Nearly everyone with a tripping RCD goes looking for a faulty appliance. Sometimes that is exactly what it is. But on a lot of the boards we open, nothing is faulty at all, and that is why people spend weeks unplugging things and getting nowhere. This guide explains both, and how to tell which one you have.
Every modern appliance leaks a small amount of current to earth by design. A 30mA RCD is allowed to trip at anything from 15mA upwards. If half your house sits on one RCD, all those tiny leakages add up until something small tips it over the edge. Nothing is broken. There is just too much on one device.
What Should I Do Right Now?
If it will reset and hold, you have time to work through this properly. If it will not, or if you can smell burning near the board, stop and call someone.
- Do not tape it on, wedge it, or keep resetting it. It has measured something real
- Move the freezer to a socket on a circuit that is still working, or open it as little as possible
- Note exactly what was happening when it tripped. Weather, appliance, time of day, what you were doing
- If you were drilling, say so straight away. That changes everything
- Leave it off overnight rather than resetting repeatedly if you cannot find the cause
What Does an RCD Actually Do?
It compares the current going out on the live with the current coming back on the neutral. If those two do not match, the difference has gone somewhere else, and the most likely somewhere else is through a person or into the fabric of the building. A 30mA RCD cuts the supply in under 40 milliseconds when it sees that imbalance.
That is why an RCD trip is never something to just reset and forget. It has measured something. The question is whether it measured a fault or measured the ordinary leakage of a modern household finally adding up.
Why Does My RCD Trip When Nothing Is Broken?
Because leakage to earth is normal, and it accumulates. Almost every appliance with electronics inside it has a filter that deliberately passes a tiny current to earth, usually somewhere between half a milliamp and three milliamps each.
- Washing machines, dishwashers and tumble dryers
- Induction hobs and electric ovens
- Computers, televisions, games consoles and routers
- LED drivers and dimmer modules
- EV chargers and solar inverters
- Anything with a switch mode power supply, which is nearly everything now
None of those is faulty. That leakage is designed in and it is perfectly normal. The problem is arithmetic. Put eight of them on one RCD and you might be sitting at 12 or 14mA before anybody switches anything on.
The bit almost nobody explains
A 30mA RCD does not trip at 30mA. Under BS 7671 it must trip between 50 and 100 percent of its rating, so a perfectly healthy, in-specification 30mA device is allowed to operate at 15mA.
So if the standing leakage on your board is already 14mA, the RCD is sitting one milliamp below its permitted trip point before the day has started. Switch on the shower, or the tumble dryer on a damp morning, and away it goes. You unplug things, the leakage drops, it holds for a while, and everybody concludes the problem is fixed. It is not. It is just back under the line.
It trips at the same time of day, or when the weather turns, or when you use one particular appliance that tests perfectly fine on its own. It goes weeks without tripping and then does it three times in a day. Unplugging things helps for a while and then it comes back. That is accumulated leakage, not a broken appliance.
Why Does One Fault Take Out Half My House?
Because of how the board is split. Most consumer units fitted between roughly 2008 and 2018 are split load, which means two RCDs, each covering a group of circuits. Half the house on one, half on the other.
It met the regulations at the time and it is a lot better than no RCD at all. But it means a single fault anywhere in that half, or the accumulated leakage of everything in it, takes out every circuit on that side. Upstairs lights, upstairs sockets, the immersion, all gone together, and you are left guessing which of them caused it.
Modern boards use an RCBO on every circuit instead. Each circuit gets its own device, so a fault on the kitchen ring takes out the kitchen ring and nothing else. It also tells you immediately where the fault is, which turns a two-hour hunt into a two-minute one.
How Do I Find Out Which Appliance Is Causing It?
Unplug everything, reset, then plug things back one at a time leaving a few minutes between each. It takes about half an hour and it either finds the culprit or tells you it is not an appliance at all, which is just as useful.
- Unplug everything on the affected circuits, and switch off anything hard wired that has a local isolator
- Reset the RCD. If it will not stay on with nothing connected, the fault is in the fixed wiring, and you need an electrician rather than more unplugging
- If it holds, plug things back one at a time, leaving several minutes between each
- Note anything that trips it immediately. That one is faulty
- Note anything that trips it only sometimes, or only alongside other things. That is leakage, not a fault
The second pattern is the one people misread. An appliance that trips the RCD only when three other things are running is not the problem on its own. It is the last straw on a board that was already close to the limit.
What Actually Causes an RCD to Trip Most Often?
Rodents chewing cable, a faulty appliance, and outdoor electrics spurred off an indoor socket. Those three account for most of what we find. Below is the full list from our Liverpool callouts, in order, rather than a list from a manufacturer's leaflet.
1. Rodents chewing through cable
More common than anyone expects, and consistently one of the top causes we find. Mice and rats gnaw cable insulation in lofts, under floors and behind kitchen units. They rarely cut all the way through, they just strip enough insulation for the conductor to start leaking. The trip is often intermittent to begin with and then becomes permanent.
