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Why MCBs trip: troubleshooting circuit breaker tripping issues

Miniature Circuit Breakers 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 mcb trips here are some things to think about before trying to switch it back on:

  • Have you had work recently that 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.

The customer wanted to put some shelves up but unfortunately drilled straight through the cable supplying a nearby socket outlet. I had to cut away the plasterboard to reveal the problem.

  • 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 mcb to trip

  • If the circuit breaker protecting socket outlets trips then unplug as much as you can on 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?
  • If the mcb 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? What is the rating of your mcb that has just tripped? To trip instantly they need many more times their rated current. I was called out one 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 mcb protecting the socket outlet was a B16 (16 amp) designed to trip instantly somewhere between three and fives times its value.  There was nothing wrong; the breaker was working as intended.

The 16 amp circuit breaker highlighted here was doing its job when tripping. The amount of heaters plugged in to the socket outlet was an overload

  • It’s possible the circuit breaker is faulty or not connected properly.  Sometimes they can buzz in these situations.
  • 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

  • Could you have a problem with rodents? Rats can chew through cables and usually die on the cable in the process.
  • Has your squeaky floor just been fixed? If cables haven’t been installed correctly maybe one has got compressed and shorting out internally.

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.

Cooker circuit problem troubleshooting

If the fuse (or mcb) for your oven or hob won’t stay on the easiest thing to do is look for the isolator.  Switch it to the off position and see if the breaker will reset to the on position.  If yes, turn the controls on the oven or hob off, and then turn the isolator on. If the breaker is still on, now try and turn the oven or hob on.  If it trips, then you may well have a faulty cooking appliance.

Shower circuit help

Showers drawer a large amount of current when working correctly.  They heat cold water almost instantly which requires a lot of power to do so. If faulty, or there is a loose connection or faulty isolator this can cause your mcb to blow with a bang! Try turning the shower isolator to the off position and retrying.

Outdoor power checks

Electrical cables and accessories are more prone to damage outdoors because of the weather (rain and 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.

More about mcb tripping

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 fault. You may need help from your local electrician.  Cardiff home and business owners contact me on a regular basis to resolve mcb tripping faults. 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 the faulty mcbs 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. The 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 using the form below.

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