How to Connect a Three-Phase Monitor Relay

Issue:

I have never connected a three-phase monitor relay. Do you have a typical wiring connection?

Solution/Resolution:

While each individual application may have its own requirements, this diagram showing a three-phase monitor relay and motor starter is fairly typical of most installations. When the three-phase voltage is present and all conditions are normal, the N.O. contact on the phase monitor relay (typically pins 1-8 on a plug-in product) closes, allowing the motor starter or contactor to be operated as normal. If a fault occurs, the phase monitor relay detects it and will open the N.O. contact, thus de-energizing the motor starter or contactor. When the fault goes away, the phase monitor relay will reclose its N.O. contact, but the motor starter or contactor will not energize until either a START pushbutton is closed or a two-wire control unit (i.e., limit switch, proximity switch, relay contact, etc.) is closed.

With the relay connected before the motor starter as shown above, the motor can be started in the reverse direction. However, the motor is unprotected against phase failures between the relay and the motor.

With the relay connected directly to the motor as shown below, the total feed lines are monitored. This connection should not be used with reversing motors, however.

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