Blowing Fuses on a Lighting Circuit

[Brian G]

Some 15 0r so years back I built an extension and garage, the lighting and socket circuits are independent of the main fuse board and on their own circuits. a few months back the lighting circuit started to blow the fuse, I suspected a faulty exterior security light as the fault first started after heavy rain, changed the security light, and disconnected a faulty pull switch in what is an outside store, which ensured that no lighting was able to draw power, replaced fuse it blowed immediately, this led me to believe that it might be a fault in the extension lighting and pulled switch from the back box to investigate if there was a fault there, nothing was visible, although the back box seems a little shallow with little room for the cables to sit comfortaby replaced fuse whilst having switch hanging loose, hey presto lights on with the exception of the outside store light as that still had no switch, fitted extension switch to back box and after a short period of maybe 10 to 20 minutes the fuse blows again.
Today I have replaced the back box and light switch, along with a new pull switch in the extension rewired fuse hey presto everything works, well for a short while because the fuse has just blown again, has anyone any suggestions what I can check next could it be a fault on the consumer unit, which was not brand new but fitted along with all the wiring by a registered electician.

[Mickangler]

Brian ?is there a chance that you could have a faulty MCB in the consumer unit..silly question but is the correct fuse rating being used for the circuit in question..both lighting and sockets.

?Have you removed all cieling roses to check for a short to earth or a bured wire somewhere.

Do a continuity test on each circuit,if you can using a multi meter,this checks for shortcircuits.

Other than that..get a man in who is a fully qualified sparky.

I'm not a sparky myself but my bro is,ans this is what he said he would do faced with your problem...HTH.

[Brian G]

Cheers Mick for your reply, following a chat with another site member it looks like I have traced the fault, it appears to be a cable fault in a part of the garage that is used as a store room ( junk hole ) I now have lights in all areas except this and have disconnected a newly fitted pull switch which replaced a faulty one, I will pull the plasterboard down tomorrow and check the wiring out, maybe I have had a squirell residing in the void, or some sort of leak, luckily its only around 2 metre by a metre so not much of a problem to rip out and reboard.

[Mickangler]

OK Bri,no problem and GL.

[luke carpstalker]

Brian, I am a spark by trade, but not domestic, same stuff applies. Rewireable fuses are quite tolerant of a slight overload, but will go more or less instantly on a dead short, i.e L>N or L>E.

I would guess that because the fuse is taking some time to blow, that there is a short somewhere, but with a reasonably high resistance. This high resistance will act in much the same way as say a filament in a lamp ( in terms of it being a load ), and will maybe be pulling 6A through the fuse, and losing it to either N or E. Out of interest, what are the lamp ratings on the circuit?

I would suggest you leave the fuse out until you find the fault, it's these types of faults that can set fires. If you could sketch up a schematic of what is on the lighting circuit, and how it is all wired together, I could maybe suggest you a point where to start fault finding it, you have to start in the 'middle' and eliminate half the wiring at a time, and so on.

You would really need an insulation resitance tester to test the cables ( they blast 500v in ), but I would imagine you could pinpoint the fault with a multimeter. If the wiring is surface clipped and exposed it's much easier, as it takes the guesswork out of what goes where.

The comment you make about this fault starting after heavy rain is also valid, it could well be that there was a potentail for a fault, say a nail or screw in a cable that would not fault in dry conditions. The water could have made the juice jump the gap, and the heat caused by this would have burnt more insulation away, and you are now in the position you are in.

Have a think back to any jobs you might have done recently around the garage...there is a cause for these 99% of the time.


Mickangler...a continuity test is for continuity only, what brian needs to look for is low resitances between LNE.

[Frisky Fish]

Get a good rodent trap

[wotnot]

Brian if he's nuts are glowing you've solved it.

[Brian G]

Heres the problem lol

[FORDY-1]

did a properly qulified butcher do that or a trainee

[Brian G]

Definitely unqualified Steve, you should see the tape joint I have used to reconnect it lol.

[luke carpstalker]

Is that just an open ended cable, or has it been severed?

[Brian G]

It had worn through the outer cover, or possibly burnt through, the earth wire was welded to the live.

[sparky 3]

Brian well done on your fault finding you might make a sparky yet lol