RCD (protective shutdown device) or differential relay are used to protect people during current leakage, in case of damaged insulation of the wiring or low -quality connection. These devices also protect against the occurrence of a fire in the presence of prolonged current leaks. RCDs 2-pole are used in single-phase networks and open the contacts if the residual current exceeds the given rated value of the turnover current. Depending on the brand of the manufacturer of the devices, the range of these values may vary from 10 to 500 miliamper. The RCD does not consume electricity and can maintain performance if there is a cliff of the zero conductor. The correct connection of the device is determined using the corresponding test button.