🔍 Key Concepts

• Phase failure protection must sense loss of one phase to de-energize the motor starter coil. • Load side voltage sensing means the relay monitors the voltage on the MOTOR side of the contactor/starter, not on the incoming line side. • Distinguish voltage sensing (direct connection to all three phase conductors) from current sensing (using current transformers or CTs).

💭 Think About

• In which schematic is the phase failure relay actually connected to the same three conductors that feed the motor AFTER the starter contacts? • Which diagram clearly shows the relay terminals tied directly to L1, L2, and L3 on the motor side, without using current transformers? • Which options can you eliminate because the relay is monitoring line side power or motor current rather than the load-side phase VOLTAGE?

✅ Before You Answer

• Verify that the phase failure relay is connected on the load side of the motor starter/contactor (M contacts), not directly to the supply lines. • Confirm that the relay connections are to L1, L2, and L3 voltage lines, and that there are no CT symbols between the motor lines and the relay. • Check that loss of any phase on the motor terminals would immediately remove the relay’s output and open the control circuit to the starter coil (M).