RCD tripping troubleshooting in Cyncoed

An RCD tripping is referred to as ‘nuisance tripping’ by many. In a large percentage of tests electricians confirm they work as designed. Residual current devices detect imbalances in ovens, sockets and showers. They remove the mains power if the live and neutral go out of balance. Most RCDs tripping under fault conditions will be between 20 and 30 milliseconds – faster than you can blink! ‘Nuisance’ arose because re-wireable fuses and miniature circuit breakers didn’t trip at all! Designed to protect the cables if overload occurred. Fuses and circuit breakers offer no protection from a person coming into contact with a live conductor. RCDs help prevent electrical related injuries and fire. They are a big part of the wiring regulations electricians adhere to.

What causes an rcd to trip: diagnostics you can do yourself.

Top Tip! The first thing to do if you loose mains power is unplug (turning the switch off isn’t the same) everything you can. If you call any electrician for an rcd tripping issue in Cardiff – this is the first thing they will do!

i. Identify what the RCD is protecting. Clarify what the device is protecting and begin the diagnostic process. The rcd device will have a test button. If your consumer unit has one RCD it is protecting all the circuits connected to it. If it has two residual devices 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) it protects one circuit.

If you don’t know what an rcd looks like you can see examples below:

Split load (dual rcd) consumer unit.

If your consumer unit looks like this; the rcd (note yellow test button) protects everything to the left of it. 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 may have a developed a fault. Faulty appliances are common. Problems with the wiring are another cause.

Split board, single rcd.

If your consumer unit looks like the one above the rcd is protecting: water heater, cooker and sockets. Something on one of the circuits is causing a problem so unplug every appliance you can get to and re-try. The circuits on the left (lights and door bell transformer – the little square box marked “Deta”) are not connected to the rcd.

Single rcd consumer unit

One rcd in this domestic installation in Whitchurch photographed above. If you have one that looks similar: it could be a problem on one or combination of all the circuits. Your best chance is to unplug everything accessable and trying to reset it.

Full rcbo consumer unit

I installed this consumer unit in a domestic property in Roath. It has residual protection (called an rcbo) on each circuit (test button in yellow). A fault on the kitchen sockets will remove power on the kitchen sockets for example. This is the best setup for reducing power cuts if earth faults arise. If your installation looks like this: whichever rcbo is tripping – something on that circuit is causing a problem. If its sockets, unplug everything you can access and try to reset. If its a shower, switch the isolator off – same with the cooker or hob. If its lights you can remove all the lamps.

ii. With a list of what the RCD is protecting, turn all electrical equipment in the property off. Switch the RCD off by the lever, or by pressing the test button. Make a list of what no longer works. The list will tell you what item(s) or area(s) could be causing the problem.

iii. If you believe the socket circuit is to blame; unplug all the appliances. Add them back one at a time to see if the rcd trips. If you believe to have found a faulty appliance, try it by itself. The combined earth leakage of two items can be enough to trip an RCD although pose no problem used by themselves. This can be a time consuming although valuable exercise. Suspected items can leave the appliance disconnected for a few hours or days if necessary to help confirm.

Appliances unknowingly faulty from a home without rcd protection move to a new one and cause tripping . Portable appliance testing should confirm the fault, but that’s with the help and test equipment from an electrician . Users of the equipment assume a problem with the wiring or electrical installation.

iv. If you think your electric shower is a cause; try turning the isolator off. If the rcd resets this would suggest a fault with the shower. A shower isolator removes the power in the same way unplugging a faulty kettle from its socket would.

Isolators like this remove the live and neutral from the shower which is what the rcd is monitoring.


Shower isolators can be outside the room, on a wall or inside a cupboard. They remove the power so if the unit is faulty will allow you to reset your rcd and get the power back on.


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

  • An extractor fan in a guest bathroom that isn’t used a lot?
  • A security light that no longer works?
  • That the oven reaches a certain temperature?
  • An outdoor light fitting affected by the weather?

Tried the suggestions to no avail? Maybe the trip is too random or localised to the upstairs lighting for example. You have carried out the first part of the electricians call out and will assist further diagnostics.

Notes that will help you track a tripping rcd .

1. The first time the tripping started.

2. If any work has been carried out on the property or at the time the tripping began. Non-related work like new shelves can cause rcd problems because the wiring has been disturbed unknowingly.

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

4. Previous unresolved electrical problems.

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

What electricians can do to resolve your RCD tripping .

An earth leakage meter measures a low amount of current. It is connected to the mains supply entering the property:

This instrument reveals the amount of current leaking. In the photo above a small amount (1.8ma) is is normal. Although rcds state 30ma, most trip closer to 25ma to ensure they disconnect in the necessary timeframe. I remove the consumer unit cover and disconnect circuits until the residual current drops. If no leak is present the rcd can be ramp tested. Electricians can check the rcd is tripping as intended like in the video below:

If the circuit protection is OK disconnected from the installation, further testing will be necessary. An rcd/rcbo that fails will need replacing. If the residual current isn’t close to the threshold it could be a security light for example. Items that aren’t on for long can be harder to diagnose as part of an rcd tripping exercise!

I’ve covered appliances although that is one aspect of residual current. Problems with wiring inside the walls, above the ceiling, or inside plastic trunking can cause rcds to trip. Have you fitted shelves or picture frames? Perhaps a screw or nail has damaged the wires inside the wall. You can expect buried cables if an accessory is close-by (like a socket or light fitting). Cables can be present without any sign of an accessory.


Rats had chewed through this telephone cable which caused the telephone to stop working.

If you ask any electrician in Cardiff they will tell you rats love chewing through cables. They chewed through this telephone cable which caused the telephone to stop working.

It was common practice at one time to get cables from one floor to another by taking a brick out. The cables passed through the cavity of the walls and went to the different floors of the property. Rats can climb – and they love eating cable.

With the amount of electronics in use today the amount of leakage is on the increase. Some items (even new) leak a small amount of current through their design. Some electricians install an RCBO unit helping prevent tripping in a safe installation for this reason.

Maybe you have followed the advice in my rcd tripping article and unable to find the reason your rcd/rcbo trips? If this is the case; i offer rcd tripping diagnostics for Cardiff home and businesses. The cost will depend on the amount of circuits and electrical accessories.

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