GFCI and RCD glossary

AFCI (arc-fault circuit interrupter)

A device that trips on the electrical signature of arcing in damaged wiring. Fire protection, not shock protection. A different device from a GFCI, though combination units exist.

Automatic reset

A GFCI or PRCD that re-closes when power returns after an outage. Chosen for refrigeration, pumps and other loads that must not stay off unattended. See guide 3.

Bonding

Connecting the neutral to the equipment ground (earth) at one point so that a fault has a low-impedance path back to the source. In a building this happens at the service. On a generator it may or may not be done inside the machine. Two bonds in one system make a GFCI trip. See guide 5.

Class A

The UL 943 class of GFCI that protects people: trips between 4 and 6 mA. Every GFCI sold for personnel protection in North America is Class A.

Class B

An obsolete UL 943 class tripping at 20 mA, for swimming-pool lighting installed before 1965. Not personnel protection.

ELCB (earth-leakage circuit breaker)

An older, voltage-operated device that measured the voltage on the earth conductor rather than the current imbalance. Superseded by the RCD; the term is still used loosely for RCDs in some countries.

Floating neutral

A source whose neutral is not bonded to earth or to the frame. Common on inverter generators and small inverters. A GFCI still works on it, because it compares hot against neutral, not against earth.

GFCI (ground-fault circuit interrupter)

The North American device that compares current on the hot and neutral conductors and opens both when they differ by 4 to 6 mA. Built to UL 943 in the US and CSA C22.2 No. 144 in Canada. See guide 1.

GFI

The same thing as a GFCI. An informal shortening, not a different device.

GFPE (ground-fault protection of equipment)

Ground-fault protection at 30 mA or higher intended to prevent fire in wiring and equipment, not to protect people. Sometimes marked GFEP.

Ground fault

Current leaving the intended circuit and returning to the source by another path: through a person, through water, through a damaged casing to earth. The North American term for what the IEC calls residual current.

IΔn (rated residual operating current)

The IEC symbol for the trip level of an RCD: 10 mA, 30 mA, 100 mA, 300 mA. An RCD must trip at IΔn and must not trip below half of it.

In-line

A GFCI or RCD housed in a module spliced into the cord partway along, rather than in the plug. See guide 4.

IP rating

Ingress protection to IEC 60529, two digits: solids then water. IP44 is splash-resistant, IP65 is dust-tight and resists jets, IP66 resists heavy seas and powerful jets, IP67 survives immersion. The North American equivalent on a device listing is "wet location" or "damp location".

Leakage current

The small current every appliance passes to ground through its filter capacitors and insulation in normal operation, typically under a milliamp. Several appliances on one GFCI add their leakage together. See guide 7.

Let-go current

The current above which a person holding a live conductor can no longer release it, commonly given as about 10 mA for adults. The Class A trip level sits below it.

Line and load

Line is the side of the device connected to the supply; load is the side it protects. Everything on the load side is covered, nothing on the line side.

Manual reset

A GFCI or PRCD that stays open after a trip or a power loss until someone presses RESET. Chosen for tools, heaters and anything that must not restart on its own. See guide 3.

Nuisance tripping

A trip with no obvious fault. Usually a real trip on a small fault: accumulated leakage, moisture, a damp heating element, a second neutral bond. See guide 7.

Plug-head

A GFCI or RCD housed in the plug itself, so the whole cord downstream is protected.

PRCD (portable residual current device)

An RCD built into a plug, an in-line module or an adaptor for portable use, to IEC 61540.

PRCD-S

A PRCD that does not re-close on power return (manual reset) and that also checks the protective earth conductor before switching on. The type specified for portable tools in Germany and much of Europe.

RCBO (residual current breaker with overcurrent protection)

An RCD combined with a miniature circuit breaker in one unit, to IEC 61009. Protects against ground faults and overload. Fitted in distribution boards, not on cords.

RCCB (residual current circuit breaker)

A pure RCD without overcurrent protection, to IEC 61008. The distribution-board device most people mean by "the RCD".

RCD (residual current device)

The generic IEC term for any device that trips on residual current: RCCB, RCBO, PRCD. The counterpart of the North American GFCI, at 10 or 30 mA for personnel protection and 230 V. See guide 2.

Residual current

The difference between the current leaving on the live conductor(s) and the current returning on the neutral. Zero in a healthy circuit. The IEC term for what North America calls a ground fault.

Rewireable / user-attachable

A GFCI or RCD with screw terminals and a cord grip, fitted to your own cable instead of a moulded cord. See guide 4.

Self-test

The automatic internal test UL 943 has required on GFCI receptacles since 2015, with a lock-out or indicator on failure. Some cord devices carry it. It does not replace the button test.

TEST and RESET

The two buttons. TEST leaks a few milliamps around the sensor to prove the device trips. RESET closes the contacts. See guide 6.

Trip time

How long the device takes to open after the fault begins. UL 943 ties it to the fault current: several seconds at 6 mA, about 25 ms at high currents. IEC 61008 requires 300 ms at IΔn and 40 ms at five times IΔn.

Type AC, A, F, B

IEC RCD grades by the waveform of residual current detected: AC only; AC plus pulsating DC (Type A, the usual default); plus mixed frequencies (F); plus smooth DC (B). Not used for UL 943 GFCIs. See guide 2.

UL 943

The US standard for GFCIs. Canada's equivalent is CSA C22.2 No. 144. The number should appear on the device.

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