Residual-current device

In electrical engineering, residual current circuit breakers (RCCB) or residual current devices (RCD) are circuit breakers that operate to disconnect their circuit whenever they detect that current leaking out of the circuit (such as current leaking to earth through a ground fault) exceeds safety limits. These devices can be tested to see if they are operational and/or they have been wired correctly. Manufacturers are now designing such devices that will always disconnect their circuit if wired improperly.

The term ground fault circuit interrupter (GFCI) is used in the United Sates.

The terms earth leakage circuit breaker (ELCB) is also used.

Operation

The device operates by measuring the balance between in-bound and out-bound currents using a differential current transformer, and operating the circuit breaker if the currents do not cancel out.

They are designed to attempt to prevent electrocution or fire by detecting the leakage current, which can be far smaller (typically milliamperes) than the trigger currents needed to operate conventional circuit breakers, which are typically measured in amperes. They are intended to operate quickly, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock.

Residual current detection is complementary to, rather than a replacement for, conventional over-current detection, as residual current detection cannot provide protection for faults which do not involve an external leakage current, for example faults that pass the current directly from one side of the circuit through the victim to the other.

See also:

• electrical safety

External links

• http://www.safety.ed.ac.uk/policy/part3/52.html
• http://www.elec-toolbox.com/Safety/safety.htm#GFCIs