The short answer
An RCD (residual current device) is a protective device that constantly monitors the current flowing out on the live conductor and the current returning on the neutral conductor. In a healthy circuit these are equal. If they differ — because current is leaking to earth through a fault, damaged insulation, or a person — the RCD detects the imbalance and disconnects within milliseconds. Domestic RCDs are typically rated at 30 mA (30 milliamps), a level chosen because it can disconnect fast enough to prevent ventricular fibrillation in most circumstances. An RCD does not protect against overload or short-circuit faults — that is the job of an MCB. Under BS 7671 18th Edition, RCD protection is required for most final circuits in a domestic installation, including socket circuits likely to supply portable equipment used outdoors.
The RCD is the device behind the test button on your consumer unit. It is the reason a modern board protects against electric shock in a way that old fuse boxes with rewireable fuses never could. Understanding how it works explains both why it is important and why it sometimes trips without an obvious cause.
RCD key facts
- Full nameResidual current device
- Typical domestic sensitivity30 mA
- Typical disconnection time40 ms at 150 mA (to BS EN 61008)
- Protects againstEarth-leakage / shock risk
- Does not protect againstOverload or short circuit (MCB does that)
- Required byBS 7671 18th Edition for most domestic circuits
How an RCD works
Inside an RCD is a toroidal transformer — a ring of magnetic core through which both the live and neutral conductors pass. When the circuit is healthy, the current flowing out on the live and the current returning on the neutral are equal and opposite, so their magnetic fields cancel out and no signal is produced. If even a small amount of current leaks away — to earth through a person, through damp insulation, or through a fault — the balance is upset. The transformer detects the residual difference, and a solenoid inside the device trips the contacts open.
A 30 mA RCD will disconnect when the imbalance reaches 30 milliamps. To put that in context, a current of around 30–50 mA passing through the chest region can trigger ventricular fibrillation; the RCD is designed to disconnect fast enough — typically within 40 milliseconds at 150 mA under BS EN 61008 — to prevent that in most exposure scenarios. It is not infallible (the path the current takes through the body matters), but it is a substantial protection against fatal shock from earth-leakage faults.
RCDs in a domestic consumer unit — how they are arranged
In a typical domestic consumer unit, RCDs are arranged in one of two ways:
- Split-load arrangement: the board is divided into two sections, each covered by its own RCD. The MCBs for one half of the circuits sit behind one RCD; the other half sit behind the second. If one RCD trips (due to an earth fault on one of its circuits), half the house loses power but the other half stays on. This is the layout of a 'dual-RCD' or 'high-integrity' consumer unit, which became common after the 17th Edition of the wiring regulations.
- RCBO arrangement: each circuit has its own RCBO (a combined RCD and MCB in a single device). A fault on one circuit trips only that circuit's RCBO — nothing else loses power. This is the more expensive but finer-grained option, and is what BS 7671 18th Edition recommends for new boards.
Some boards also use an RCD incomer — a single RCD covering all circuits. This was common in older boards but is less favoured now because a single fault trips the whole installation.
| Arrangement | How it works | Trip impact |
|---|---|---|
| Single RCD (older) | One RCD covers all circuits | Whole board trips |
| Dual-RCD / split-load | Two RCDs, each covering half the board | Half the installation trips |
| Full RCBO board | One RCBO per circuit | Single circuit trips only |
RCD arrangements in domestic consumer units. Sources: BS 7671:2018+A2:2022; Electrical Safety First best practice guide.
Why an RCD trips — and when to investigate
An RCD trips when it detects 30 mA or more of residual current. Common causes include:
- A faulty appliance with damaged insulation letting current leak to earth — unplug appliances one at a time to identify the culprit.
- Damp or moisture ingress in wiring, sockets or light fittings — common after flooding or in outdoor circuits.
- A nuisance trip from the natural sum of small leakage currents across many circuits exceeding the trip threshold — more common on boards with a single RCD covering many circuits.
- An actual wiring fault in the fixed installation — a chafed cable, a damaged connection.
An RCD that trips repeatedly without an obvious cause should be investigated by a registered electrician rather than just reset. Resetting restores the power, but it does not fix the underlying fault.
Frequently asked questions
Why does my RCD keep tripping?
Usually because it is detecting a genuine earth-leakage fault — either in an appliance or in the fixed wiring. Try unplugging all appliances on the affected circuits before resetting; if the RCD holds, plug appliances back in one at a time to identify the faulty one. If it trips with nothing plugged in, the fault is likely in the wiring and needs a registered electrician to investigate.
What does pressing the test button on an RCD do?
The test button introduces a small artificial imbalance to simulate an earth fault, confirming that the RCD can detect and respond to a fault. It should be pressed every three to six months to confirm the device is operating. If it does not trip when you press the button, the RCD may have failed and should be examined by an electrician.
Is a 30 mA RCD the same as a 100 mA one?
No — the sensitivity differs. A 30 mA RCD is the standard for domestic shock protection and disconnects at 30 milliamps of earth-leakage. A 100 mA or higher RCD is used in some fire-protection or supply-discrimination applications and does not provide the same level of shock protection. Domestic final circuits require 30 mA RCD protection under BS 7671 18th Edition.
Sources & further reading
- Electrical Safety First — RCDs (residual current devices)
- IET — BS 7671:2018+A2:2022 (18th Edition Wiring Regulations)
- NICEIC — understanding RCDs
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.