What you can do when an rcd trips

Rcds usually trip because of faulty appliances or wiring.  If your residual current device has tripped with no reason you are aware of; this article may help you turn it back on.  As a professional electrician I should start by saying there are occasions where you shouldn’t, or where further checks are necessary before trying to switch it back on. Some scenarios i can think of include:

❌ Someone has just drilled a hole in the wall
❌ You have just replaced a light fitting or socket outlet
❌ Part of the electrical installation has become damaged – wiring or accessories
❌ Heavy rain (and you have outside lights/socket outlets)
❌ An appliance no longer works

If there is no reason you are aware of; the best chance you have of resetting your residual device is to unplug (switching off isn’t always the same*) as many appliances as you can. Showers, ovens and hobs can be isolated in the same way by switching their isolator to the off position.  If you have a fuse or circuit breaker problem my mcb tripping article may help you (these are devices without a test button).

How to identify an rcd tripping device (and it’s current state)

how to identify an rcd tripping
Contactum rcds have a red window when the device is switched in the ON position

The residual device on the left is in the on position (lever up). The one on the right has tripped and in the off position (lever down). RCDs can be identified by a test button. In the photo above they are yellow and marked with a “T” (for test).

Why is my rcd tripping? Five diagnostic steps you can do for nothing

Step 1. First identify what circuits the device is protecting. If your consumer unit has one it maybe protecting all circuits. With two residual current devices; each one monitors half the installation to help minimize inconvenience in the event one trips. If you have an RCBO (a residual current device and overload protective device in one unit) it protects one circuit and therefore the easiest to troubleshoot.

Dual rcd consumer unit (probably the most common modern type installed in 2020)

If your consumer unit looks like the one above ⇈; the residual current device (the device with the yellow test button) protects everything to the left of it. The first one (from the right) is protecting the cooker, first floor sockets and first floor lights. The second device from the right is protecting the kitchen sockets, central heating, ground floor sockets and ground floor lights. Both residual devices in the image above (made by British General) are in the on position (up). When on, the little window is coloured red. In the off position the toggle would be in the down position and the window is green.

Split board, single rcd

If your consumer unit looks like the one above the residual current device is protecting: sockets (two circuits), cooker and water heater. Something on one of those circuits will be causing a problem so unplug every appliance you can get to and re-try. If you know where the isolator for the cooker and water heater is – turn those off too. The circuits on the left (lights and door bell transformer – the little square box marked “Deta”) are not connected to the device monitoring residual current and therefore not a cause of the tripping issue.

Single rcd consumer unit

The consumer unit photographed above has one residual device protecting the installation. If you have one that looks similar: it could be a problem on one, or a combination of all the circuits and the most time consuming to diagnose without the test equipment electricians carry. Your best chance is to unplug everything accessible, switching the cooker off and checking outdoor sockets and lights for damage or water ingress.

This is what a full rcbo consumer unit looks like

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 just the kitchen sockets. This is the best setup for reducing power cuts if earth faults arise and the quickest for troubleshooting. 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.

Step 2. Turn all electrical equipment in the property off (to prevent damage to televisions, computers and other sensitive electronic devices). Switch the residual current device off by the lever, or by pressing the test button. Now make a list of what no longer works that should. The list will tell you what circuits and appliances could be causing the problem.

Step 3. If you believe the socket circuit is to blame; unplug all the appliances – not just the easy ones, or the old appliances. Everything. Sometimes customers say things like: “i didn’t unplug the washing machine because its difficult to move”.  Or, “i didn’t unplug the kettle because it is only a few months old.” Well, new things can be faulty. I’m happy to attend, but if i unplug your washing machine and the fault clears, you have a call out fee and the cost of a new washing machine! And turning the switch off might not be enough – *many socket outlets are single pole which means just the live is switched off leaving the neutral connected (residual devices monitors both). So unplug.  Yes, it can be a pain, but you want the power back on right? With as much unplugged as you can – see if the device will reset.  If  it will, connect the appliances back one at a time. If you believe to have found a faulty appliance, try it by itself. Two appliances with a high combined earth leak can be enough to trip a residual protective device although not when used individually.  And this can be a big red herring when customers are troubleshooting tripping faults. As a professional electrician I was hired to troubleshoot an rcd trip in Cardiff Bay recently. The customer purchased a new microwave because every time they switched it on the residual current device switched off.  Unfortunately they had the same problem with the new one and called me.  The problem was with the wiring inside the socket the microwave was plugged in to.

What you have done so far may be time consuming but a valuable exercise. Even if you don’t find the cause of the tripping – write down what you have done and show it to the electrician so they don’t waste time (and your money) doing the same.  Any suspected appliances that you can do without; leave disconnected for a few hours or days if necessary to help confirm.  Sometimes appliances unknowingly faulty lived in a home without residual 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 would understandably assume a problem with the wiring or electrical installation.

Step 4. If you think your electric shower is causing a problem; try turning the isolator off.  A shower isolator removes the power in the same way unplugging a faulty kettle from its socket would. If the residual device resets this would suggest a fault with the shower.

Pull cord isolators like this remove the live and neutral from the shower unit which the residual current devices monitor. If the shower is faulty, removing the power from it should help to get your power back on and you need to get the shower problem resolved.

