Hunting shorting fault in Garden Lamps

[D. T. Green]

We have an electric cable plugged into a wall socket in a rear living room that then goes through a hole in the wall and travels to a junction box in the garden. Which splits it into two wires that individually go to a small garden lamp each.

This arrangement has worked satisfactorily for over fifteen years, but is suddenly blowing 13 amp fuses in the plug in the wall.

I've looked at the garden lamps and can see nothing wrong. What would be the best way to track down where the fault lies? Thanks.

[Gazz]

Replace the fuse with a suitable length of brass rod, plug it back in, and see which wire melts,

or, you could disconnect at the junction box, then use a multimeter to see which one is shorted.... or if it's the cable through the wall, then follow the faulty cable till you find the teeth marks from where a rat or rabbit has chewed it.

[John Rumm]

Have you got a multimeter?

If so, you can disconnect things at the junction box (when not plugged in obviously!), and measure the resistance between L and N and L & E on each cable. That should identify which of them has a problem. If its on a light, then open the light and disconnect the cable there now check the light and the cable - again eliminating one of them.

[NT]

Divide & conquer, using a multimeter to detect the short

NT

[Brian Gaff]

Look for the dead rat?

Brian

[Tim Watts]

You jest but that is exactly what the London Electricy Board did somewhere around the 1950's - they called it the "solid system" and all 300A road link box fuses were replaced with copper bars.

[D. T. Green]

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Thanks. On one garden lamp I get a resistance of about 20 ohm between the two bulb holder contact points and the same reading with hose contacts each connected to the brass earthed lamp holder casing.

The other lamp has no earth wire, and I get about the same 20 ohm resistance reading between the two bulb holder contacts as on the other garden lamp.

Not sure how to interpret these findings, grateful for any further simple advice please.

[NT]

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Sonds like you've still got the whole circuit connected up, thus no way to know which part's ok and which isnt. You dont have any bulbs in too do you?

NT

[John Rumm]

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Have you disconnected the circuit at the supply end?

Have you removed the bulbs (sorry, should have said that in my last post)?

With the circuit disconnected, and the bulbs out, you should see infinite resistance between L&N, and L&E and N&E measured anywhere. If you still find a low resistance then, that would indicate a problem.

Next, isolate each section of the cable - at the junction box would be the best place. Make the same measurements again on each of the free ends. You will likely see a difference on one of them.

(if you were looking for a broken connection, then you could twist LN&E together at the end of one wire, and then measure resistance at the other end, where you would then expect to see a very low resistance)

[D. T. Green]

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Thanks to all. I finally found the junction box which was hiding behind some bricks. Looks like it had a minor nuclear explosion, with wires blasted out of the connecting strip. Guessing that the rain had got in there.

[Jeremy Nicoll - news posts]

I hope you're replacing it with a proper outside waterproof junction box.

You mentioned originally that the power to this box came from an inside 13A socket - is the cable to the outside junction box properly protected? And what about the cables running from junction box to lamps?

You also mentioned a cable to one lamp not having an earth - that sounds potentially lethal.