RCD (safety switch)
Detects earth leakage and disconnects fast to protect people from electric shock.
What it is
An RCD (called a safety switch in homes) protects people against electric shock by detecting current leaking to earth and disconnecting in a fraction of a second.
How it works
The active and neutral conductors pass through a sensing ring (toroid). In a healthy circuit, the current going out (active) equals the current coming back (neutral), so their magnetic effects cancel.
If some current leaks to earth — through a fault or a person — the active and neutral no longer balance. The imbalance induces a current in the sensing coil, which trips the RCD. It's detecting the leakage, not the total load.
Where it's used
Protecting socket and lighting circuits, wet areas, and outdoor circuits. An RCBO combines this earth-leakage protection with overload/short-circuit protection in one device.
Never bypass an RCD to stop nuisance tripping — it's protecting against a shock risk. Trips come from real leakage, cumulative leakage from electronics, moisture, or neutral-earth faults.
What it looks like when it fails
Random or repeat tripping is usually the circuit, not the device: cumulative leakage from a board full of electronics, moisture in a fitting or buried joint, a neutral–earth fault downstream, or one failing appliance. Trips that follow rain, or only when a particular load switches on, are telling you where to look.
The device fails too: it won't reset with everything disconnected (worn mechanism — or a hard N–E fault still present), or the test button does nothing. An RCD that won't trip on its own test button gets replaced, not excused.
How to test it
The test button only proves the mechanism moves — it says nothing about trip current or speed. A proper check needs an RCD tester: it should hold at half rated residual current and trip at rated current (30 mA for personal protection) within the time limits in AS/NZS 3017 — verify the exact limits you test to. A ramp test tells you the actual trip point.
Chasing nuisance trips: unplug everything on the circuit, reset, then reintroduce loads one at a time. A leakage clamp around active and neutral together reads the standing leakage directly — see how much of the trip threshold is already spent before anything 'faults'.
Gotchas
A neutral–earth fault trips RCDs with every load unplugged — that leakage path doesn't need an appliance connected. And leakage adds up: a circuit full of switch-mode supplies and EMC filters can idle close enough to the threshold that one more device tips it over.
Type matters: a standard Type AC can be blinded by the DC-component leakage from VSDs, EV chargers and some inverters — those need Type A or B (the equipment manual says which). And shared or borrowed neutrals between circuits cause 'impossible' trips on the wrong RCD.
Safety first
RCD trips indicate earth leakage — a shock risk. Never defeat or bypass one; find the leakage. Test it operates using 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 test button doesn't trip the RCD
Pressing the RCD's test button doesn't trip it — a serious sign the RCD may not operate on a real earth fault, even though the circuit is live and working.
RCD trips only in wet weather or after wash-down
The RCD holds fine when dry but trips after rain, washdown, or in damp conditions — pointing to moisture creating an earth-leakage path somewhere outdoors or in wet areas.
RCD trips the moment a specific load is switched on
The RCD is fine until a particular appliance/circuit is switched on, then it trips immediately — clearly pointing at that load or its switch-on behaviour.
Related definitions
RCBO
Combines an RCD and a circuit breaker in one device — earth-leakage plus overload/short-circuit protection.
Circuit breaker (MCB)
Automatically disconnects a circuit on overload or short circuit, and can be reset rather than replaced.
Earthing & bonding
Connecting exposed metal to earth so a fault blows protection fast and metalwork can't become live.
AC vs DC
Alternating current reverses direction many times a second; direct current flows one way. Why it matters on site.