Residual-current Device - Purpose and Operation

Purpose and Operation

RCDs are designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 5–30 milliamperes) and disconnecting quickly enough (<30 ms), they may prevent electrocution. There are also RCDs with intentionally slower responses and lower sensitivities, designed to protect equipment or avoid starting electrical fires, but not disconnect unnecessarily for equipment which has greater leakage currents in normal operation. To prevent electrocution, RCDs should operate within 25-40 milliseconds at leakage currents (through a person) of 30 milliamperes, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would not cause the total current to become high enough for a fuse or circuit breaker to break the circuit, let alone do so fast enough to save a life.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between the current flowing through the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to another circuit), and the device will open its contacts.

Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents, except for the special case of a short circuit from live to ground (not live to neutral).

For a RCD used with three-phase power, all live conductors and the neutral must pass through the current transformer.