I'm starting a small project involving 3-pole circuit breakers and 3-ph motors. There's this custom built machine with multiple 3-ph motors, each connected to circuit breakers. The purpose of the circuit breakers is only to prevent damage caused by overload of the motors, which in this case, a motor being stuck. I'm trying to build a supplementary system for this machine.

First off, the machine lacks an emergency stop button(you know, that big red button that stays on when pressed). I want to hook the stop button with the circuit breakers so all the circuit breakers are open(tripped?) when pressed.

I also want to make a controller(probably a Raspberry Pi Compute Module in a shielding running Android) which supervises all the other components of the machine. The most important role of the controller would be stopping the machine when a circuit breaker trip is detected. Currently, a human intervention is needed when this happens(flipping down the main switch, that is).

Other functions of the controller would be: controlling other components of the machine via an RS-485 bus, displaying information derived from sensor data, telling other out-of-scene info(time, weather, news and so on)... all of which are non-essential and supplementary.


I can get all the parts except for this: a 3-pole circuit breaker with DO for circuit on/off status and DI for circuit closure(trip).

Does this thing exist or should I build one from the scratch?

  • \$\begingroup\$ How about a 3 phase circuit breaker? \$\endgroup\$
    – Andy aka
    Commented May 24, 2020 at 9:41
  • \$\begingroup\$ Welcome to EE.SE, sure such a thing exists, its called a safety-relay (but they are very expensive). That said this question is off topic IMO. \$\endgroup\$
    – user173292
    Commented May 24, 2020 at 10:39

2 Answers 2


"... that big red button that stays on when pressed."

You want the button to stay latched when pressed and the circuit to remain off. I think that's what you meant to say but your wording shows muddy thinking which is never a good thing when designing a safety circuit.

I want to hook the stop button with the circuit breakers so all the circuit breakers are open(tripped?) when pressed.

Machinery safety circuits are not designed this way. What you are proposing is that you require a trip coil to work when an e-stop command is given. If the coil or wiring fails you have lost your safety. Instead depending on the classification of the safety risk one or two sets of relay / contactor contacts should be wired in series with each motor so that when the e-stop is pressed that power is removed from the coil and the relays drop out making the circuit safe. This is basic fail-safe design (and yours is not).

The e-stop button should be monitored by a safety relay with a separate reset circuit with button in series with a normally-closed contact of each contactor checking that the contactors all dropped out.

I wrote a detailed explanation of the workings of a safety relay in my answer to Symbol or marking on safety relay.

Be very, very careful.

  • \$\begingroup\$ Thank you for your concern. We all know something. But you should understand that condescending tone is as good as no answer. \$\endgroup\$
    – user253361
    Commented Jun 7, 2020 at 11:20
  • \$\begingroup\$ I know what E-Stop is. I was just trying to be blunt. Maybe it wasn't apt for this community. Sorry about that. \$\endgroup\$
    – user253361
    Commented Jun 7, 2020 at 11:21
  • \$\begingroup\$ I already knew I had to use a safety relay. I can always find workarounds. I just wanted to let my thoughts out to see what others would think. The project didn't take off anyways. Too much work and cost. The problem was kinda non-existent. \$\endgroup\$
    – user253361
    Commented Jun 7, 2020 at 11:23
  • \$\begingroup\$ I was not being condescending. Your question is about safety of machinery so there is no room for the ambiguity shown in your question. There is nothing in your user profile to indicate what level of experience you have so I could only respond to the question itself. \$\endgroup\$
    – Transistor
    Commented Jun 7, 2020 at 11:32

There are molded-case circuit breakers available with the accessories described:

Alarm Switches — Used for remote indication of automatic trip operation. The switch automatically resets when the circuit breaker is reset. These switches mount internal to the breaker in the right side accessory cavity.

Shunt Trip — Provides capability to trip the breaker by remote control. Shunt trips are designed to be applied at specific AC or DC voltages. These devices are installed internal to the breaker in the left side accessory cavity.

The above descriptions are accessories for the Eaton Power Defense line of circuit breakers. Similar features are likely offered by other manufacturers.

  • 1
    \$\begingroup\$ There are also undervoltage lockouts, those will trip the breaker when the voltage is removed. Opposite of the shunt trip. \$\endgroup\$
    – Jeroen3
    Commented May 24, 2020 at 13:51
  • \$\begingroup\$ Using the undervoltage lockout would satisfy the fail-safe requirement mentioned by @Transistor. \$\endgroup\$
    – user80875
    Commented May 24, 2020 at 14:25
  • \$\begingroup\$ Everybody makes shunt trip breakers. There are some other features only Eaton makes, like remote control breakers (basically an RR7 relay built into a breaker as a separate mechanism). \$\endgroup\$ Commented May 24, 2020 at 15:45

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