What is the primary cause of premature relay failure with AC inductive loads: inductive kickback causing arcing across the contacts or the inrush surge current exceeding the capacity of the contacts? Specifically I'm trying to find the right relay to control a 120vac, 7.7Amp (1/2-hp) furnace motor. The relay would be driven by smoke detectors.

EDIT: To clarify, the furnace has a main power switch and is also turned on and off by the thermostat. The relay itself is not used to turn the furnace on or off EXCEPT in case of a fire, in which case the smoke detectors activate the relay, opening the N/C contacts to shut off the furnace.

An engineer at a major relay manufacturer recommended a model of theirs that would handle the high inrush current but made no mention of any voltage spike suppression. Discussions on this board, however, focus mostly on RC snubber solutions.

Given the nature of my application, the N/C relay contacts would remain closed in normal operation except in the case of fire, monthly detector tests or the occasional burnt slice of toast. So there should be minimal (but not zero) arcing occasions. On the other hand, inrush surges would occur several times a day through the closed contacts as the furnace cycles on and off.

This leads me to think that over-sizing the relay capacity is, indeed, the most important thing. But do you think it would also be advisable to look at a snubber?

OK, no resolution yet. But here's another possible approach... Doesn't a furnace thermostat use a relay to turn the furnace on and off? If so, instead of using the smoke-alarm-connected relay to cut out the furnace motor directly, might it be possible to use this (new) relay to control the current to the existing relay that's already installed somewhere in the furnace? Presumably this existing relay is already properly spec'd and/or surge-protected. I don't have a circuit diagram for this furnace but does that sound like a possibility?

  • \$\begingroup\$ With relays it is only the state change that is of consequence. A surge current through already closed contacts is like a surge current through wire, nothing happens. The troublesome parts is a surge current when the contacts are closing and inductive kick when the contacts are opening. \$\endgroup\$ – vini_i Mar 23 '17 at 15:29
  • \$\begingroup\$ If the relay operates several times a day to cycle the furnace on and off, all of the contacts would open and close. Are you asking about more than one relay? Normally, the motor current is carried by only the normally open contacts of one relay or contactor. That relay or contactor should have a motor power rating. That should be used to select the relay, not the current rating. Relays that do not have motor power ratings should not be used to switch motor current. You need to revise the question to make it clear. A diagram would help. \$\endgroup\$ – Charles Cowie Mar 23 '17 at 15:34
  • \$\begingroup\$ It sounds like you are adding a relay that has a normally closed contact in series with the main power switch. The relay is energized in case of fire to shut down the furnace. Energizing something for emergency shutdown in case of fire is generally not good practice. Snubbers are often used across relay and solenoid coils, but not across motors. Any relay that must make or break motor current should be designed and identified as suitable for that purpose except for very small motors. \$\endgroup\$ – Charles Cowie Mar 23 '17 at 17:57
  • \$\begingroup\$ Yes, that's it. A relay in series with a main power switch. It may not be good practice but it's required by the Building Code here in Ontario if I want to have a legal apartment in my home. I've certainly seen mention of snubbers across motors. @MEZLAW (below) provides a link to such a solution. But I'd love to find a relay specifically designated for breaking a motor current. Does anybody know of one? \$\endgroup\$ – doozit Mar 23 '17 at 23:33
  • \$\begingroup\$ @vini_i Thanks for that excellent point. State change is the key, then. Since the contacts are normally closed (and normally STAY closed), the inrush current should not really be an issue. Except that the contacts do occasionally open and then close again. I'm wondering why the relay manufacturer is ignoring the arcing. \$\endgroup\$ – doozit Mar 23 '17 at 23:48

What is not good practice is to have a relay that must remain energized to shut off a system that must be shut down in case of fire. Cutting off power to the furnace may be ok.

The thermostat probably doesn't immediately shut off the fan. It tells the furnace to shut off the heat. That will first shut the fire off, then some time later, shut off the fan. That allows the remaining heat in the furnace to be circulated through the building.

You don't want to just shut off the fan without shutting off the fire in the furnace. The furnace must be designed to shut down safely in the event of power loss, so I would think that it would be ok to just cut off the power to the furnace for an emergency shut-down.

However, I would think that the furnace should have an emergency shut down provision that would allow connecting a low-current relay to a special place in the furnace control unit. You should probably investigate that. I would think that a heating contractor would know exactly what is the recommended way to accomplish what you want to do.

If you want to cut off the furnace power with a normally open relay contact, search for Hp (horsepower rated) relays. Note that a modern furnace will have both an air circulation fan motor and a forced draft fan motor. Since normally closed contacts are not usually used in motor power circuits, you may not find a Hp rated relay with NC contacts.

The smoke detectors deliver 24V to the relay when they are triggered. Not sure how I could use them to keep the relay energized.

Use a latching circuit as shown below.

enter image description here

  • \$\begingroup\$ Excellent points, Charles. We actually had an NC relay installed for this purpose 2-1/2 years ago by the electrician who installed the smoke detectors. But it failed after several months as did its identical replacement. Turns out it was designed for non-inductive loads only. Your suggestion to talk to a heating contractor makes a lot of sense! \$\endgroup\$ – doozit Mar 24 '17 at 12:19
  • \$\begingroup\$ Charles, thx for your diagram. I assume that Smoke Alarm is the N.C. contacts of a relay connected to the alarm. I'm still hoping for a one-relay solution so I'm searching for that HP rated relay with NC contacts. \$\endgroup\$ – doozit Mar 25 '17 at 22:38
  • \$\begingroup\$ Yes. The NC contact would be from a relay energized by the alarm and would only need to handle the coil current of the motor-rated relay. \$\endgroup\$ – Charles Cowie Mar 25 '17 at 23:50

You need to make your own contact protection circuit, this is something I learned back in the electro-mechanical days at the Bell System.

This should help you design what you need; https://www.industrologic.com/mechrela.htm

  • \$\begingroup\$ Thanks, MEZLAW. That link does help but it's a bit lacking in detail for AC motors. Not sure what values to use for R & C. :-( \$\endgroup\$ – doozit Mar 23 '17 at 23:39

The answer to my original question is: inrush surge current was the cause of my two furnace relays frying. This is based on the observations that (a) the normally closed contacts opened only a few times before failure - therefore not many arcing events; (b) on the other hand the furnace motor would have stopped and started hundreds of times leading up to the failures - with huge surges running through the NC contacts at each startup.

Also, I must thank Charles Cowie for pointing me in the right direction to actually solve my problem. Charles suggested talking to a furnace contractor. Bingo! The SM120X relay is fine to use but not to interrupt the 120V power to the furnace. Instead it should be used to interrupt the 24V control circuit within the furnace which, in turn, will cut power to the furnace. In practice this means putting the relay's normally closed contacts into the path of the Red wire from the thermostat.


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