Earth Problem - fuse blows

[paulhetherington]

I've just installed a set of LV kitchen downlighters as a spur from the lighting circuit. This was all very easy, nothing too challenging for an advanced DIY'er.

The problem arose when I wired up to provide their own switch to an existing wall box. The exisiting switch was earthed ok, so I didn't need to add an extra earth, especially so as the lights dont require earth.

But having routed out for a 3 core cable down the wall to reach this switch, I thought I'd make use of an extra earth. At the other end it connects into the Earth of the lighting ring, at the switch end it earths the metal box along with the earth that was there already.

When both earths are connected BANG goes the 5amp fuse. When the existing earth is left alone, OK, all works fine. When the new earth is connected alone (old earth disconnected), OK, all works fine.

I've checked as much wire as I can see to try and find a fault, but everything looks fine. If either one of the earths had a problem then surely that would blow the fuse, so why is it only when both earths are connected?

Can anyone help define the problem?

 

[andy]

whats the voltage between the 2 earths?

 

[paulhetherington]

Wouldn't have a clue. I've never had to check voltage, so I don't have the means to do so.

Should I get it proffessionally checked, or get a voltage tester and pursue the problem myself?

 

[andy]

Wouldn't have a clue. I've never had to check voltage, so I don't have the means to do so.

Should I get it proffessionally checked, or get a voltage tester and pursue the problem myself?

you need a multimeter. if you dont have 1 of these then go get 1. even a cheepo will do for DIY. how else do you expect to fault find w/o 1?

 

[Steve]

led screwdriver

 

[ban-all-sheds]

an advanced DIY'er.

I've never had to check voltage, so I don't have the means to do so.

Bit of a contradiction there...

The exisiting switch was earthed ok,

Are you sure?

At the other end it connects into the Earth of the lighting ring,

Are you sure?

why is it only when both earths are connected?

Because only one of them is actually an earth - one of them is live.

 

[BJS_Spark]

If either one of the earths had a problem then surely that would blow the fuse, so why is it only when both earths are connected?

If for example the original earth is damaged, say by nailing through, and is now broken from origin but shorted to live at the point of damage the fuse would not blow. This could make the switch end live and only blow the fuse when shorted to a 'good' new earth. Same could also be true of the new cable.

I have seen this before in a cable. I have also seen ring cable damage where the earth is broken at one point and one end of the break is shorting to live and the other is shorting to neutral! A multimeter was how I found the fault.

 

[paulhetherington]

It sounds like it could be one of the things you describe. I can't say that the existing Earth is ok - just that there were no previous problems. I can say that the new earth IS earth, and that I haven't wired into an earth cable being used as live or neutral, as I am able to follow the circuit.

Actually I do have a multimeter, but I gave up trying to work out how to use it.

If I was to take each of the earths and connect them in turn to another earth, say a power ring, and see which them causes a short, would this be any more dangerous to test this way than it has already proved to be?

 

[andy]

It sounds like it could be one of the things you describe. I can't say that the existing Earth is ok - just that there were no previous problems. I can say that the new earth IS earth, and that I haven't wired into an earth cable being used as live or neutral, as I am able to follow the circuit.

Actually I do have a multimeter, but I gave up trying to work out how to use it.

If I was to take each of the earths and connect them in turn to another earth, say a power ring, and see which them causes a short, would this be any more dangerous to test this way than it has already proved to be?

you said you have a multimeter. now go get it and use that. really wanna put your safety as risk, fukin about causing direct short circuits when you have the equipment needed to test it properly

 

[Qedelec]

I don't wish to rude but I would strongly suggest calling a qualified electrician, considering your replies. You wouldn't want to add to the 12,500 fires each year from faulty electrics.

 

[ban-all-sheds]