Two names for one measurement
Both devices measure the residual current, meaning the difference between what leaves on the live conductor and what returns on the neutral, and open the circuit when it passes a threshold. "Ground fault" is the North American way of describing that event and "residual current" is the IEC way. Everything around the principle is different.
Trip levels
| Device | Standard | Trips at | Purpose |
|---|---|---|---|
| GFCI, Class A | UL 943 (US), CSA C22.2 No. 144 (Canada) | 4 to 6 mA | protection of persons |
| RCD, 10 mA | IEC 61008 / 61009 / 61540 | 10 mA | additional protection where required (schools, hospitals, wet trades) |
| RCD, 30 mA | IEC 61008 / 61009 / 61540 | 30 mA | general protection of persons |
| RCD, 100 / 300 mA | IEC 61008 / 61009 | 100 to 300 mA | fire protection, selectivity upstream |
The gap between 5 mA and 30 mA is the question we get asked most. Both levels are below the current at which a shock becomes fatal for most people in the fraction of a second the device takes to trip. The IEC settled on 30 mA because it protects a person and still keeps nuisance tripping low on circuits where several appliances each leak a little through their filters. North America chose the lower figure and accepted more nuisance trips. You cannot shop for the "safer" one. Each is what its own code requires.
Voltage and plugs
A GFCI cord for North America is built for 120 V (NEMA 5-15P, 5-20P, L5 series) or 240 V (6-15P, 6-20P, L6 series). An RCD cord is built for 220 to 240 V single-phase and follows the country's plug pattern: Schuko CEE 7/7 for Germany and most of Europe, BS 1363 for the UK, Ireland and the Gulf states, CEI 23-50 for Italy, AS/NZS 3112 for Australia and New Zealand. Plugging a 120 V GFCI into 230 V through an adapter destroys it. Plugging a 230 V RCD into 120 V leaves a device that may never trip.
Types AC, A, F and B
IEC RCDs are graded by the waveform of residual current they can detect. Type AC senses sinusoidal AC only. Type A adds pulsating DC, which is the leakage from rectified loads such as variable-speed drives, LED drivers and switch-mode power supplies. That is most modern equipment, so Type A is the usual default. Type F adds mixed-frequency leakage from single-phase inverter drives. Type B adds smooth DC for three-phase drives, EV chargers and solar inverters. A portable RCD (a PRCD to IEC 61540) is normally Type A. A UL 943 GFCI is tested against the fault types the North American market presents and does not carry these letters.
Approvals to look for
On a GFCI: UL Listed, or ETL or CSA to UL 943 / C22.2 No. 144, printed on the device itself and not just on the cord. On an RCD: VDE, KEMA-KEUR, ENEC, the BSI Kitemark, SAA / RCM depending on the market, with the standard number on the body. An RCD cord for export equipment should carry the mark of the destination country. Guide 9 is about RCD cords for export equipment and goes country by country.
Where each is required
US: NEC 210.8 for receptacle locations and OSHA 1926.404 for construction. UK: BS 7671 requires 30 mA RCD protection for socket-outlets up to 32 A and for mobile equipment used outdoors. Australia and New Zealand: AS/NZS 3000 requires 30 mA RCDs on final sub-circuits supplying socket-outlets. Germany and most of Europe: DIN VDE 0100-410, the national edition of IEC 60364-4-41, with 30 mA on socket-outlets up to 32 A. Many sites and insurers ask for 10 mA on top.
The rule we work by
Buy the device for the country the equipment will run in, not the country it ships from. A machine built in Ohio and installed in Hamburg needs a 230 V Schuko RCD cord with a VDE-marked device. A UL GFCI on a travel adapter is neither legal nor functional there. We build both families to the destination's standard, and we will tell you when a customer's drawing asks for the wrong one.