Circuit design of three-phase power electronic and electrical protector

This is the circuit design of a three-phase power electronic and electrical protector. Many electronic devices require three-phase AC power to operate. If any failure occurs at any stage, then it will make the device prone to unstable functionality and may even lead to failure. Therefore, it is crucial to monitor the availability of three-phase power and shut down electronic equipment if any fault is detected in one or two phases. To avoid surges and transient fluctuations, the power supply to an appliance should resume with a certain time delay when all mains phases are available.

The electrical protector requires three-phase AC power, three 12V relays and a timer ICNE555, and a four-pole 230V coil contactor to turn the electrical appliance on and off.

How the protection circuit works

Relays RL1 and RL2 are used as sensing devices for the Y phase (yellow) and B (blue) phase respectively. These relays are connected in such a way that each relay acts as an enabling device for the following relay. The combination of relays therefore forms a series logic AND gate.

The availability of phase R (red) energizes relay RL1 and its normally open (N/O) contact closes to connect phase Y to the input of transformer X2. The availability of phase Y energizes relay RL2 and its N/O contact closes to connect phase B to the input of transformer X3, thereby applying the trigger input to timer ICNE555 (IC1).

Therefore, the delay timer around the NE555 will only fire when all phases (R, Y and B) are available. It provides a delay of approximately 4 seconds, allowing relay RL3 to energize and its N/O contacts to close, connecting the line to the energized coil of four-pole contactor relay RL4. Contactor RL4 is closed to ensure that three-phase power to the device is available.

The rating of contactor RL4 can be selected based on the full load current rating of the appliance. The four-pole contactors here have contact current ratings up to 32A. The availability of R, Y and B phases is monitored by appropriate LEDs connected to the secondary windings of transformers X1, X2 and X3 respectively. Therefore, this circuit does not require separate indicator lights to monitor the availability of the three phases. LED1 emits light when phase R is available. LED2 emits light when the Y phase is available. When phase B is available, LED3 lights up.

The main advantage of this protection circuit is that it protects three-phase appliances from any faults installed on the back of the cabinet. Connect the device via external wires.

WARNING: Circuit contains mains high voltage. Make sure to disconnect AC power during circuit assembly and double check everything before connecting the circuit to AC power.