a line - side circuit interrupter protection system monitors the current flowing in a circuit in order to determine whether any current is flowing outside of the circuit upstream of the point at which a power supply is connected to a plurality of circuits . the system is located between the power supply and the main circuit panel , which is advantageous in certain applications in which an auxiliary power supply is used when a utility power supply is unavailable . in the event that the system detects current flowing outside of the circuit , the circuit will be broken to eliminate any potentially unsafe conditions .