Tripping Circuit

[pramplin]

Hi all.  Hope someone can help me figure this out.

One of the lighting circuits on my consumer unit is tripping.  It won't even stay on - it instantly trips off when I try to turn it back on.  I have removed all light switches and pendants and inspected them for problems but there is nothing.  I have also turned absolutely everything off around the house - lights, sockets etc - in the hope of finding something that may have been wired into the light circuit without my knowledge.  But even with all of these things turned off, the switch is still tripping off.

Could I have missed something obvious?

Please offer up some suggestions.

(I will give more information if needed - I've been intentionally brief and given an outline for fear of bombarding with too much of the wrong information.)

Additional information:  work has recently been carried out in the house but that work was on a different lighting circuit and that circuit works fine.

More additional information: the consumer unit is a new-style model with two sets of RCDs, each one controlling four or five MCBs.

Thanks in advance,

Paul.

 

[Murdoch]

Burnt insulation on cable above a downlight or rodent damage

 

[Essex1]

Does the MCB go 'BANG!' Or does it just click off?

 

[ProDave]

In these situations, as an electrician investigating, I first ask has ANYTHING been done in the house recently?  Absolutely anything, even as simple as hanging a new picture on the wall, or any work, any floor boards up, put anything up in the loft, added extra insulation etc etc etc.

You haver a wiring fault, a short circuit, so it's a process of checking everything associated with that circuit.

How old is the house? or more importantly if it's been rewired when was that done?

Think of ANYTHING done recently.  I wish my customers would when asked, e.g the time when I traced the fault to a short in a cable above the kitchen ceiling, only then did the customer say "oh we had the flat roof replaced last week"

EDIT:

Re reading the post, you HAVE had work done. That is where I would be looking for the fault.  Even if the rooms that have been worked on are different circuits, it is quite possible the feed to your circuit passes through where the work has been done, and the cable now has a nail through it.

 

[Doc Hudson]

Is it the MCB or the RCD that is tripping?

Doc H.

 

[pramplin]

Hi all and thanks for the replies.

Murdoch - I did find a cable with insulation that didn't look great and I changed that cable for new - still tripping though.

Essex1 - the MCB just clicks off - there's no bang at all.

ProDave - I share your worries about the recent work but I can't find anything there that could be connected.  I'm not an expert, but there just doesn't seem to be anything that would be interfering with the problem circuit.  That said, I will try and look there again tomorrow.

Doc Hudson - It is the MCB that is tripping.

Thanks everyone for their help.  I'll be getting back to it tomorrow, probably laying awake in the meantime trying to figure things out...!

 

[Essex1]

No bang. Could be a faulty MCB.

 

[Doc Hudson]

[SIZE=10.5pt]Disconnect the wire from the MCB, can the MCB be reset without any load connected. (or turn the main switch OFF to remove power from the installation can the MCB be reset with no power on) A few continuity and insulation resistance tests would quickly confirm if there is a fault somewhere on the circuit. Did a lamp blow when the MCB first tripped? As Essex suggests it could be faulty MCB, after genuine trip caused by lamp blow, MCB now not able to reset. do you have another lighting circuit MCB you could swap your "suspect" circuit onto another MCB to prove the circuit?[/SIZE]

[SIZE=10.5pt] [/SIZE]

[SIZE=10.5pt]Doc H.[/SIZE]

 

[NozSpark]

I've just had a faulty MCB on a fault that I've been looking at.....

I was called out to a "shower tripping" and was told that they'd not long replaced the isolator switch...

so I look in the switch and find damaged insulation and excessively cut back sheathing so I sort that out with a new switch, carry out testing and every things fine... turn shower on and it's fine

I get a call that evening saying that it's still tripping,, on further investigation I find that the shower is leaking internally so I replace that and test again.... everything tests out ok

I get another call... still tripping... so I go out and test it again..... test out fine.... I leave the shower running for 10 mins and it's still fine... I put my clamp meter on the shower circuit 32.6A on a 40A MCB and it continues to run fine...I tap the MCB and it trips!!! (even with the shower turned off)!!!!!! this is repeatable so I replace the MCB...

I hope that this is the last I see of this job!!

 

[pramplin]

Well, I figured it out.... kind of.

I undertook the frustrating process of disconnecting wires and then reconnecting them again in the hope of rectifying a hidden-loose-connection type scenario.  And that's exactly what happened.  At least I now know where the problem was located - I just don't know exactly what the problem was.  I will, at some point, pull the problem wires out and have a closer look/replace but for now I'm happy that things are working again.

Thanks to all who advised - it was much appreciated.

 

[sasse1989]

that was lucky bro