RCD or residual current device, an additional means of protection from electric shock. In addition, they provide protection against fire and fires that occur due to possible damaged insulation, wiring and electrical faults. If you violate the zero level of isolation, direct contact with one of the live parts or on loss of protective conductors RCD is almost the only quick way to protect people from electrical shock. RCD is useful for damaged wire insulation in electrical devices, careless handling of electrical wiring or appliances. Short-circuit, usually develop from defects in isolation, earth-fault current leakage on the ground (a situation where part of the current does not return to the RCD), etc. RCD track leak current and prevent a short circuit, disabling the electrical installation of power supply and thereby preventing unacceptable heating of conductors, arcing and subsequent fire. Since the inception of leakage auto shut off all phases of an emergency electrical system is part of the time, usually not exceeding a period of 0.03-0.3 s depending on the leakage current. In simple terms, compares the current RCD, who went to the apartment, with current, which returned to the apartment.
If these currents are different, the RCD disconnects the voltage. The first active device based on a differential transformer and a polarized relay, which had sensitivity of 0.01 A and speed of 0.1 s was founded in 1937 by Schutzapparategesellschaft Paris & Co. In 1960-1970. worldwide, primarily in Western Europe, Japan, the U.S. began the active introduction of RCD into broad practice.