A tripping RCD is one of the most common electrical faults in UK homes. This guide explains every cause — from a faulty kettle to deteriorated wiring — and walks you through the diagnostic process step by step.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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A Residual Current Device (RCD) monitors the balance between the current flowing out through the live conductor and the current returning through the neutral conductor. Under normal conditions, these are equal. If some current leaks to earth — through a person, a fault in an appliance, damaged cable insulation, or moisture — the RCD detects the imbalance and disconnects the circuit. A standard domestic RCD trips when the leakage reaches 30 milliamps (30 mA), which is the threshold considered safe enough to prevent fatal electric shock in most circumstances.
When an RCD keeps tripping repeatedly, it means current is consistently leaking to earth somewhere on the circuits it protects. The challenge is finding where. The leakage could be in one of dozens of appliances plugged into the circuit, in the fixed wiring behind the walls, in an outdoor fitting exposed to rain, or even in the RCD itself. This guide covers every common cause and the systematic process for identifying the source.
It is important to understand that the RCD is doing its job — it is detecting a genuine imbalance and disconnecting to protect you. The temptation to bypass or remove the RCD is extremely dangerous and illegal. The correct approach is to find and fix the source of the leakage.
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The single most common cause. An appliance with damaged insulation, a faulty heating element, or a worn motor allows current to leak from the live conductor to the earthed metalwork of the appliance. Kettles, washing machines, dishwashers, tumble dryers, and immersion heaters are the most frequent culprits because they combine electricity with water or heat, both of which degrade insulation over time. The fault may be intermittent — the appliance works fine when cold but develops a leak when the heating element expands, or it only leaks during the spin cycle when vibration moves a damaged cable.
Water is an excellent conductor. When moisture gets into an electrical enclosure — an outdoor socket, a junction box in the loft, a bathroom light fitting, or a garden lighting connection — it creates a path for current to leak to earth. This is particularly common after heavy rain, when condensation builds up in cold weather, or when a bathroom extractor fan seal has failed. The tripping pattern often correlates with weather conditions: the RCD trips during or after rain, then stays on during dry spells.
In older properties (pre-1970s), the cable insulation may be rubber rather than modern PVC. Rubber insulation degrades over time, becoming brittle, cracked, and losing its insulating properties. This allows current to leak through the insulation to the cable sheath or surrounding materials, particularly where the cable is in contact with damp plaster or masonry. Even modern PVC cables can develop insulation faults if they have been damaged during building work (a nail through a cable is a classic example) or if they have been subjected to excessive heat from spotlights or appliances mounted too close.
Every electrical device has a small amount of earth leakage during normal operation — this is called standing leakage. Individually, each device leaks far less than 30 mA, so no single appliance would trip the RCD. However, on a split-load consumer unit where one RCD protects multiple circuits, the leakage from all connected devices adds up. If the total standing leakage exceeds approximately 10 mA (one-third of the 30 mA trip threshold), the RCD becomes susceptible to tripping from any small additional transient — a fridge compressor starting, a light switch being operated, or a motor drawing inrush current. This is sometimes called "nuisance tripping" because there is no actual fault.
Although less common, the RCD itself can become faulty. The toroidal core can degrade, the trip mechanism can become overly sensitive due to corrosion or mechanical wear, or internal component failure can cause the device to trip at currents well below its rated 30 mA. A faulty RCD may trip at half-rated current (15 mA or less), which would show up as frequent tripping even when there is no significant earth leakage. The only way to confirm this is to test the RCD with a calibrated multifunction test instrument — if it trips at half-rated current, it needs replacing.
A Type AC RCD (the traditional type) can only detect sinusoidal AC earth leakage. Modern electronic equipment — EV chargers, inverter washing machines, heat pumps, LED dimmers, and computer power supplies — can produce pulsating DC or mixed-frequency earth leakage that a Type AC device cannot properly detect. This can cause erratic tripping behaviour. Upgrading to a Type A or Type F RCD often resolves tripping caused by these types of load.
Distinguishing between nuisance tripping and a genuine earth fault is critical. The approach to fixing each is very different. Nuisance tripping means the RCD is tripping due to the cumulative effect of normal standing leakage from multiple devices, with no actual insulation failure or dangerous condition. A genuine fault means there is real insulation breakdown, a damaged cable, a faulty appliance, or moisture creating a dangerous leakage path.
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If you are a homeowner, you can carry out some basic diagnostic steps before calling an electrician. If you are a qualified electrician carrying out fault-finding, this section outlines the systematic approach and the instrument tests that confirm the diagnosis.
