Why does my RCD keep tripping? Causes and what to do

Short answer: A residual current device (RCD, RCCB) trips when the current going out on a circuit does not match the current coming back, meaning some of it is leaking to earth or through a person. Under the product standard a 30 mA RCD opens when this difference reaches roughly 15 to 30 mA. The most common causes are damp sockets, junction boxes or outdoor lights; appliances with failing heating elements (water heater, washing machine, dishwasher, oven); neutral and earth touching somewhere; and many small appliance leakages adding up. If it keeps tripping, unplug everything first, then switch the circuit breakers back on one by one to find the faulty circuit. Never bypass or remove the RCD; the rest is a job for a qualified electrician.

What is an RCD and how does it work?

An RCD monitors the line and neutral conductors of a circuit at the same time. Normally the current that leaves on the line returns on the neutral. If a conductor touches a damp surface, an appliance case or a person and part of the current flows to earth, the two no longer match. That difference is the residual (leakage) current; when it approaches the rating on the label (IΔn) the RCD disconnects the circuit within a fraction of a second.

• 30 mA: Protection of people. Used on socket, bathroom, kitchen and outdoor circuits in homes.
• 300 mA: Fire protection. Not enough to protect people; see 30 mA or 300 mA.

The RCD type (AC, A, F, B) defines which kinds of leakage it can detect; see RCD types.

Which one tripped: the breaker or the RCD?

When the power goes off, look at the consumer unit and find the device whose lever has dropped. They do different jobs:

• Circuit breaker (MCB, overcurrent protection): Trips on overload, when a circuit draws more current than its cable can carry, or on a short circuit. Typically it happens when several powerful appliances run together. See curve B vs curve C breakers.
• RCD: Ignores how much current flows and looks only at the difference between outgoing and returning current. It trips on leakage even with a single lamp on, but it does not trip on overload.

The RCD is usually the 2- or 4-module device with a "T" or "TEST" button. Combined devices (RCBOs) protect against both; their indicator or an electrician tells you which function operated. For the board layout see home circuit breakers and cable sizes.

Why does an RCD trip?

• Damp sockets, junction boxes or switches: Water or condensation in boxes in bathrooms, kitchens, basements or on outside walls creates a small path from line to earth. Joints buried in a damp wall do the same.
• Appliances with heating elements: The insulation of elements in water heaters, kettles, washing machines, dishwashers, ovens and hobs degrades over time. Some elements absorb moisture when unused and leak when they first heat up, so the RCD often trips the moment heating starts.
• Outdoor lights and garden circuits: Garden lights or wall lights with tired seals, buried cable joints, pond and pool pumps, outdoor sockets.
• Damaged cable: Cable crushed, pierced by a nail or screw, chewed by rodents, or aged by heat.
• Neutral-earth fault: If the neutral touches or is connected to earth anywhere after the RCD, part of the return current flows through earth and the RCD trips. Linking a socket's earth terminal to neutral in a house without earthing has the same result and is dangerous. Mixing up the neutrals of two different RCDs is another common wiring mistake.
• Cumulative leakage: Filters in computers, TVs, LED drivers and inverter air conditioners leak a tiny current to earth even when healthy. If one 30 mA RCD covers the whole house, these add up. A common recommendation is to keep the standing leakage under one RCD below about a third of its rating (around 10 mA for a 30 mA device). The fix is to split circuits across two or more RCDs.
• Lightning and surges: A nearby strike or a mains surge can create a brief leakage current through cables and appliance filters and trip the RCD. Equipment is protected against surges by a surge protective device; where it sits relative to the RCD is a decision for the electrician. See what is a surge protector.
• Wrong RCD type: A type AC device may not detect the pulsating DC leakage that electronics produce; the problem then is not nuisance tripping but missing protection. Circuits with inverter appliances and electronic loads need at least type A. On an EV charging circuit a type A RCD is enough only if the charger has built-in DC leakage detection; otherwise type B is needed. See type A RCD for EV chargers.
• Faulty RCD: RCDs age too; the mechanism wears and moisture or dust can get in. An RCD that trips with no circuits connected, or does not drop when you press TEST, is suspect. An RCD also gives no overload protection; if its rating is lower than the breaker in front of it, it can overheat. For wiring and sizing see single-phase and three-phase RCDs.

