400 VAC low voltage Incoming Power Supply and Main Cabinet Protection example

This example shows the incoming electrical power supply to the cabinet, including the main circuit breaker, current measurement, surge protection, and the main power distribution busbars.

The external 3-phase 400 VAC / 50 Hz supply enters the cabinet through the incoming cable -WD1. The three phases L1, L2 and L3 are first connected to the main molded-case circuit breaker -Q1. The circuit breaker provides the main protection and switching point for the cabinet incoming power supply.

After the main circuit breaker, the three phases pass through the current transformers -TC1, -TC2 and -TC3. These current transformers are used to measure the current flowing in each phase of the protected cabinet supply.

The measured three-phase supply is then connected to the cabinet's main busbar system -BSB1, from which the electrical power can be distributed to the individual circuits and equipment within the cabinet.

The incoming neutral conductor N is connected to the cabinet's N busbar, while the protective earth PE is connected to the cabinet's PE rail. These provide the required connection points for the neutral and protective earth conductors used throughout the cabinet.

The incoming supply also includes a surge protection device -Q3. The surge arrester provides protection against transient overvoltages, such as those caused by switching events or lightning-related disturbances. The surge protection circuit is provided with its own protective device -Q2 and is connected to the protective earth system.

The schematic also shows feedback signals that allow the control system to monitor the condition of the incoming power protection devices. The status of the main circuit breaker -Q1 is monitored through PLC input I300.2. The surge protection system provides additional monitoring signals, including the SPD protection trip signal at I300.0 and the surge arrester fault signal at I300.1.