Post by Francis Keke

PHASE/SEQUENCE FAILURE RELAY A lot of electrical technicians sees this device below on an electrical panel but can't fathom it's function. The device below is a phase/sequence failure relay. It is used to protect an electrical motor whenever there is a phase failure or when phase sequence changes. CONNECTION Looking at the circuit diagram by the side, pin 2 is the common while pin 3 and pin 1 are normally open (NO) and normally close(NC) respectively. When the relay is powered ON, pin 3 which supply live wire to the contactor will turn to NC. This will remain this way untill an issue arise along the phases which will de-energise the relay making pin 3 to switch to NO. Note: Phase failure is when one or two of the three phases (L1, L2 & L3) failed. If a phase fail, the motor will lose it's rotating magnetic field required to run properly and as a result produces a humming sound. It also draws more current leading to overheating and motor burnout. On phase sequence, imagine what would happen if your motor is connected in order of RYB which is meant for your conveyor to move in clockwise/forward direction but all of a sudden the phase sequence changes to BYR and the conveyor begins to move in anticlockwise/reverse direction. In some case, a lot of damage will be incurred.