Edited by Leland Teschler

A core-balance protection device serves as an example of how these switches operate. Phase and neutral power conductors pass through a toroidal (differential) current transformer. Under normal conditions, the vector sum of the currents in the live and neutral conductors will equal zero. In an unhealthy circuit a residual current flows in the system. The toroidal transformer detects this current and generates a signal in response that feeds to a highly sensitive electromagnetic relay. The relay mechanically actuates the tripping mechanism thereby instantaneously isolating the supply system.

The National Electrical Code demands GFCI devices designed to protect people interrupt the circuit within 25 msec if the leakage current exceeds a range of 4 to 6 mA (the exact trip setting can be chosen by the GFCI manufacturer and is typically 5 mA). The limit of current imbalance for GFCIs designed to protect only equipment (not people) can be as high as 30 mA.

The reason GFCIs are deployed is that the current levels that trigger them are enough to electrocute people or cause damage but not enough to actuate conventional circuit breakers. The GFCI response-time spec is designed to cut power before the heart goes into ventricular fibrillation, the most common cause of death through electric shock.

A point to note about GFCIs is that they can't protect against faults which do not involve an external leakage current, as when current passes directly from one side of the circuit through the victim to the other. Thus they don't protect against overloads or short circuits between the live conductor and neutral or phase-to-phase.

The term earth-leakage circuit breaker (ELCB) is sometimes used interchangeably with GFCI, though strictly speaking an ELCB is a different type of device. ELCBs are generally used only with what are called TT earthing systems, where the earth connection protecting the consumer is through a local connection to earth (often near the receptacle), independent of any earth connection at the generator. TT earthing systems have largely been replaced but the ELCB moniker for protective devices has remained.

Circuit Breaker Industries Inc. (cbibreakers.com) provided information for this article.