Shower isolators can be outside the room, on a wall or inside a cupboard. This type of isolator is often used for showers, hobs, ovens and immersion heaters

Step 5. If your device trips are infrequent (the trickiest to diagnose), make a note of when it happens. Could it be:

  • An extractor fan in a guest bathroom?
  • A security light that no longer works?
  • The oven reaches a certain temperature?
  • An outdoor light fitting affected by the weather?
  • A rarely used appliance?
  • If you wake up in the morning with the residual current device in the off position – think what might be happening when you are asleep.  Do you have an immersion heater that powers on at midnight and off at 6am? Do you have heating in a greenhouse that comes on?  Do you have security lights that may have become damaged and trying to come on?

Referred to as  ‘nuisance tripping’ although most electricians working in the Cardiff City Centre area will tell you most residual devices tested do work as designed. Residual current devices detect earth leakage (electrical current leaking somewhere it shouldn’t be). They remove the power if the live and neutral go out of balance protecting users from electrocution and buildings from fire. Most trip correctly under fault conditions (faster than you can blink – between 20 and 30 milliseconds) ‘Nuisance’ arose because re-wireable fuses 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. Residual current devices trip to prevent electrical related injuries and fire. They are a big part of the wiring regulations electricians adhere to.

If you have tried everything listed above to no avail don’t worry.  Perhaps the trip is too random or not local to one area. Don’t despair, what you have done will have saved the electrician you do call out time and enable them to continue with more involved troubleshooting.

What will help you (and the electrician if you contact one) track the cause.

1. The first time the tripping started.

2. If any work has been carried out on the property. New lights, sockets, shelves, flooring, kitchen units can all cause tripping if the wiring has been disturbed unknowingly by a screw, nail or not terminated correctly.

3. If you have fitted any additional electrical appliances or had any repaired.

4. Previous unresolved electrical problems.

5. If you have experienced power cuts.

6. If you have power supplying outdoor fixtures or buildings.

Time to call an electrician? What they can do to resolve unwanted tripping :

The instrument below reveals the amount of current leaking. In the photo above a small amount (1.8ma) is normal.

Residual current devices are manufactured to prevent 30ma of current leaking somewhere it shouldn’t. Most trip, switch off and remove power closer to 25ma to ensure they disconnect in the necessary time frame. If no leak (or rather nothing near the tripping threshold) is present the residual device can be ramp tested. This is when we can check its tripping exactly like the manufacturer designed it to.

I’ve covered appliances although that is only one aspect of residual current. Problems with wiring inside accessories (like light switches and socket outlets), cable buried inside the wall, above the ceiling, or inside plastic trunking can cause tripping . Have you fitted shelves, a picture frame or a mirror? Could a screw or nail have 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 and often damaged unintentionally when something is fitted to the wall.

If you ask any electrician in Cardiff they will tell you rats love chewing through cables. So if you have cabling clipped along an outside wall or under decking – is it possible they have become damaged by rodents? It’s not just outdoors cables are at risk unfortunately. Rats can climb and sometimes cables inside the loft space can become damaged if you have had a problem with them.  It was common practice at one time to get cables from one floor to another by taking a brick out at both levels. The cables passed down the cavity of the walls as a route from first floor to ground floor. We don’t install cables like this anymore, but that doesn’t mean you can rule it out until it has been checked.

When i am troubleshooting i usually make a list of the circuit descriptions from within the consumer unit and measure the leakage across each one. With the amount of appliances in use today the amount of residual leakage can be close to the tripping threshold with not one particular fault.  Sometimes its a little leak on one circuit, bit more on another, bit higher on the circuit supplying items outdoors.  When you add them all together it’s not far off the amount the device is designed to trip. Some items will leak small bits of current through their design.  Its not dangerous to the user, but in large premises (or, and) with lots of appliances it can be a problem. It is for this reason, my usual advice when replacing a consumer unit, or troubleshooting problematic residual current device faults is to fit a full rcbo consumer unit (see photo above).  With this configuration you are allowed the same amount of residual current per circuit opposed to half the installation on a dual rcd or the entire premises when one rcd is protecting everything.

I have listed the results of a recent installation experiencing random tripping.  With everything disconnected from the residual device it passed test after test.  I had no doubt the device itself was not the problem (it rarely is).  It was an accumulation of residual current:

Circuit descriptionLeakage (in milliamps)
Upstairs Lights0.7
Downstairs Lights0.4
Sockets Ground floor2.2
Sockets First Floor2.9
Sockets Kitchen1.5
Kitchen Appliances1.9
Immersion Heater1.7
Central Heating1.3
Garden lighting and pond equipment2.4
Total Leakage18.4

The total leak for the installation was 18.4 milliamps at the time of test.  Because there was only one residual current device protecting the whole installation it was getting close to the tripping threshold with not one particular problem that could have been (easily) removed on the day.  The easiest part of the installation to break down further with a high overall reading was the socket circuit on the first floor.  This is what i found:

ApplianceLeakage (in milliamps)
Computer 10.4
Computer 20.5
Electric blanket0.4
Games room0.9
Total Leakage2.2

I found 2.9ma leak at the consumer unit and 2.2 whilst on the first floor.  The missing 0.7 could be due to the wiring supplying the socket outlets, something intermittent or something i could not find attached to the installation.  My point is this: the bigger the installation, often it means more appliances. More appliances equals more natural residual current leaking to earth. The purpose of residual current devices is to protect us from current leaking to places it shouldn’t.

If you have followed my advice and still unable to find the cause of your rcd tripping then get in touch so i can help you (if you live in the Cardiff and Vale area).  The cost to resolve it will depend on the amount of time to find the cause and then what is involved to rectify it.