If you have heard anything in the loft or under the floor in the last few months, say so when you call. It changes where we look first.
2. A genuinely faulty appliance
Still very common, and this year it has been fridges and microwaves more than anything else. Both run constantly or near enough, both have heating or cooling elements, and both tend to fail by leaking rather than by stopping.
A fridge is the awkward one because nobody unplugs it during fault finding. It sits there quietly leaking while you work through everything else in the house.
3. Outdoor electrics spurred off an indoor socket
Garden lights, a socket in the shed, a supply to a garage or a hot tub, all taken from the nearest downstairs socket because it was the easiest thing to do. Now anything that goes wrong outside, in the weather, trips the circuit inside your house.
Outdoor work should be on its own circuit with its own protection. We find this one constantly, and it is usually the reason the tripping follows the rain.
4. Water ingress
Outdoor sockets with failed seals, garden lighting, bathroom fittings, anything in a garage or outbuilding. Water gets in, the RCD trips, it dries out and holds for a week, then it rains again.
5. Damp behind sockets on external walls
A specific one worth knowing about. On solid external walls, particularly north facing ones and older properties without a cavity, damp works through and sits behind the back box. There is nothing visibly wrong on the wall and nothing wrong with the socket, but the moisture provides a path to earth and the RCD sees it.
It is often seasonal. Fine all summer, tripping from October onwards.
6. Slugs and insects inside the back of a socket
This sounds like a joke and it is not. We have opened sockets to find a slug that has made its way inside and positioned itself across the live and earth terminals, or the neutral and earth. It is a wet body bridging two terminals, which is exactly what an RCD is built to detect.
It happens most often in ground floor sockets on external walls, garages and outbuildings, and it goes with the damp problem above. Nothing shows on any test until you take the faceplate off and look.
7. A loose or snapped conductor touching the back box
Two versions of the same problem, and both are workmanship rather than age.
The first is a termination that was never tightened properly. It works for years, then thermal movement finally loosens it enough that a conductor comes free inside the box. If that conductor is the live and the box is a metal one, which is earthed, you have a direct live to earth fault. The RCD does exactly what it is supposed to.
Cables cut too short at first fix leave the conductors under tension every time the faceplate is pushed back into the box. The copper flexes a fraction each time the circuit warms and cools. Ten or fifteen years of that and it work hardens and snaps, usually right at the terminal, and the broken end springs back against the earthed metal box.
It is often intermittent to begin with, because it depends whether the loose end happens to be touching. Push the socket back after checking it and it might hold for a month before it goes again.
It is also the reason we leave generous tails on a rewire. Cable is the cheapest part of the job and cutting it short creates a fault that turns up a decade later, long after whoever did it has moved on.
A slug, a damp back box, a snapped conductor or a chewed cable behind a unit will not always show up cleanly on a meter reading taken at the board. Some of these are only ever found by working along the circuit and taking faceplates off. If someone tests your board, shrugs and leaves, they have not finished the job.
8. The socket faceplate itself
Sometimes the accessory has simply failed internally. No damp, no rodents, no appliance, just a socket that has broken down inside. Replacing the faceplate fixes it. It is a five minute job once you have found which one it is, and finding it is the entire job.
9. Drilling through a cable
This one is different from everything above, because the person already knows what they have done. Everything was fine, they were putting a shelf up or mounting a telly, there was a flash and a bang, and now the sockets are dead. The call usually starts with "I think I have gone through a cable."
They are almost always right. A drill bit into a live conductor is an instant fault, and depending on how your board is arranged you can lose several circuits at once rather than just the one you hit.
The good news is that it is the quickest fault we deal with, because you know exactly where it is. We cut in, joint or replace the damaged section, make it accessible or reroute it properly, and test the circuit before it goes back into service. Most of them are done the same day.
Cables are run in prescribed zones, and the most likely place to find one is directly above or directly below a socket or a switch, running vertically, or within 150mm of the ceiling or of a corner. That is precisely where people drill, because it is where shelves, radiators and televisions go. If you are drilling in those areas, use a detector and go slowly, and if you are unsure, do not guess.
A slower version of the same thing is a screw that only nicks the insulation rather than hitting the conductor. There is no flash and nothing appears to happen. The fault turns up weeks later as damp works in, and by then nobody connects it to the shelf that went up last month.
10. Deteriorated cable insulation
Insulation resistance drops slowly over decades until the leakage it allows holds the RCD near its trip point permanently. Common in properties not rewired in 25 to 30 years, and the point at which fault finding stops being worth paying for and a rewire becomes the answer. We have written separately about how to tell when wiring has reached the end of its life, including how to date your own installation from the cable colours.
