The residual current circuit breaker is the circuit element that detects the fault current caused by the insulation error and cuts the phase (phases) and neutral in the circuit to which it is connected if the detected leakage current value is more than the specified rated leakage current value. Residual current circuit breakers perform life (human) protection at 30 mA and operating (fire) protection functions at 100-300 mA. Due to the lack of protection functions against over current or short circuit currents, automatic fuse must be connected to the circuit. Residual current circuit breakers with rated current ranges from 25 A to 125 A are produced with 2k and 4 poles with a breaking capacity of 10kA. Residual current circuit breakers can go on after reaching 50% of the positive current (residual current) value declared in accordance with the standards. For healthy operation, the neutral and ground line must never touch each other after the residual current circuit breaker. There is no strict rule in the number of lines to be connected to the leakage current switch in the installation. The natural leakage currents of the electrical devices in the circuit should be calculated and the sum of the natural leakage currents should not exceed the rated leakage current value (30/300 mA) of the switch.