I want to add some protection to an ac 3/4 hp motor. I did some research and found that I could use one of these motor circuit breakers:

enter image description here

My question is, if I use one of these, can I use the on/off buttons it has to start/stop the motor?

A few guys told me I could, but others tell me I should add a contactor with a couple of no/nc push buttons. But if I need to add a contactor, shouldn't I be using a thermal relay instead of the motor circuit breaker?


Yes, you can use that directly all by itself. The other more descriptive term for it is a "Manual Motor Starter" (MMS), because it is specifically designed to be used exactly the way you want to use it. There are however two things it does NOT do for you:

  1. Operate remotely. With a contactor and push buttons, you can mount
    those push buttons somewhere else. With this, you are right there
    where the power is being switched, it's a little less safe overall
    (but done all the time).
  2. Low Voltage dropout. With a contactor and push buttons wired for what is called "3 wire control", if the utility supply power fails, the contactor drops out and does NOT come on again when power is restored until you hit the Start button again. With an MMS like this when power comes back on, the buttons remain in their last state and your motor starts, suddenly and unexpectedly. THAT can be a very serious safety problem. You can add an accessory to the MMS however called an "Under Voltage Release" coil that will accomplish the same function. If it's a dangerous machine that can hurt someone (as most are), I highly recommend that option.

Another consideration is the lifetime of the breaker. It is designed to provide a safety function, to break the circuit under abnormal conditions of excessive load, not for routine operation.

Motor connection circuits normally use these in conjunction with a contactor, for the reasons in J. Raefield's answer (especially NVR - No Volt Release - the undervolt protection he mentions).

As a result of this, the contactor is expected to perform the main switching, and is rated for many operations, while the breaker may only be rated for a relatively small number of operations. Whether this applies to the pictured one I can't say, but the relevant information will be in its datasheet. If in doubt, link to the datasheet in the question.

It is also very useful to have a second means of breaking the circuit for that surprising moment when the first one's contacts weld themselves shut!

So check the breaker's datasheet very carefully before you design it into your circuit as the main switching mechanism. Or follow conventional practice and use a contactor too.

  • 1
    \$\begingroup\$ Thanks for your reply, I do have the usual thermal magnetic breaker plus a rcd in the circuit. I checked the datasheet of the one I bought (different from the pic) and it's rated for 100000 operations. I will probably end up adding a contactor in the future but for now it seems it's safe if I use it manually. \$\endgroup\$ – user135908 Jan 18 '17 at 3:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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