RCD, GFCI, RCBO and AFCI: How Protective Devices Keep You Safe

A few tens of milliamps through the body can be fatal. A cable, on the other hand, can carry many amps safely. Protective devices exist because no single device can watch for both kinds of danger, plus the separate risk of fire from arcing.

Four devices, four jobs

Circuit breakers: protecting the wiring

A breaker has two trip mechanisms. A thermal element responds to sustained overload that would slowly overheat the cable. A magnetic element trips almost instantly on a short circuit. Fuses do the same job by melting.

Both are sized to protect the cable. A 16 A breaker will happily let 100 mA flow forever, and 100 mA through a person’s chest can be deadly. That is the gap residual-current devices fill.

RCDs and GFCIs: protecting people

In a healthy circuit, every amp that goes out on the live conductor comes back on the neutral. A residual-current device passes both conductors through a small ring-shaped transformer (a toroid). As long as the two currents are equal, their magnetic fields cancel.

If some current leaks to earth, perhaps through a damaged appliance or a person, the currents no longer balance. The difference induces a signal in a sensing coil, and the device trips quickly: a general-type RCD within 300 milliseconds at its rated 30 mA, and faster at higher leakage. A GFCI follows a curve too, tripping in about 25 milliseconds at 250 mA or more and taking up to a few seconds close to its 6 mA threshold.

• GFCIs that protect people in North American homes are Class A devices: they trip at a ground-fault current of 6 mA or more and do not trip below 4 mA.
• RCDs used for personal protection in the UK and IEC countries are rated 30 mA. BS 7671 requires this “additional protection” for socket-outlets rated up to 32 A and for mobile equipment up to 32 A used outdoors, among other circuits. The only exception is for socket-outlets in locations not used by ordinary people or children, where a documented risk assessment by a skilled person shows the RCD isn’t needed.
• In India, RCDs are sold as RCCBs (or RCBOs), and are still often called ELCBs; the product standard is IS 12640. India’s National Electrical Code, SP 30:2023 (Part 3, Section 1), notes that the CEA safety regulations make a 30 mA RCD mandatory for installations of 1 kW and above. It recommends one for every home whatever its load, plus a 300 mA RCD at the point of supply to guard against fire from leakage currents.

RCD types

Modern electronics can produce leakage currents that aren’t pure AC. RCDs are classified by the waveforms they can detect:

• Type AC: sinusoidal AC only. Since Amendment 2 of BS 7671 (2022), Type AC may only be used for fixed equipment known to produce no DC components.
• Type A: AC plus pulsating DC, the usual choice for homes today.
• Type F: Type A plus mixed frequencies, for equipment with single-phase variable-speed drives such as some washing machines and heat pumps.
• Type B: all of the above plus smooth DC, needed for some three-phase inverters and EV chargers that lack built-in DC fault detection.

RCBOs: one device per circuit

An RCBO combines a breaker and an RCD in a single module. The benefit is selectivity: a leakage fault on one circuit trips only that circuit, instead of an RCD shared by several circuits switching off the lights, fridge and outlets together. That is why RCBO-based consumer units are now common in the UK. In North America, GFCI circuit breakers and dual-function AFCI/GFCI breakers do a similar job. (A “combination” AFCI is something else: an arc-fault breaker that detects both series and parallel arcs.)

AFCIs and AFDDs: protecting against fire

Damaged cords, crushed cables and loose terminals can create small electrical arcs. They may draw little current, so a breaker doesn’t notice, but they run hot enough to ignite nearby material.

Arc-fault devices analyze the current waveform for the signature of arcing and disconnect the circuit.

• United States: the NEC requires AFCI protection for most 120 V, 15 A and 20 A branch circuits supplying living areas of homes, such as bedrooms, living rooms and kitchens. Exact locations depend on the code edition adopted locally.
• UK: BS 7671 requires AFDDs on socket-outlet circuits rated up to 32 A in higher-risk residential buildings, houses in multiple occupation, purpose-built student accommodation and care homes, and recommends them elsewhere.

How to look after them

• Press the test button as the manufacturer recommends. For GFCIs, that is commonly monthly. The device should trip immediately and reset.
• Don’t keep resetting a device that keeps tripping. Unplug everything on the circuit, reset, then reconnect one item at a time. A persistent trip with nothing connected points to a wiring fault.
• Outdoor trips after rain often mean moisture in an outdoor outlet or fitting.
• Never bypass or remove a protective device. If it trips for no obvious reason, have a qualified electrician investigate.

For the other half of the safety system, read earthing, grounding and bonding explained.