Switch wiring question

Hi,
I am having a problem with double light switch in my kitchen...and would appreciate any advice before I call out an electrician.
In the kitchen I have a double switch (in a single gang faceplate) that separately operates a ceiling light and a set of under cupboard lights. The kitchen was rewired by an electrician about 6 years ago when we had a kitchen fitted.
For a while this one switch has been playing up - depending on how hard/fast you switched off the under cupboard lights would either blow the lighting circuit fuse (5A) or possibly trip the whole house circuit - I say possibly because I have recently fitted a plug in trip fuse for the consumer unit (just on this lighting circuit) to avoid having to fit a new 5A fuse in the dark each time the fuse went - and now the house earth leakage circuit breaker trips out as well.
This week the switch has started tripping the electric almost every time it is used. There is some soot/ burning to the inside of the undercupboard switch.
The new fuse/trip device in the consumer unit is the only change.Thsi repalced the old style 5Amp fuse holder at the consumer unit with a new plug in trip type which was at 6A (Wylex to match consumer unit). This was to avoid having to replace the fuse which used to go in the lighting circuit when the undercupboard lights
On inspection of the switch wiring there are two cables entering the switch backplate - both earths are connected to the metal back box. On the switch for ceiling light cavle ones red and black wire are connected to switch one COM and L1. There is a jumper wire from this switch L1 to switch 2 L1. The 2nd incoming mains cable has the neutral cable taped up (not connected) and the red cable enters switch 2 COM.
There is a junction box above the kitchen cupboard that the electrican has used to extend the old switch cable to the current position. On inspection the two switch cables (from switch back box) go to this junction box and connect separately to two further cables that go to the ceiling (upstairs floor ) - connections in this junction box are cable1 neutral to neutral / cable 1 live to live ; and cable2 neutral to neutral and cable2 live to live. Earths are all connected together. This junction box looks to do no more than extend the switch cable run,
The cable to the ceiling light is just that - a junction box / ceiling rose is not visible. When the kitchen was rewired the electrians had the floor in the bedroom above the kitchen up so I presume any junctions boxes are under the floorboards...and I haven't considered removing the carpet / floorboards yet to look for them. .
I guess my questions are ... - Does the cabling sound right (jumper etc from switch 1 to switch 2) - Any ideas what would cause it to trip ...(this happens even if all the undercupoard lights are turned off (they each have a built in switch). - Should I go back to the old style cartidge fuse ?
The trip unit on my house wiring is an earth leakage circuit breaker (my earth comes from a ground rod) .
Many thanks... Nick
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snipped-for-privacy@yahoo.co.uk wrote:

is there four twin and earth cables into this junction box?

switch sounds ok, one of the black wire as a "switched"(Live) wire, have the lights ever worked?if they have it prob a faulty light, have you done/had done any work in kitchen that could have damaged a cable?
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Yes - 4 twin and earth into the junction box - but in two pairs . Yes the lights have all worked - for atleast 5 years. No work done in kitchen or anywhere else. Ref faulty light - see below for it still tripping with lights disconnected.
I have tried disconnecting all the under cupboard lights - the electrician actually put in three drops from ceiling - which terminate in junction boxes fixed to underside of wall units. The inidivual light fittings (3) are connected to a separate junction box at the bottom of each of the three cable drops.
The lights (link lights) are connected to the junction boxes - so I have disconnected each light fitting in case one of these has gone bad ...but this didn't make a difference. The only thing I can think is that one of the junction boxes under the wall cupboard has corroded inside due to moisture ingress and when the switched live is applied this trips the circuit ??
Additionally today I tried bypassing the two "extension" cables (both twin and earth) that drop from this junction box to the wall switches. For the main ceiling lights the twin and earth is suppling a live (preumably looped in from a junction box under the upstairs floor) and the black wire is as expected a switched live feed back to the light (it's not marked with red tape though). A simple test joining the black and red from cable one (which is all the switch one would do) and as expected the ceiling lights are on (once power back on of course).
Based on the use of the jumper from switch 1 COM to switch 2 COM I believe the second twin and earth is actually only supplying a switched live back up to the junction box supplying the under cupboard lights. If I join the red (this must have been acting as switched live before as black wire is taped up unconnected in switch) from cable 2 to the live red from cable one I would expect the under cupboard lights to be on.....but this trips the earth leakage cicuit breaker the second the power is restored.
I also tried going back to standard fuse on the circuit instead of the MCB - no differnce apart from I'm now a gew 5A fuses down.
Apart from the moisture what about mice eating throug a cable - I had a brief look under floor upstairs today but no signs of any damage.
Thanks so far... Nick
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On 8 Jan 2006 17:09:57 -0800, snipped-for-privacy@yahoo.co.uk wrote:

Junction boxes dont corrode, they are plastic and brass. Why havn't you looked inside them yet?

