The Capacitor Trip Unit (CTU) is usually installed in substations and operates the shunt trip coil of a circuit breaker during a loss of power supply to the transformers. This prevents transformers being automatically energized following restoration of the main power supply.

The unit can also be used to initiate alarms or provide standby excitation.The unit also serves as a continuous DC power supply or stored energy .The unit is housed in a high impact polycarbonate enclosure.