alarms are hardwired Kidde alarms and we found the SM120X device to perform the relay. We were able to put the smoke alarms and the whole house fan on their own 15A circuit.† The relay seemed to work the first time we turned on the fan (on the normally closed / NC wire) but the second time the fan lasted less than one second before shutting off. It appears as though the relay has been killed since the NC wire now no longer carries any current as tested with a multimeter and after having reset the entire circuit. When the relay is bypassed, the fan works fine.

Then doing more research after the fact, the problem is likely that the relay is only rated for 10A of non-inductive load and a whole house fan is a fairly large inductive load... this also fits with the fact that the second time it lasted a moment before dying since the inductive spike had some time to travel back and fry the relay.

One suggestion I read was that if the inductive load is only a "few" amps that an RC snubber (a resistor and capacitor in series placed in parallel to the load) could protect the relay. However, they did not mention what a "few" was or what size snubber.

So, what should be used to protect the relay from the inductive load?

† We made sure to install the relay in the living space (a closet) and not the unfinished attic since we noted that the documentation states "Do not exceed the temperature or humidity limits of +40°F (4.4°C) to 100°F (37.8°C) (such as in garages and unfinished attics)". So, we didn't completely ignore the manual...

thaimin
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    If the goal is to reduce fires, mashing together homebrew components is not the way to go LOL... – Harper - Reinstate Monica Jun 26 '22 at 00:32
  • @Harper-ReinstateMonica Not planning to use homebrew components for this, they make pre-packaged RC snubbers meant for this (at least I found a few). – thaimin Jun 26 '22 at 01:33
  • @knowitall The SM120X is not being used for that. The fire alarms are all there and interconnected without the relay. We added the relay afterward. Also, there is no fire alarm panel. It is being used as a relay (its marketed purpose). – thaimin Jun 26 '22 at 01:35
  • Confused, so who or what is giving power to that relay, if it is not the fire sensors – Ruskes Jun 26 '22 at 01:56
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    @knowitall it is, but what you quoted from the document was that SM120X should not be used to bridge otherwise independent groups of alarms together. This is a single group of alarms and the relay is using their interconnect signal to flip another device that isn't a fire alarm panel. In fact, the circuit we are using is nearly Figure 1 on the second page (except we are using NC instead of NO and we have an additional alarm in the main circuit). – thaimin Jun 26 '22 at 02:37
  • Thank you, so you are using the Normally Closed output (NC), not the Normally open (NO). Would mean when the really is not energized the 120 V. goes to Fan. Now tell me about the Inductive load – Ruskes Jun 26 '22 at 02:51
  • How many HP is the motor in your whole-house fan? – ThreePhaseEel Jun 26 '22 at 03:47
  • @ThreePhaseEel It doesn't say on the device, but 6.5 A @ 120 V is ~780 watts which is ~1 HP. However, I don't know if you can directly convert like that with AC (I feel like some consideration of phase or something has to be taken into account). – thaimin Jun 26 '22 at 16:03
  • @thaimin yeah -- by the Full Load Amps tables in the NEC (430.248 to be precise) -- a 6.5A 115VAC motor is somewhere between 1/4th and 1/3rd HP...anyway, it's pretty fractional horsepower, although NC contacts tend not to have very good ratings, so you'll need a somewhat beefy RIB for this, such as a RIB01P30-S-NC – ThreePhaseEel Jun 26 '22 at 16:23
  • When you take *components* (UR Recognized) and install them directly in house wiring, that's what I mean by homebrewing. Equipment is UL listed (ETL, MET, CSA being equivalents). When a manufacturer is getting a UL Listing, it helps if the internal components are UR Recognized, as those components won't need to be tested again. That's all that does. UL must still check the equipment for design usage and fitness for purpose, and to ensure the provided instructions comply with Code. Also the instructions define the scope of testing. – Harper - Reinstate Monica Jun 26 '22 at 22:23
  • @Harper-ReinstateMonica Thanks for clarifying. I definitely want to do this right. Thanks. – thaimin Jun 29 '22 at 14:16
  • @ThreePhaseEel Would a RIB01BDC work as well? It has the same 1 HP @ 120V rating as the RIB01P30-S-NC and has an NC connection. They don't give a reduced amount for NC contacts (except for tungsten and electric ballast). – thaimin Jun 29 '22 at 14:56
  • @thaimin the RIB01BDC has no HP rating for its NC contacts at all, so no – ThreePhaseEel Jun 30 '22 at 01:10
  • @ThreePhaseEel The datasheet at https://www.functionaldevices.com/wp-content/uploads/2021/08/RIB01BDC.pdf has some that are specific for NO and some for NC, but the 1 HP @ 120 V has neither NO or NC classification, wouldn't that imply that it supports that much for both? Otherwise, why would they have an NO classification on some like electronic ballast which definitely has no NC rating? – thaimin Jul 01 '22 at 02:14
  • @thaimin -- I reckon that a call to Functional Devices is the only way to be sure on this one – ThreePhaseEel Jul 01 '22 at 02:58
  • @ThreePhaseEel Already sent in a request. May call tomorrow. The one you found just seems so much like overkill and also not available from Grainger (but is available from other suppliers, just many times more expensive). I found another one that has a clear 1/3 HP @ 120 V for N/C and 1/2 HP @ 120 V for N/O that would "work" but would be very tight (fan is likely 0.3 HP from interpolating the NEC chart). Also, thanks for all your input during this! – thaimin Jul 01 '22 at 03:01
  • It is rated for both N/C and N/O if not labeled with either. This does not fully apply to any "ballast" rating, however, but HP ratings are fine. – thaimin Jul 12 '22 at 16:11