Can you get access to the junction box now? Its going to be crowded in there with the 3 feeds to the other junction boxes, maybe the switched live has come lose in this one or one of the others and is touching an earth.
If there is a fault in the cable that caries the switched live you could maybe use the currently tapped over core to be the switched live if it turns out the short is in between the red core and earth.

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snipped-for-privacy@yahoo.co.uk wrote:

stick with the mcb, you defo have a short on cable to undershelf lights best way to test is with a 500v insulation tester, prob light switch is now faulty, don't try to switch on again until fault fixed, will damage mcb next, fault is prob under flooring somewhere, maybe cable trapped under floorboard? best to lift flooring and try to trace route of cable, really you should now get a electricin in tho
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snipped-for-privacy@yahoo.co.uk wrote:

any fitting with any signs of charring at all should be replaced, however slight.

that'll be the unswitched live

since both mcb and rcd trip, sounds like a toast situation. Find out whats toasted and replace it before something catches fire.

no
You need to find out where the fault is. Trips on switch off typically result from a switch being in the neutral line. You can use a multimeter (in most cases) to find what is conducting slightly when it shouldnt be. Maybe switch, light fitting, or cable. With item under test disconnected, measure from earth to L&N connected. Also measure L to N with bulb removed. You should get no conduction anywhere if things are ok.
Obviously turn supply power off before disconnecting anything, leave alone if you dont know what youre doing etc.
But dont leave it like this, it sounds like a fire starter.
NT
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On 7 Jan 2006 04:31:31 -0800, snipped-for-privacy@yahoo.co.uk wrote:

I guesse you mean the faceplate is 2 gang and the back box is single width.

You have a short to earth and as its taking out the mcb aswell its probably from a live.

The soot is the clue!
Have the earths got yellow/green sleeving over them??? If not then put some on as this is probably the problem.
Otherwise replace the switch and make sure the wires are not lose ie tighten the screw terminals and make sure the cables have not had the core exposed maybe the electrician had accidentaly scrapped some off the live wire when he was removing the outer sheathing, this could be fixed with some insulation tape.

Its OK

There is an earth short from the feed to the undercupboard lights - that is the red wire that goes to the cable that has got its black core taped up.

No, nothing wrong with the mcb, its telling you there is a problem.

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snipped-for-privacy@yahoo.co.uk wrote:

Sounds a bit rum to me.
Normally a light switch gets a live in, and a switched live out.
If you are taking these in using a normal twin-and-earth cable the black wire needs some red sleeving (or red tape) near the end to signify that it caries live nor neutral.
When doing this it is usual to put the live on the red and the switched-live on the black-with-red-bit.
For one thing it looks as if your setup has it the other way around.
Also, you seem to have live coming in via one cable only and switched live going out in both. This is a bit strange, especially as the junction box would be a great place to common together the lives if it was not available up-stream for the under cupboard lights.
Why would the electricial have joined cable-2 neutral in the junction box when he wasn't using it in the light switch? I wonder what is connected to the other end of it!
I don't like the sound of this at all - a slightly odd wiring setup that keeps blowing fuses. I would either check the whole lot myself (including checking what is under that floor) or get in someone that can be trusted to check it all for you. I wouldn't ask the original guy back!
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snipped-for-privacy@yahoo.co.uk wrote:

any fitting with any signs of charring at all should be replaced, however slight.

that'll be the unswitched live

since both mcb and rcd trip, sounds like a toast situation. Find out whats toasted and replace it before something catches fire.

no
You need to find out where the fault is. Trips on switch off typically result from a switch being in the neutral line. You can use a multimeter (in most cases) to find what is conducting slightly when it shouldnt be. Maybe switch, light fitting, or cable. With item under test disconnected, measure from earth to L&N connected. Also measure L to N with bulb removed. You should get no conduction anywhere if things are ok.
Obviously turn supply power off before disconnecting anything, leave alone if you dont know what youre doing etc.
But dont leave it like this, it sounds like a fire starter.
NT
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snipped-for-privacy@yahoo.co.uk wrote:

any fitting with any signs of charring at all should be replaced, however slight.

that'll be the unswitched live

since both mcb and rcd trip, sounds like a toast situation. Find out whats toasted and replace it before something catches fire.

no
You need to find out where the fault is. Trips on switch off typically result from a switch being in the neutral line. You can use a multimeter (in most cases) to find what is conducting slightly when it shouldnt be. Maybe switch, light fitting, or cable. With item under test disconnected, measure from earth to L&N connected. Also measure L to N with bulb removed. You should get no conduction anywhere if things are ok.
Obviously turn supply power off before disconnecting anything, leave alone if you dont know what youre doing etc.
But dont leave it like this, it sounds like a fire starter.
NT
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