RCD Troubleshooting You Can Do Before Calling An Electrician

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.

See below for example rcd configuration.

There was only one rcd on this consumer unit taken from a property in Whitchurch which means any, or a combination of the circuits could cause it to trip.

The rcd can be identified by the yellow test button in the photo above. In this configuration, it is only protecting the circuits highlighted in green. The lights and door bell in this Fairwater property (on the left in red) are not protected by RCD.


I installed this consumer unit in a property in Canton. Each rcd protects three circuits (separated for illustration purposes with the green line), so a fault on the ground floor lighting would not cause power to be lost on the first floor lights for example.


The Contactum consumer unit above has 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 property such as Ty Mawr with no residual protection to a new build in say, The Chantry 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.

v. If the RCD trips are very infrequent, make a note of the time. Could it be an extractor fan in the bathroom coming on coincides with losing power, for example? I found a problem with a security light once that caused the rcd to trip every time a the number 57 Cardiff bus drove past the customers house!

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 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.

There was a small amount of earth leakage from this property in Fairwater although not enough to trip the rcd

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.