help fault finding a tripped fuse

[Mr Red]

Overnight a fuse in the consumer box of my house tripped. The fuse is for the sockets in 3 rooms. If I turn the fuse on then the RCD trips. I unplugged everything from all the sockets but still when I turn the fuse on the RCD trips. What is the next step in fault finding?

[Neelix]

Call somebody competent with proper test kit?

[aeromech3]

serious problem, just my 2 penny worth: assuming Mrs Red has not been hanging pictures up on the wall with hammer and nail overnight what high load items did you have on that circuit ? you could well have a cable (ring or spur) overheated with insulation melted and there is now a track to the earth wire.

[Someone-Else]

Although this is a DIY forum, there are some jobs that are either not really DIY or above the capabilities of the poster, particularly where electricity is concerned a lot of people assume "Well it works, so it must be OK"
I have to say that abrupt that the answer is, what Neelix said is the next step, unless you have an MFT and know how to use it, but if you did have one, you would not be asking here, so call an electrician it is then.

[Mr Red]

serious problem, just my 2 penny worth: assuming Mrs Red has not been hanging pictures up on the wall with hammer and nail overnight what high load items did you have on that circuit ? you could well have a cable (ring or spur) overheated with insulation melted and there is now a track to the earth wire.

It was in the middle of the night so I can't blame Mrs Red unfortunately. The only load of any note was the gas combi boiler but that only has a 3A fuse. My initial thought was that perhaps a mouse had chewed through a cable somewhere. So I figured that testing with an insulation tester might be in order. I've always wanted one so maybe now's the time to buy although I'm not sure where to stick it so to speak. (I'm an electronics engineer, so not a complete dullard in the ways of the electron. I'm not familiar with the circuitry of a house though)

[Neelix]

My money is on a boiler problem.

Did you isolate the boiler? And beware lots of fused spurs are single pole so that type won’t isolate the neutral and the live

[aeromech3]

Though I agree with all the above and not a dullard too, I would isolate all the electrical supply at the C.U. yes master off as you don't know for sure that another circuit is involved, and go around each of the wall sockets, starting with the most used on fault side, and check the back connections, I don't think this is outside the realms of a DIY'r.
I have had houses where due use or vibration, a clamping screw has loosened and the wire potentially could have made contact with the earthed metal back box.

[Mr Red]

My money is on a boiler problem.

Did you isolate the boiler? And beware lots of fused spurs are single pole so that type won’t isolate the neutral and the live

yes, I isolated the boiler by removing its fuse. Will have to do some research on fused spurs - it means nothing to me!

[Mr Red]

Though I agree with all the above and not a dullard too, I would isolate all the electrical supply at the C.U. yes master off as you don't know for sure that another circuit is involved, and go around each of the wall sockets, starting with the most used on fault side, and check the back connections, I don't think this is outside the realms of a DIY'r.
I have had houses where due use or vibration, a clamping screw has loosened and the wire potentially could have made contact with the earthed metal back box.

I could certainly do that.

[Neelix]

My money is on a boiler problem.

Did you isolate the boiler? And beware lots of fused spurs are single pole so that type won’t isolate the neutral and the live

yes, I isolated the boiler by removing its fuse. Will have to do some research on fused spurs - it means nothing to me!

It this confuses you then get somebody in

[Grumps]

I had a similar problem and, although we rarely use it, it turned out to be the cooker hood/extractor.

I'd been meaning to take it down and check for obvious defects until, one day, SWMBO switched on the overhead light of the cooker hood/extractor, even though it was isolated at the fused spur.

Seemingly, the motor/extraction part is controlled by the fused spur but the illumiation portion is on a lighting circuit.

Damn. That could have been somewhat lethal!

[Neelix]

I had a similar problem and, although we rarely use it, it turned out to be the cooker hood/extractor.

I'd been meaning to take it down and check for obvious defects until, one day, SWMBO switched on the overhead light of the cooker hood/extractor, even though it was isolated at the fused spur.

Seemingly, the motor/extraction part is controlled by the fused spur but the illumiation portion is on a lighting circuit.

Damn. That could have been somewhat lethal!

That single product should not have 2 points of isolation …

[Mr Red]

I had a similar problem and, although we rarely use it, it turned out to be the cooker hood/extractor.

I'd been meaning to take it down and check for obvious defects until, one day, SWMBO switched on the overhead light of the cooker hood/extractor, even though it was isolated at the fused spur.

Seemingly, the motor/extraction part is controlled by the fused spur but the illumiation portion is on a lighting circuit.

Damn. That could have been somewhat lethal!

I have a similar situation - the extractor hood and lights have their own socket which I'd forgotten all about when unplugging everything else as it's hidden away at the back of a cupboard. I unplugged it but that's not the problem.

[Mr Red]

My money is on a boiler problem.

Did you isolate the boiler? And beware lots of fused spurs are single pole so that type won’t isolate the neutral and the live

yes, I isolated the boiler by removing its fuse. Will have to do some research on fused spurs - it means nothing to me!

It this confuses you then get somebody in

I've done some reading. Unfortunately I'm not at home to check what type of FCU I have. What do you think could be happening if the FCU is single pole?

[Someone-Else]

What do you think could be happening if the FCU is single pole

You will need to do some more reading (Sorry, but you did ask) You have not (yet) grasped the "complexity" of the problem and why single pole is not ideal.

If the answer/reason was given to you on a plate, you would not learn.