How an RCD protects you
An RCD compares current out and back; any imbalance means leakage to earth, so it disconnects fast.
The principle
In a healthy circuit, all the current that flows out through the active comes back through the neutral — they're equal. An RCD works by continuously comparing the two.
How it detects danger
The active and neutral pass through a sensing ring. Equal and opposite currents cancel, so the ring sees nothing. But if some current leaks to earth — through a fault, or through a person touching a live part — less comes back via the neutral than went out via the active. That imbalance is detected and trips the RCD in a fraction of a second, before the leakage can be lethal.
Why it matters
An RCD protects against shock in a way a fuse or breaker can't — it responds to small leakage currents, not just big overloads. That's also why it trips on real leakage, cumulative leakage from electronics, moisture, and neutral-earth faults — and why it must never be bypassed.
Safety first
An RCD trip means real leakage and a shock risk — find it, never defeat the device. Test it operates with its test button periodically.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
RCD / RCBO keeps tripping
An RCD or RCBO trips repeatedly — immediately on reset, randomly during the day, or only when certain equipment runs. The earth-leakage protection is doing its job; something is leaking.
Safety switch (RCD) keeps tripping at the switchboard
The safety switch trips repeatedly — instantly on reset, randomly, or when certain appliances run. It's detecting earth leakage somewhere; the job is to find where.
RCD passes test but doesn't trip on a real leakage test
The RCD's own button trips it, but a proper instrument test shows it doesn't trip within the required time/current — so it may not protect adequately in a real fault.
Related definitions
RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
Neutral vs earth
The neutral carries return current; earth is for safety. Confusing them causes real, dangerous faults.