If it trips in the rain, at night or at a certain time

When the RCD trips is the best clue. Write it down:

• In the rain: Outdoor lights, garden or balcony sockets, facade lighting, the outdoor unit of an air conditioner, or a junction box taking in water from the roof or a wall. It may stop once things dry, but the insulation is damaged and it will happen again.
• At night: Garden or sign lights on a time switch, a water heater on a night tariff, a fridge defrost heater and condensation in the early morning.
• When one appliance runs: The fault is most likely in that appliance (element, motor, lead or plug).
• At random: Total leakage may be close to the limit, so any small extra leakage trips it. A neutral-earth fault also trips at random as the load changes.

What to do if your RCD keeps tripping

These steps only involve switching levers and pulling plugs. Do not remove the board cover or touch any terminals, and never touch the board with wet hands or on a wet floor.

1. If you notice a burning smell, smoke, a hot or blackened socket or water ingress, do not reset the RCD. Call an electrician.
2. Unplug every appliance on the part of the house the RCD supplies. Switch off the breakers of fixed appliances such as the water heater, oven and hob.
3. Reset the RCD once. If it stays on, plug in and run appliances one at a time. Stop using the one that trips it and have it repaired.
4. If it still trips with everything unplugged, switch off all breakers below the RCD, reset the RCD and switch the breakers back on one by one. The breaker that makes it trip points to the faulty circuit. Leave that breaker off, use the rest of the house and show the circuit to an electrician.
5. If it trips even with all breakers off, the fault may be in the RCD itself, between the RCD and the breakers, or in a circuit's neutral. A breaker only switches the line; a circuit with a neutral-earth fault can still trip the RCD with its breaker off. From here on it is a job for an electrician.

Do not keep resetting the RCD again and again. Every trip means current is going somewhere it should not.

What the electrician does

• Insulation resistance test: With the circuit dead and electronics disconnected, tested at 500 V DC. IEC 60364-6 expects at least 1 MΩ on domestic circuits. A low value shows damp or damaged cable, boxes or joints.
• Measuring the standing leakage of each circuit with a leakage clamp meter, and splitting circuits onto a second RCD if the total is high.
• Checking that neutral and earth are separate, separating mixed neutrals and removing incorrect links in sockets.
• Testing the RCD's trip current and time with an RCD tester. A general-type RCD must trip within 300 ms at its rated residual current.
• Measuring earth resistance; see what is earthing and how is it measured.
• Replacing damp boxes, sockets and outdoor connections and reviewing the RCD type or the wiring; see rewiring a house.

Never bypass the RCD

Removing an RCD that trips often, bridging it, moving circuits in front of it or replacing a 30 mA unit with 300 mA must never be done. A tripping RCD means there is leakage; disabling the RCD does not remove the leakage, it only leaves you unprotected. The next contact can be an electric shock, and leakage in a damp junction box can start a fire.

Is the RCD working? The TEST button

Every RCD has a "T" or "TEST" button. Pressing it creates an artificial leakage and the RCD must drop at once. The common recommendation is to test it monthly; RCDs that never operate can stick over time. Testing cuts power to the circuits behind it, so shut down computers first. An RCD that does not drop when tested is not protecting you and must be replaced by a qualified electrician.

RCDs in our shop

We stock Legrand XC10 residual current devices in type AC and type A, 2-pole (single-phase) and 4-pole (three-phase), 25, 40 and 63 A, 30 mA and 300 mA. They comply with EN/IEC 61008-1; 2-pole models take 2 modules, 4-pole models 4 modules. Choose type A for inverter appliances, washing machines and electronic loads and type AC for general household circuits. Replacing an RCD or splitting circuits must be done by a qualified electrician; local wiring rules apply. You can also see the products at the ENGN store in Nilüfer, Bursa.

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