11. The RCD itself
They do fail, and they become oversensitive with age. It is the last thing to check rather than the first, but we have replaced plenty of them. An electrician can measure the actual trip current and trip time and tell you in two minutes whether the device is still within specification.
Heating elements are worth a mention too. Immersion heaters, ovens and electric showers often fail by degrading their insulation rather than stopping. A shower that only trips once it has been running a few minutes and the element has warmed up is a classic.
We measure the standing leakage on each circuit and tell you exactly where it is coming from. Same day across Liverpool.
What Is the Difference Between a Type AC and a Type A RCD?
What they can detect. A Type AC device only sees alternating current leakage. A Type A device sees that plus the pulsating DC leakage that modern electronics produce, and that difference has become a real problem in ordinary houses.
Anything with a switch mode power supply or an inverter in it leaks a small amount of DC to earth. EV chargers, induction hobs, solar inverters, LED drivers, variable speed washing machines. None of it existed in quantity when most split load boards were fitted.
DC leakage can saturate the core inside a Type AC device. At best it causes nuisance tripping nobody can explain. At worst it stops the device operating at the current it is supposed to, which means it may not trip when somebody actually needs it to. If you have had an EV charger or an induction hob fitted to an older board, this is worth checking.
Every consumer unit we fit uses Type A RCBOs throughout, one per circuit, as standard. It is not an upgrade on our quotes, it is what goes in.
How Does an Electrician Actually Diagnose It?
By measuring, not guessing. A clamp meter around the live and neutral of a circuit together reads the leakage on that circuit directly, in milliamps, with everything still running.
That turns the whole thing into numbers. If the kitchen circuit is sitting at 9mA and everything else is under 1mA, we know where to look. If six circuits are each sitting at 2 or 3mA and none of them is faulty, we know the board is the problem rather than the wiring, and no amount of unplugging will fix it.
We also test the RCD itself, measuring the current at which it actually operates and how long it takes. If it is tripping at 11mA it is out of specification and needs replacing regardless of what else is going on.
What Is the Difference Between an RCD, an RCBO and an MCB?
An MCB protects the cable, an RCD protects people, and an RCBO does both in one device. Knowing which you are looking at tells you a lot about how your board will behave when something goes wrong.
If your board has two big switches with test buttons and a row of smaller switches beside them, you have RCDs covering groups of circuits. If every switch has its own test button, you have RCBOs and your board is already doing the right thing.
Should I Just Replace the Fuseboard?
If the leakage is spread across several circuits and none of them is faulty, then yes, and it is usually the cheapest real fix. A consumer unit with an RCBO on every circuit is £700 fitted, including the certificate and building control notification under Part P.
Splitting the leakage across separate devices means no single RCD is carrying the whole house, and each circuit gets its own 30mA of headroom instead of sharing it. It also means a fault takes out one circuit rather than half the property.
If one circuit is sitting at 20mA on its own, moving it to its own RCBO just gives you a circuit that trips by itself. There is a genuine fault on that circuit and it needs finding first. We will tell you which situation you are in before you spend anything.
How Much Does Fault Finding Cost?
£80 an hour Monday to Friday between 9am and 5pm. Weekends are £120 an hour between 9am and 1pm, and outside those hours it is £150 an hour. No callout charge on top and no minimum beyond the first hour.
Most RCD faults are found and repaired inside that first hour, particularly the ones where you already know what happened. Accumulated leakage across several circuits takes longer, because it means clamping each circuit in turn and testing the device itself.
If the answer turns out to be a new board, a consumer unit with Type A RCBOs is £700 fitted including the certificate and Part P notification, and we put the fault finding time towards it rather than charging you twice.
How Often Should I Test My RCD?
Every three months, using the test button on the device itself. It takes ten seconds and almost nobody does it.
Press the button. The RCD should trip immediately. Switch it back on. If it does not trip, or it hesitates, the device is not protecting you and needs replacing. Do it when the house is not mid wash cycle and warn anyone using a computer first.
A tripping RCD in a rental is not just an inconvenience, it is something that will appear on an EICR. Landlords in England must hold a satisfactory report renewed at least every five years, and a device that will not hold or is out of specification is a straightforward fail. Better found by us than by an inspection.
When Should I Stop and Call Someone?
Call an electrician if the RCD will not reset with everything unplugged, if it trips within minutes of resetting, if you cannot identify a culprit after working through the isolation steps, or if there is any burning smell, scorch mark or visible damage on any fitting.
Stop straight away and do not reset it if you can smell burning near the board. Repeatedly resetting an RCD that keeps operating is not a neutral act. It is overriding a safety device that has measured something real.
If you need an emergency electrician in Liverpool, we are available 24/7 and aim to be with you within 30 minutes. For fault finding that is not urgent, we offer same day appointments across all Liverpool areas, and out to the Wirral, St Helens, Warrington and Formby.
We measure the leakage rather than guessing at it. NICEIC approved, same day available.