Residual-current device

A residual-current device disconnects an electrical circuit when the current returning through neutral differs from the current supplied through live, indicating leakage to earth or another path.

Current transformer: A transformer that produces an output from the combined magnetic effects of currents in its conductors. Live and neutral pass through its core, so their currents cancel when no leakage occurs.
Ground-fault circuit interrupter: A North American protective device that disconnects a circuit when it detects current leaking from its intended path. It performs the same basic shock-protection function under a regional name and standard.
Miniature circuit breaker: A resettable protective switch that interrupts a circuit when excessive current flows. An MCB responds to overloads and short circuits, not small leakage currents.
Electric current: The rate at which electric charge flows through a conductor or circuit. An RCD compares currents rather than directly measuring voltage or shock.
Electrical safety: The practices and systems used to prevent harm from electrical energy. RCDs are one layer of protection within broader electrical safety measures.
Residual current: The difference between currents flowing through the live and neutral conductors of a circuit. This imbalance is the quantity an RCD detects.
Residual-current circuit breaker: An RCD that provides residual-current protection without built-in overcurrent protection. This form needs separate protection against overloads and short circuits.
Fuse: A sacrificial protective component that melts and opens a circuit when excessive current flows. A fuse usually cannot detect the small earth leakage that an RCD is designed to catch.
Live conductor: A conductor intended to carry supply voltage and current to electrical equipment. It carries the outgoing current that the RCD compares with the return.
Touch voltage: The voltage between conductive parts that a person can touch simultaneously. A fault can create dangerous touch voltage before an RCD disconnects the supply.

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