2 Answers2

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Sounds like a job for a RiB or Aube relay. These are both UL-Listed general purpose relays made specifically for applications like this. You must use UL listed equipment (not UR-Recognized components) NEC 110.2, and you must install in all respects according to labeling and instructions NEC 110.3.

Any snubber you might use would need to be UL Listed for use in residential AC electrical, and that will narrow your product selection to nil most likely.

The relay will take care of any snubbing issues via its rating, which you will assure is sufficient when you conform with 110.3.

The Aube contains its own transformer and you simply shunt 2 wires together to throw the relay.

The RiB requires power be supplied from elsewhere, such as 120V from the fire alarm circuit. The RiB must be chosen to have the appropriate coil voltage.

Both of them are designed to be mounted in a standard steel junction box with knockouts, with Romex cables entering through cable clamps. As required by the electrical code.

The Aube intentionally keeps the low voltage terminals on the outside of the junction box and the high voltage pigtails on the inside. This doesn't matter here since the fire alarm module does not provide separation, therefore if 24V relay coil voltage was used, it would still need to be in AC rated cable (Romex).

Harper - Reinstate Monica
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  • Yea. Don't start messing around with snubbers, use a relay that's properly rated for the job instead. – Mast Jun 26 '22 at 10:28
  • Yeah, all of the snubbers I saw were only UL-Recognized which is kind of what led me to ask if that was at all right. I am looking into the RIB relays right now, but getting a bit lost in all of their parameters (Grainger has 16 different ones with 120V coil voltage). – thaimin Jun 26 '22 at 16:11
  • @Thaimin well you don't have to closely match anything but the coil voltage; all other parameters only need to be >=. In fact, an abundance of switching amps only helps durability. – Harper - Reinstate Monica Jun 26 '22 at 22:25
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Motors need beefier switching to maintain controllability. In industrial situations, devices called 'contactors' are used, essentially relays that can handle the inductive kickback from a motor turning off.

The contactor has a small relay coil that can probably be safely driven by your SM120X, and the contactor can disconnect power to the whole-house fan.

One thing of concern might be this:

We were able to put the smoke alarms and the whole house fan on their own 15A circuit.

If I accurately understand this to mean that you run both the fan and the smoke detectors from a single circuit breaker, that may be a code violation and dangerous. I'd suspect that the smoke detectors need their own separate circuit. If the fan should happen to trip the breaker, then the detectors will have to rely on the batteries.

I would run a separate circuit for the fan. Get a good contactor for the fan that has a 100% duty cycle, that is, the contactor coil can stay powered on all the time. It will be on all the time so the contactor stays closed, and so the fan motor can be powered on as needed from a wall switch etc in series with the contactor.

Then drive the contactor coil with power routed through the NC contacts on the SM120X switch. This power should also probably come from the fan circuit.

The power for the SM120X coil will come from the smoke detector circuit.

Under normal operation the SM120X switch would be closed, power goes through the SM120X to the contactor coil, which stays energized and closes the contactor, letting power go to the fan motor as desired through the wall switch.

In a smoke alarm, the SM120X triggers, opens the NC switch and de-powers the contactor coil, which opens the contactor, which in turn cuts power to the fan motor and absorbs the kickback.

Triplefault
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    Residential smoke *detectors* do *not* need their own circuit -- in fact, it's best to put them on a hall lighting circuit or such, so you know if the circuit ever trips – ThreePhaseEel Jun 26 '22 at 03:46