A residual current device (RCD) is a safety switch that automatically cuts off the electricity supply when it detects current leaking along an unintended path, such as through a person’s body or into the ground. It works by continuously monitoring the current flowing out through the live wire and back through the neutral wire. In a healthy circuit, those two values are equal. When they’re not, the RCD trips in milliseconds, cutting power before the leaking current can cause serious harm.
How an RCD Detects a Fault
Inside every RCD is a ring-shaped transformer that both the live and neutral wires pass through. Under normal conditions, the current flowing out on the live wire and returning on the neutral wire creates two equal but opposite magnetic fields, which cancel each other out. The transformer’s net output is zero.
When current escapes the circuit, perhaps because someone touches a live component or water creates a path to earth, less current returns through the neutral wire than left through the live wire. That imbalance, called the residual current, generates a small signal in the transformer. Once this signal reaches a preset threshold, the RCD triggers a mechanical switch that opens the circuit and kills the power. The whole process happens fast enough to prevent a lethal shock.
RCD vs. Circuit Breaker vs. RCBO
A standard miniature circuit breaker (MCB), the type found on most consumer unit switches, protects wiring from overcurrent caused by short circuits or overloaded circuits. It does not detect earth leakage, so it offers little protection against electric shock. An RCD, by contrast, specifically monitors for current leaking to earth but does not protect against overload or short circuits on its own.
An RCBO (residual current breaker with overcurrent protection) combines both functions into a single device. It protects against earth faults, overloads, and short circuits. In a modern consumer unit, you’ll often see either a combination of RCDs and MCBs, or individual RCBOs on each circuit.
Trip Sensitivity and Speed
RCDs are rated by how much residual current triggers them. Two thresholds cover most residential situations:
- 30 mA (personal protection): This is the standard sensitivity for protecting people. A current of 30 milliamps flowing through the body is dangerous but not instantly lethal, so tripping at or below this level prevents fatal electrocution. These devices must disconnect within 40 milliseconds.
- 300 mA (fire protection): A higher-rated RCD won’t necessarily prevent a shock, but it catches larger earth faults that could heat wiring or components enough to start a fire.
Time-delayed RCDs also exist. These are used upstream to allow a downstream RCD to trip first, preventing an entire building from losing power over a single fault. A time-delayed device cannot be used for personal shock protection because it doesn’t meet the 40-millisecond disconnection requirement.
Where RCDs Are Required
Under current UK wiring regulations (BS 7671), 30 mA RCD protection is required on nearly every circuit in a domestic property. That includes all socket outlets, all lighting circuits, any circuit supplying equipment used outdoors, all circuits in a bathroom, and any cables buried in walls at a depth of less than 50 mm or running through walls containing metal parts. In practice, this means almost every circuit in a modern home should have RCD protection.
Older homes wired before these regulations may have limited or no RCD coverage. If your consumer unit still uses rewirable fuses and has no RCD, your installation predates these requirements.
Three Ways RCDs Are Installed
Fixed RCDs sit inside the consumer unit and protect entire circuits, covering all the wiring, sockets, and connected appliances on that circuit. This provides the most comprehensive protection because it guards against faults in the fixed wiring itself, not just in appliances.
Socket-outlet RCDs are special sockets with built-in RCD protection. They protect anyone using equipment plugged into that specific socket, but they don’t cover the wiring behind the wall leading to the socket.
Portable RCDs are plug-in adapters that go between a standard socket and an appliance. They’re useful as a stopgap when neither of the other types is available, for example when using power tools outdoors on an older property. Like socket-outlet RCDs, they only protect the appliance plugged into them.
Different Names Around the World
The technology goes by different names depending on where you are. In the UK, Europe, and Australia, the terms RCD, RCCB (residual current circuit breaker), and RCBO are standard. In the United States and Canada, the same concept is called a GFCI (ground fault circuit interrupter) or GFI. The underlying principle is identical: detect a current imbalance between live and neutral, then disconnect.
RCD Types: AC, A, F, and B
Not all earth leakage looks the same electrically. Simple resistive appliances like heaters and incandescent bulbs leak pure AC current. Electronics with rectifiers, inverters, and variable-speed motors can produce pulsating or even smooth DC leakage that a basic RCD won’t detect. This is why RCDs are classified into types.
- Type AC: Detects only pure AC leakage. Suitable for simple loads like heaters, ovens, and traditional motors. Cannot detect any form of DC leakage.
- Type A: Detects AC plus pulsating DC leakage. Covers most household electronics: washing machines, refrigerators, computers, TVs, LED lighting, and microwaves. This is the most commonly specified type in modern domestic installations.
- Type F: Covers everything Type A does, plus higher-frequency pulsating DC from single-phase inverter equipment. Designed for inverter-driven air conditioners, heat pumps, variable-speed washing machines, and UPS systems.
- Type B: The broadest detection range, including smooth DC with no pulsation at all. Required for EV chargers, solar panel inverters, battery storage systems, and three-phase industrial drives.
Installing the wrong type can leave a gap in protection. An EV charger on a Type AC device, for example, could produce a DC fault that the RCD completely ignores.
Testing Your RCD
Every RCD has a small test button, usually marked “T.” Pressing it sends a controlled current through an internal resistor that bypasses the transformer, simulating a fault. If the device is working, it should trip immediately and cut power to the protected circuits. You then reset it by pushing the switch back to the “on” position.
Manufacturers generally recommend pressing the test button once every six months in a normal household environment. Some older guidance suggests monthly testing. If the RCD fails to trip when you press the button, it needs replacing. The test button only confirms the mechanical tripping mechanism works; it does not verify the sensitivity is still within specification. A qualified electrician can perform a more thorough test using calibrated equipment.
Why an RCD Trips When Nothing Seems Wrong
Nuisance tripping is one of the most common frustrations with RCDs, and the cause is often cumulative leakage. Every appliance allows a tiny amount of current to leak to earth through its filters and insulation. Individually, these amounts are harmless, often between 1 and 4 milliamps per appliance. But when several appliances share the same RCD, their leakage currents add up. If the combined total approaches 30 mA, even a brief spike from a motor starting or a compressor cycling can push the total over the threshold.
Other common causes include moisture in outdoor sockets or junction boxes, a deteriorating appliance with failing insulation, and neutral-to-earth faults somewhere in the wiring. A neutral-to-earth fault is particularly tricky because it can cause tripping only when other appliances on the circuit are drawing current, making the pattern seem random. Unplugging appliances one by one and seeing whether the tripping stops is a reasonable first step, but persistent or unexplained tripping usually needs professional diagnosis. Redistributing circuits across separate RCDs can also help by reducing the cumulative leakage on any single device.