RCD tripping

An RCD tripping is often referred to as ‘nuisance tripping’, although when tested by electricians they usually work within range exactly as intended.  Residual current devices (or RCD for short) detect an imbalance in the power supply to your lighting, sockets, showers, etc. and disconnect the power extremely quickly if a fault occurs. Most RCDs tripping correctly under fault conditions will disconnect the supply somewhere between 20 and 30 milliseconds – quicker than you can blink! They are only a nuisance when they are faulty. The term ‘nuisance’ came about because the older re-wireable fuse boards and later miniature circuit breakers only protected the cables, and only switched off if overload occurred: when a bulb shorted out internally, for example. Fuses and miniature circuit breakers offer no protection from a person coming into contact with a live conductor. RCDs were designed to help prevent electrocution and fire and are a big part of the wiring regulations professional electricians adhere to.

What causes an rcd to trip: troubleshooting you can do yourself

i. Identify what the RCD is protecting. Once you have confirmed what the device is protecting, you can begin the troubleshooting process. The rcd itself can be identified by the presence of a test button. If your consumer unit has one RCD then it is protecting all the circuits connected to it. If it has two residual devices then it protects everything to the left or right of it. If you have an RCBO (this is an RCD and overload protective device in one unit) then it will only protect the one circuit.

Not sure what an rcd looks like? See below for example rcd configurations:

If your consumer unit looks something like this then the rcd (in this example with an oval yellow test button) protects everything to the left of it. So, the first rcd (from the right) is protecting the first floor lights, first floor sockets and cooker. Whilst the second is protecting the ground floor lights, ground floor sockets, kitchen sockets and central heating. If your rcd won’t stay on; something it is protecting usually has a fault that needs to be removed – such as a faulty appliance or problem with the wiring.

If your consumer unit looks like the one above then it is protecting: water heater, cooker and sockets. So something on one of those circuits is causing a problem – best to unplug (not just switch off) every appliance you can get to and re-try. The circuits on the left (lights and smoke detectors) are not protected by the rcd

There is only one rcd in this installation so it could be a problem on one or more than one of the circuits. Your best chance is to unplug as much as you can before trying to reset it.

This consumer unit has residual protection (called an rcbo) on each circuit (test button in yellow). So a fault on the kitchen sockets will only remove mains power on the kitchen sockets for example. This is the best configuration for reducing inconvenience when earth faults arise.

ii. Once you have determined what the RCD is protecting, make sure all electrical equipment in the property is switched off. Now switch the RCD off by the lever, or by pressing the test button. Now walk around the property and make a list of what no longer works. Once you have this list, you can see if you can isolate the problem to one area or appliance.

iii. If socket outlets are involved, you can unplug (remove from the socket, not just switch off) all the appliances and add them back one at a time to see if there is a pattern when an appliance is reconnected and the RCD tripping. If you believe to have found a faulty appliance, then you could try it by itself to see if it consistently causes tripping, although it’s possible to have more than one faulty appliance, whereby the combined value of two items is enough to trip your consumer unit’s RCD when used together, but not enough by themselves. This can be a valuable although time-consuming exercise. Once you have narrowed down your suspicions you can leave the appliance disconnected for a few hours or days if necessary to help confirm.

Often people will take hairdryers or vacuum cleaners unknowingly faulty from an older part of Cardiff with no residual protection to a new build in The Bay with rcd protection and then experience a tripping rcd . Portable appliance testing should confirm the fault, but that’s with the help and test equipment from a professional electrician although its not always obvious to the user of the equipment who often assumes there is a problem with the wiring inside the property.

iv. If you believe your electric shower is causing the problem and switching the isolator off resolves it, this would suggest a fault with the shower. The isolator removes the power in the same way unplugging a faulty kettle from its socket would.

If you think your shower maybe causing your rcd problem then you can remove the power from it by pulling the cord so the neon light goes out. Then, try to reset the rcd.


Shower isolators are sometimes mounted outside the shower room on a wall or in a nearby cupboard. Whichever type you have, both should remove the power to the shower so if it is faulty should allow you to reset your rcd and get your power back on.


v. If the RCD trips are very infrequent, make a note of the time. Could it be:

  • An extractor fan in a guest bathroom that isn’t used very often?
  • A security light that only comes on when you go in to the garage?
  • When the oven is initially turned on?
  • An outdoor socket/light fitting that has been affected by the weather?

If you have tried what has been suggested so far to no avail, or the trip is very random, localised to one area (such as the upstairs lighting circuit) and you cannot resolve it, fear not because what you have narrowed it down to will help the electrician fault find from here onwards.

Notes that will help you track down a tripping rcd

1. When the tripping started.

2. If any work has been carried out on the property recently, or around the time the tripping began. Sometimes, non-related electrical work such as shelves being mounted on the walls, or plastering can cause rcds to disconnect because the wiring has been disturbed unknowingly.

3. If you have purchased any new electrical appliances or had any repaired.

4. If there have been any previous unresolved electrical problems you or the previous owner are aware of.

5. If you, or your neighbours have experienced power cuts.

What electricians can do about your RCD tripping

A test meter that can measure a very low amount of current can be connected where the mains supply enters the property. This will reveal exactly how much current is leaking. It would then be an exercise of removing the consumer unit cover and disconnecting circuits until the drop in residual current is noted. If there is no leak at the time of testing; the rcd can be tested.  Test equipment electricians use will check the residual current device is not tripping too soon and exactly as intended by the manufacturer.

If the circuit protection tests OK when disconnected from the electrical installation, further testing and inspection will have to be carried out to identify the fault. Sometimes the rcd/rcbo itself fails and in this situation will need to be replaced. If the amount of residual current isn’t near the disconnection threshold it could be something used infrequently like a security light, or bathroom towel rail.

So far i have covered appliances and accessories from a user and electricians perspective although that is only one aspect of residual current. There could be a problem with the fixed wiring inside the walls, above the ceiling, or inside plastic trunking supplying power to lights and sockets. Have any shelves or picture frames been fitted recently? If so, it’s possible a screw or nail has damaged the wires inside the wall. If you ask any electrician in Cardiff they will tell you mice and rats love chewing through cables. It’s not uncommon to discover damaged pvc cable inside the roof space and wiring that has been run vertically in the wall cavity which was common installation practice at one time to get power from the ground to first floor.

If after following the advice in this article you are unable to find the reason your rcd trips, I offer rcd tripping investigation and a complete domestic and commerical electricians service in Cardiff. The cost will depend on how many circuits and electrical accessories are involved. As mentioned previously, rcds normally test OK; it is almost always one or more faults that when combined leak enough current to disconnect the rcd leaving you without power.

With the number of electronic items in use today the amount of leakage can easily add up – even on items which are not faulty, but leak a small amount of current through their design. It is for this reason most electricians recommend installing a full RCBO consumer unit instead of a split-load rcd model which helps prevent tripping in an otherwise OK installation.

If you have constant or intermittent RCD tripping and live in Cardiff, please get in touch to see how I can help throughout the week or weekend if that’s better for you.

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