Unplug everything. Go around every socket on the affected circuits and unplug every appliance. This includes anything that might be hidden — behind furniture, under worktops, in garages and outbuildings.
Reset the RCD. Push the RCD switch back to the ON position. If it stays on with nothing plugged in, the fault is in an appliance. If it trips immediately, the fault is in the fixed wiring — call an electrician.
Reconnect one appliance at a time. Plug in each appliance individually, switch it on, and wait a few minutes. When the RCD trips, the last appliance you plugged in is the likely culprit. Leave that appliance disconnected and reconnect everything else.
The systematic approach for fault-finding on an RCD that keeps tripping involves isolating circuits one by one at the consumer unit and carrying out insulation resistance tests on each circuit.
Also check the earth fault loop impedance (Zs) on each circuit. An abnormally low Zs reading can indicate an earth fault, as the fault path provides a lower-impedance route to earth than the normal protective conductor.
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Start free trialThe type of RCD installed can directly cause tripping problems if it does not match the type of load connected to it. BS 7671 Regulation 531.3.3 requires the RCD type to be selected based on the waveform of the residual current likely to occur under fault conditions.
Detects sinusoidal AC residual currents only. The most basic type. Cannot detect DC leakage from electronic equipment. May behave erratically when DC fault currents are present, causing both missed trips and nuisance trips. Now largely superseded by Type A for new installations.
Detects sinusoidal AC and pulsating DC residual currents. Now the standard for most domestic circuits. Handles the leakage waveforms from washing machines, dishwashers, computers, and LED drivers. Required by BS 7671 Regulation 531.3.3 for circuits supplying equipment with rectifiers.
Detects sinusoidal AC, pulsating DC, and composite residual currents from single-phase frequency-controlled equipment. Designed for circuits supplying heat pumps, air conditioning units, and inverter washing machines. Resolves tripping caused by mixed-frequency leakage that Type A cannot handle cleanly.
Detects all of the above plus smooth (pure) DC residual currents. Required for EV chargers without built-in DC detection, three-phase rectifier loads, and certain industrial equipment. Significantly more expensive but essential where smooth DC fault currents are possible.
If you have a Type AC RCD and are experiencing tripping when using modern electronic equipment, upgrading to a Type A RCBO (or Type F for inverter-driven loads) often resolves the problem completely. This is not a fault — it is a compatibility issue between the RCD type and the load characteristics.
While basic appliance isolation can be done by a homeowner, many RCD tripping scenarios require a qualified electrician with calibrated test instruments. You should call an electrician if:
A competent electrician will carry out insulation resistance tests, earth fault loop impedance measurements, and RCD testing to identify the exact cause and location of the fault. If a periodic inspection is needed, the electrician will produce an Electrical Installation Condition Report (EICR) documenting all findings and recommendations.
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Try it free for 7 daysIf the RCD keeps tripping and an electrician cannot attend immediately, there are some temporary measures to keep essential circuits operational. These are temporary measures only — they do not fix the fault and the installation must be properly repaired as soon as possible.
Never bypass, bridge out, or remove an RCD. This removes life-saving protection and is illegal under the Electricity at Work Regulations 1989. The measures below reduce the load on the RCD — they do not remove it from the circuit.
Go to every socket outlet on the circuits protected by the tripping RCD and unplug every appliance. Include anything hidden behind furniture, in cupboards, garages, and outbuildings. This removes all appliance-based earth leakage from the circuits.
Push the RCD switch firmly to the ON position. If it stays on with all appliances disconnected, the fault is in an appliance (proceed to step 3). If it trips immediately, the fault is in the fixed wiring — do not proceed further; call a qualified electrician.
Plug in each appliance individually, switch it on, and wait at least two minutes. If the RCD trips, the last appliance connected is the likely cause. Disconnect it and continue testing the remaining appliances. Some faults are intermittent (e.g., a washing machine may only leak during the heating cycle), so run appliances through a full cycle if possible.
If the RCD trips with no appliances connected, isolate each circuit individually at the consumer unit by switching off all MCBs on the RCD side, then switching them on one at a time. When the RCD trips, the last circuit energised contains the fault. This narrows the search to a single circuit.
With the faulty circuit identified, perform a 500V DC insulation resistance test between Live-Earth and Neutral-Earth with all loads disconnected. A reading below 1.0 MΩ confirms an insulation fault. Test at each accessory point along the circuit to pinpoint the exact location of the breakdown.
Once the fault location is identified — whether it is a damaged cable, a faulty connection, moisture in a fitting, or a deteriorated section of wiring — carry out the repair. After the repair, re-test insulation resistance to confirm the fault is cleared, then re-test the RCD to verify correct operation.
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