A power cut affecting only part of a property is frustrating, particularly when the consumer unit shows an RCD in the off position. So, why do RCDs trip? In most cases, they have detected electricity taking an unintended path to earth. That is exactly what they are designed to do: disconnect the supply quickly to reduce the risk of electric shock and fire.
An RCD trip is not automatically a sign of a major fault, but it should not be ignored or repeatedly reset without thought. The cause may be as simple as moisture in an outside fitting or a faulty appliance. Equally, it may point to damaged wiring or a developing issue that needs professional investigation.
What an RCD does
An RCD, or residual current device, monitors the flow of electricity in a circuit. Under normal conditions, the current travelling out through the live conductor should match the current returning through the neutral conductor. If there is a difference, some electricity may be flowing elsewhere – potentially through an appliance casing, a cable fault, building fabric or, in the worst case, a person.
When that imbalance reaches its operating threshold, the RCD disconnects the circuit. This is different from a circuit breaker or fuse, which is primarily there to protect cables against overloads and short circuits. Modern consumer units may use RCBOs, which combine both functions on individual circuits. Although the arrangement differs, the purpose is the same: to provide effective protection where an earth leakage fault occurs.
Why do RCDs trip in homes and workplaces?
The answer depends on what was connected and what conditions were present when the trip occurred. A one-off trip after a period of heavy rain may suggest an external fitting or cable is affected by water. A trip that happens whenever the washing machine starts heating points more strongly towards that appliance. If it happens with nothing obvious switched on, the fault can be less straightforward and may require testing.
A faulty appliance
Appliances with heating elements, motors or water connections are frequent causes of RCD trips. Kettles, washing machines, tumble dryers, dishwashers, ovens, immersion heaters and portable heaters can all develop earth leakage faults as components age.
Portable equipment deserves attention too. A damaged extension lead, lawnmower, power tool or phone charger may look harmless while having an internal fault. In commercial settings, kitchen equipment, cleaning machinery, IT equipment and temporary supplies can create the same issue. The timing of the trip often provides the first useful clue.
Moisture and weather exposure
Outdoor sockets, garden lighting, garage supplies, external signage and EV charging equipment are exposed to changing weather conditions. Water ingress can allow leakage to earth, especially where fittings are cracked, glands are loose or enclosures are not suitably rated for their location.
Bathrooms, utility rooms, plant rooms and kitchens can also create damp conditions. Condensation inside an older fitting or enclosure may be enough to cause intermittent tripping. This can be particularly difficult to diagnose because the circuit may appear to work normally once the area has dried.
Damaged or deteriorating wiring
Cables can be damaged by drilling, nails, rodents, heat, movement or previous alterations. In older properties, insulation can deteriorate over time. A fault may only become apparent when a cable warms up, when a particular circuit is used, or when dampness affects part of the installation.
For businesses and industrial sites, cable damage is sometimes associated with changes to equipment layouts, vehicle movements, vibration or maintenance work. A professional inspection helps establish whether the problem is local to one item of equipment or part of the fixed wiring installation.
Cumulative leakage on a busy circuit
Not every RCD trip is caused by one failed item. Many modern devices have a small amount of normal earth leakage during operation. On a circuit serving numerous computers, LED drivers, appliances or electronic power supplies, those small amounts can add up.
This is more common in offices, retail spaces, workshops and properties with extensive electrical additions. The solution may involve identifying higher-leakage equipment, redistributing loads or improving circuit protection arrangements. It is not simply a case of fitting a larger RCD, as protection must remain suitable for the installation and its users.
Incorrect connections or recent electrical work
A tripping RCD can also follow an alteration, new appliance connection or recent building work. Neutral conductors connected incorrectly between circuits, damaged cables during refurbishment, or unsuitable equipment can all cause immediate or intermittent operation.
If the issue began after electrical work, report it promptly to the contractor. Do not assume the device itself is at fault. Proper testing is needed to confirm the cause and ensure the installation remains safe.
What to do when an RCD trips
Start by checking whether the trip affects the whole property or only certain circuits. At the consumer unit, identify the device that has moved to the off position. Avoid touching exposed wiring or attempting repairs inside the consumer unit.
If it is safe to do so, switch off the individual circuit breakers supplied by that RCD, then reset the RCD. Re-energise the circuit breakers one at a time. If the RCD trips when a particular circuit is turned back on, leave that circuit off. This narrows down the area of concern without bypassing the protective device.
Where a circuit remains on, unplug appliances connected to it before reconnecting them individually. If the RCD trips when a particular appliance is plugged in or switched on, stop using it. Do not continue resetting the RCD to keep the appliance running. Arrange repair, replacement or appropriate testing.
There are sensible limits to this basic process. Hard-wired appliances, fixed lighting, outdoor circuits and commercial equipment should be assessed by a qualified electrician rather than dismantled or disconnected by an untrained person. Never bypass an RCD, hold it in position, or replace it with a device of a different rating in an attempt to stop tripping.
When to call an electrician straight away
Some circumstances call for prompt professional help. These include repeated trips with no appliance connected, burning smells, buzzing, visible damage, water around electrical equipment, a shock or tingling sensation, or tripping after a flood or leak. Keep the affected circuit switched off until it has been checked.
For landlords, facilities managers and business operators, recurring RCD trips can also affect compliance, productivity and the safety of staff or visitors. Repeatedly restoring power without identifying the fault may turn a manageable repair into unexpected downtime. Planned inspection and fault finding is usually the more efficient option.
An electrician can carry out insulation resistance testing, earth leakage measurements and circuit checks to locate the issue accurately. This avoids replacing appliances or components by guesswork. It also confirms that any repair has addressed the underlying fault rather than merely stopped the immediate symptom.
Preventing avoidable RCD trips
Good maintenance reduces the chance of nuisance and fault-related tripping. Keep external electrical accessories in good condition, use equipment appropriate for the environment, and replace damaged leads promptly. Do not overload extension leads or rely on temporary wiring as a permanent solution.
In workplaces, routine inspection and testing of portable appliances and fixed wiring can identify deterioration before it disrupts operations. The right frequency depends on the environment, the type of equipment and how it is used. A busy workshop needs a different approach from a low-use office, just as an older home may benefit from more attention than a recently rewired property.
Consumer units should also be clearly labelled so that occupants or staff can identify circuits quickly during a trip. Clear information makes a stressful situation easier to manage and helps an electrician diagnose the issue efficiently.
An RCD that trips is performing a safety function, not creating the problem. Treat it as useful warning, leave any suspect circuit isolated, and arrange competent fault finding where the cause is not immediately clear. A careful response protects people, property and the reliability of the electrical installation.