13 steps to resolve mcb tripping

MCB tripping occurs when:

  • A circuit overload is present – like a nail through a wire causing a direct short, or a motor is seized.
  • The rated current of the circuit breaker is not inline with the circuit it is protecting.
  • The breaker is not connected properly or faulty

Step 1. Have you had any work carried out (DIY or professionally) which involved drilling holes in the wall, ceiling or floor? Is it possible a cable has been damaged? Circuit breaker trips like this can be tricky to diagnose without further investigation because cables are not always inline with electrical accessories. But if your breaker was on until a hole was drilled; you may have just found the problem! A drill bit going through a cable might not cause a burning smell. Short circuits generally occur so quickly; the mcb trips almost instantly as the circuit overloads ever so quickly.

Be warned cables are not always inline with electrical accessories, so beware! If you think you have hit a cable with a drill bit, or nail – keep the circuit breaker switched off until you can get an electrician to investigate for you. All the electrical devices on the circuit won’t work, but for safety it’s best electrical appliances supplied are not in use. It’s quite common for cables to get damaged whilst fitting shelves or curtain poles for example. If this has just happened to you: I would strongly advise contacting your local, registered electrician for advice.

Step 2. In the event of foul weather; check outdoor lights, sockets and garden equipment for water ingress. Pond pumps and lights are also prone to faults. A metal outlet box (or plastic, electrical outlets) can cause mcb trips frequently.

Step 3. If the breaker protecting a socket circuit trips; 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.

Step 4. 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.

Step 5. Is it possible the breaker is doing its job and there is an overload present? What is the rating of the miniature circuit breaker for the electrical circuit 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 house with an electrical fault (or at least that’s what the customer thought). They unplugged their recently condemned boiler and connected several fan heaters to its socket – each rated at 10 amps. The circuit breaker was doing its job by tripping (protecting the cable from becoming overheated) due to the electrical load being drawn by all the fan heaters. If you call out an electrician in Cardiff they will carry test equipment to confirm how much current appliances are drawing (and if an overload is present). I helped resolve this particular problem by plugging the heaters in different socket outlets on different circuits and temporary heating was restored!

Step 6. When was the last time you had your consumer unit inspected? It’s possible there is no short circuit, but the mcb is not connected to the busbar properly.  Sometimes the circuit breaker trips, sometimes it can make a buzzing noise – depending on the amount of bare copper connected. The more electricity going through it; the more noise or tripping likely to be experienced. Appliances with high inrush currents (like appliances with a motor for example) can pull a lot of power instantly before settling down to it’s actual capacity.

Step 7. Could it be rodents? I’ve seen short circuits when rats have chewed through cables. Common in lofts where they can hide inside insulation and outside where they roam around in the dark looking for things to nibble on!

Step 8. Do you have a squeaky floor, or has it just been fixed? If cables haven’t been installed correctly its possible one has got compressed and shorting out internally. If you suspect a certain area of your home has been damaged; unplug any devices and see if the mcb trips stop.

Step 9. Does your cooker work as it should? For suspected oven or hob problems (or any electrical devices connected for that matter); the easiest thing to do is find the isolator.  Switch it to the off position and see if the breaker will switch on.  If it does your cooker maybe faulty.

Step 10. Showers drawer a large amount of current; they heat cold water almost instantly. They should always have their own electrical circuit. If faulty, 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.

Step 11. Electrical cables and accessories are more prone to damage outdoors because of the weather (especially rain driven in by strong wind), damp 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. There is more risk of electrocution when outside because you are in contact with the ground wire which is how electricity returns back to the supply transformer. You would actually be grateful of your rcd tripping in this situation!

Step 12. 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.

Step 13. Intermittent mcb tripping faults are the hardest to find. Sometimes professional electricians are called out but unable to clarify the problem because short circuits are not present 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 consumer 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.