Hi, during the night one of the fuses tripped. It won't reset with everything still plugged in but with less load on the circuit it stays on after flipping it a few times. Nothing had changed on the circuit, nothing was even turned on or off.

My question is: is this likely a circuit problem or does the switch just need changing?

Thanks, James.

Hi James,

Most likely a fault with an appliance/circuit..is it an mcb or mcbo tripping?

From your description could be a kitchen appliance (Fridge/freezer) ???

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Thanks for the reply.

It's an mcb tripping and there are no kitchen appliances on the circuit.
The circuit supplies a small office/computer room so the only things on the circuit are computers and reuters.

The fault isn't specific to one computer, extension lead or socket. With everything plugged in it won't reset (it did previously) and with less load it takes a few tries to reset it (which it didn't before). As far as I can tell nothing at all changed between yesterday and today.

The standard MCB is really two units built together. One is temperature controlled the other is magnetic controlled. The temperature bit is a little slow to react with a short circuit which is why the magnetic part is also included.
Both can go faulty but not that common to go faulty.
With the temperature part once allowed to cool it will normally allow a reset even if there is an overload but if kept hot due to overload the time taken to trip does tend to become shorter and shorter.
The Magnet part will often prevent one from resetting due to the inrush until the load is removed. It is rare for this part to degrade.
So it would seem you are having a problem with inrush. In which case going around count the watts and calculating amps is unlikely to help. Even using a clip on ammeter set to measure max amps may not register quick enough to show inrush peak value.
Clearly however you need to find if it's the MCB or an item or items connected to the circuit. The clamp-on ammeter reading both max and average are of course a good start and that must be first thing to do. But if the readings are within limits then still worth swapping the MCB to see if it improves.

As to nothing changing you do not know that. With routers and computers they will all be likely to be running from switch mode power supplies. The inrush to charge the capacitors will depend even in A1 condition on how long the power has been off and how discharged the capacitors are. It will also depend on the soft-start design of the power supply and if that is working A1.
So components could fail in a switch mode supply which can increase the inrush on switch on and of course what it uses.

In the past I have had problems with IT equipment using far more than it's rated value and sometimes this has been found to be due to a fault like an electrolytic capacitor having exploded.

Today buying little plug in meters to measure what each item uses is not expensive and one can without too much effort test each item. They even show power factor which is a good guide to component failure with switch mode supply.

In the past this was only done during PAT testing and one hopes the guy doing the testing had the time to read and analyse the results. But with computers I have argued many times where the label on the supply has been obscured during the building. The supply will normally have a spare label which should be stuck on outside of the computer but often this does not happen. Even when it does often the china built supply is lacking information. I have one on my lap at the moment which has no info as to it being an isolation unit normally shown with a shield and two interlinking circles and it should be rejected on a PAT test as many items which plug into the USB port have no isolation from computer supply.

That's if ever PAT tested so often I have been told you can't unplug that it supplies the router. As if that means it can never fail or go faulty.

So really the only thing a non electrician can do is use these plug in energy meters and check the item which says it 1000VA is in fact 1000VA and has a power factor of 0.9 or better. Then add up all the watts (Or VA) and divide by 230 and see if the total is less than the amps marked on the MCB. After that it's down to an electrician to find the problem.