14 Amethyst Road | Cardiff | CF5 3NU | 029 21660580

Why MCBs trip: troubleshooting circuit breaker tripping issues

MCBs trip on overload or short circuit. They are designed to remove power when the circuit it is protecting draws more current than designed.  They do this to stop the cable from becoming damaged and posing a fire risk.

If your miniature circuit breaker trips this webpage might help you get it switched back on.

Here is what circuit breakers that are on and off look like. In the photo above, the smoke alarms are switched off and the kitchen sockets are on. Contactum have a little green window for off and red for on.

Thirteen ideas why your miniature circuit breaker just switched off

✅ Have you had any work recently carried out (DIY or professionally) which involved drilling holes in the wall, ceiling or floor? If so, it’s possible a cable has been damaged. This can be tricky to diagnose without further investigation because cables are not always inline with electrical accessories.


Cables are not always inline with electrical accessories, so be aware! The customer from the property this photo was taken had drilled a hole in the wall to fit some shelves. The cable had been damaged and caused the circuit breaker to trip (switch off).

✅ In the event of foul weather; check outdoor lights, sockets and garden equipment for water ingress.

The security light pictured above filled up with water causing the circuit breaker to trip

✅ If the breaker protecting a socket circuit trips then unplug as much as you can from that circuit. So if you know “upstairs sockets” protects all the sockets and boiler – unplug everything you can upstairs and switch the boiler off if connected via a switched fuse spur.  Check for security lights which are sometimes connected via a switched fused spur next to a bedroom socket outlet. Has a socket outlet been replaced recently? Is it possible a wire has been trapped?

Central heating boilers and immersion heaters can be isolated by switching the fused connection unit (highlighted in green) to the off position. If they are faulty, your circuit breaker might reset once they have had the supply to them removed.

✅ If the miniature circuit breaker for any lighting circuits is problematic then remove as many lamps as you can before attempting to reset it. You could also ensure the switches / dimmers are in the off position.

✅ Is it possible the breaker is doing its job and there is an overload present? What is the rating of the miniature circuit breaker that has just tripped? To trip instantly they need many more times their rated current. I was called out during a cold winter to a home where the boiler had failed.  The customer unplugged the boiler and connected several fan heaters each rated at 10 amps. The circuit breaker was doing its job (protecting the cable from becoming overheated) and tripping in time.

✅ When was the last time you had your consumer unit inspected? It’s possible the circuit breaker is faulty or not connected properly.  Sometimes they can make a buzzing noise in these situations.

I was called out to a circuit breaker tripping problem on the upstairs socket circuit in the Welsh Capital. I discovered the breaker wasn’t clamped correctly around the busbar. If you look closely, you will notice the first two breakers (green ticks) have the clamp correctly around the busbar. The third breaker supplying the upstairs socket outlets did not. The customer advised the breaker would randomly switch off and sometimes buzzed. Electrical inspections will help prevent problems associated with loose connections inside the consumer unit. When was the last time your consumer unit was inspected?

✅ Could you have a problem with rodents? Rats can chew through cables and usually die on the cable in the process.

Rodents in this rear garden had bitten through most of the cable and died in the process. I’ve spared you from seeing that part! The breaker switched off removing power to the garage and the whole contents of a chest freezer had to be thrown away. Armoured cable should be used in such circumstances. The twin and earth cable photographed above is not suitable to be used outside like in this way.

✅ Do you have a squeaky floor, or has it just been fixed? If cables haven’t been installed correctly maybe one has got compressed and shorting out internally like they were in this property photographed below:

The cables for the socket outlets on the first floor in this property were sat on top of the joist. Someone had chiseled some of the floorboard away to make space, but not quite enough to clear.

✅ For suspected cooker or hob problems the easiest thing to do is look for the isolator.  Switch it to the off position and see if the breaker will switch on.  If it does your cooker maybe faulty.

✅ Showers drawer a large amount of current when working correctly.  They heat cold water almost instantly. If faulty, they have a loose connection or faulty isolator; it can cause your circuit breaker to blow with a bang! Try turning the shower isolator to the off position and retrying.

✅Electrical cables and accessories are more prone to damage outdoors because of the weather (especially rain driven in by strong wind) and garden tools coming in to contact with them.  If your circuit breaker for outdoor lighting or socket outlets supplying items like ponds and waterfalls trips then ensure it stays off until you find the cause.

✅ Do you know if there is a sub-floor? Could water have entered the foundation of your home and come in to contact with a junction box perhaps? Cables below the ground floor of a property can become damaged by water or rats.

✅ Intermittent electrical faults are the hardest to find. Sometimes professional electricians are called out but unable to clarify the problem because the fault is not there at the time of test. Make a note of when the breaker trips if you can.  Could it be when the immersion heater switches on by timer for example? As a customer you can only go so far with this type of electrical fault. You may need help from your local electrician.  Cardiff home and business owners contact me on a regular basis to resolve circuit breaker tripping faults. In some cases the circuit breaker itself can fail and replacing them isn’t a DIY job.

I attended two properties recently in The Bay where the circuit breaker wouldn’t stay on.  After removing the loads from the breaker it still wouldn’t stay in the on position.  After testing the circuits were healthy (no shorts) I replaced it and got the circuits back on for the customer.

If you find yourself in the position where it trips every few hours; it could be a bad connection, load issue or faulty part. Miniature circuit breakers are only a few pounds each, but you would need to add on an electricians call out and troubleshooting time. Usually its one circuit that has a problem, but if two breakers are tripping it could be a more serious problem – like two circuits are interconnected. If you need help and live in the Cardiff area please get in touch.

Service Areas: Adamsdown | Butetown | Caerau | Canton | Cathays | Creigiau & St Fagans | Cyncoed | Ely |  Fairwater   | Gabalfa | Grangetown | Heath | Llandaff North | Llanishen | Lisvane | Llandaff | Llanrumney | Pentwyn | Pentyrch | Penylan | Plasnewydd | Pontprennau & Old St Mellons | Radyr & Morganstown | Rhiwbina | Riverside | Rumney | Splott | Trowbridge | Whitchurch & Tongwynlais

Sitemap | NICEIC Registration: 609099 | Competent Person Registered | Zurich Insurance : CK781909/501699