An RCD, or residual current device, is a safety device in your electrical system that cuts power almost instantly when it detects current leaking where it shouldn’t, such as through your body during an electric shock. It provides a level of personal protection that ordinary fuses and circuit breakers cannot. While fuses protect wiring from overheating, an RCD protects people from electrocution and can also help prevent electrical fires.
If you’re in North America, you probably know this device by a different name: a GFCI, or ground fault circuit interrupter. The technology and purpose are essentially the same.
How an RCD Works
An RCD constantly monitors the current flowing out through the live wire and returning through the neutral wire of a circuit. In a healthy circuit, those two values are equal. Every bit of electricity that goes out comes back. But if some current escapes through an unintended path, like through a person who has touched a live part, or through damaged insulation into a wall, the outgoing and returning currents no longer match.
Inside the RCD, a component called a differential current transformer detects this imbalance. When the difference exceeds a preset threshold, the magnetic force generated becomes strong enough to trigger a switching mechanism that breaks the circuit. For personal protection, the standard trip threshold is 30 milliamps (mA). That’s a tiny amount of current, but it’s chosen for a critical reason: currents above 30 mA passing through the human body can send the heart into ventricular fibrillation, which is the most common cause of death from electric shock.
The speed matters just as much as the sensitivity. A standard RCD must trip within 25 to 40 milliseconds, fast enough to cut power before that dangerous current can do fatal damage.
RCDs vs. Fuses and Circuit Breakers
A fuse or miniature circuit breaker (MCB) protects your wiring. It trips when too much current flows through a circuit, preventing cables from overheating and starting a fire. But if you touch a live wire and current passes through your body to the ground, the amount of current involved is far too small to blow a fuse. A 30 mA leakage is lethal to a person but invisible to a device designed to trip at 6, 16, or 32 amps.
That’s the gap an RCD fills. It watches for the specific kind of fault that endangers people, not just property.
Types of RCD Protection
RCDs come in several forms depending on how they’re installed and what additional protection they include.
- Fixed RCD (or RCCB): Installed in the consumer unit (breaker panel), it protects a group of circuits or an entire installation. It only detects earth leakage, not overcurrent or short circuits, so it works alongside separate fuses or MCBs.
- RCBO: Combines an RCD and an MCB into a single device. It protects against earth leakage, overcurrent, and short circuits all at once. Because each RCBO covers an individual circuit, a fault on one circuit won’t trip the power to the rest of your home.
- Portable RCD: Plugs into a socket and protects whatever is connected to it. Useful when you’re using power tools or outdoor equipment and aren’t sure whether the fixed wiring has RCD protection.
RCD Sensitivity Ratings
Not all RCDs are set to the same threshold. The sensitivity rating determines what the device is designed to protect against.
A 30 mA RCD is the standard for personal shock protection. It’s required on circuits serving bathrooms, outdoor sockets, and other areas where the risk of contact with live parts is higher. A 300 mA RCD is used for fire protection. It won’t trip fast enough to prevent a shock, but it catches larger leakage currents that could heat up insulation or combustible materials enough to start a fire. Electrical codes typically require 300 mA protection wherever wiring comes into contact with combustible materials.
Type AC, A, F, and B
Beyond sensitivity, RCDs are also classified by the kind of electrical fault they can detect. This matters more than it used to, because modern homes are full of electronics that convert standard AC power into other forms.
Type AC devices detect only standard alternating current faults. These have largely been phased out because many household appliances (chargers, LED drivers, washing machines) convert AC to pulsing DC internally. If a fault occurs on the DC side, a Type AC RCD may not detect it, and the DC current can even saturate the device’s core and prevent it from working at all.
Type A is now the baseline standard in most countries. It detects both AC faults and pulsing DC faults up to 6 mA, covering the vast majority of household appliances. Type F adds the ability to handle high-frequency faults, the kind produced by variable-speed motors in air conditioning units or some washing machines. Type B handles everything a Type F does, plus pure DC faults, making it necessary for EV chargers, solar panel inverters, and other equipment that produces significant DC current.
Testing Your RCD
Every RCD has a test button on its front face. Pressing it simulates a leakage fault and should cause the device to trip immediately. If it doesn’t, the RCD may have failed and is no longer protecting you. Most manufacturers and safety organizations recommend pressing the test button every three months. After testing, simply switch the RCD back on to restore power.
Professional testing goes further. An electrician can verify both the exact tripping current and the trip time using specialized equipment. During these tests, a Type 1 RCD should trip within 40 milliseconds, while a Type 2 should trip within 300 milliseconds at its rated current. Periodic professional testing, typically every one to five years depending on local regulations, catches degradation that the test button alone might miss.
Where RCDs Are Required
Modern wiring regulations in most countries require RCD protection on all circuits in new installations, or at minimum on circuits considered higher risk. These include bathroom circuits, outdoor sockets, circuits supplying portable equipment, and any circuit where cables are embedded in walls at shallow depths. Older homes may have no RCD protection at all, or only on selected circuits. If your consumer unit still uses rewirable fuses and has no RCD, upgrading the board is one of the most impactful electrical safety improvements you can make.
For anyone working with power tools outdoors, mowing over a cable or cutting into a live wire is a realistic hazard. A portable RCD plugged in at the socket provides protection even if the fixed wiring predates modern standards.