Switched Line dropping to 0v when tested

[Uffledust]

Good morning all. Got a bit of a problem that is stumping me.

Called to investigate an external light not working. This light is on the same circuit as about 5 other lights and is controlled via a photocell which switches in a contactor to bring all lights on. Every light is working apart from this one. This is an emergency light that has been given new batteries lamp and ballast. What happens is it will not work in mains.

When I put a tester on the SL it shows an AC voltage. My multimeter says 60v. When I put the testers probe to Neutral or Earth and test it correctly it drops to 0v.

Also seems when I only put one probe on(I know this isn't good practice) It will only register when I use the positive lead. Despite being AC..

Any ideas what could be causing this as every other light tests fine. The permanent live to charge the batteries tests fine at 230v to Neutral and Earth

Any info would be greatly appreciated

 

[snowhead]

When I put the testers probe to Neutral or Earth and test it correctly it drops to 0v.

You're missing the S-Live into that light.
Is it looped from another?
Are all the lights through the same contact on the contactor, or individual or grouped?

Is the faulty light fed with a flex and junction box from SWA or T&E with a connector in it?

 

[Uffledust]

All lights are on the one contactor. I believe it is looped from one above it. Just going to continuity check it now.

Why would this cause the voltage to drop to 0?

 

[Leesparkykent]

There isn’t a S:

All lights are on the one contactor. I believe it is looped from one above it. Just going to continuity check it now.

Why would this cause the voltage to drop to 0?

the impedance of your tester.

 

[Uffledust]

Do you believe that the 70v is an induced voltage?

 

[The Ghost]

Maybe you've lost your live supply. The rest is phantom voltage I guess.

 

[James]

Agree with both above, you have either an induced voltage in the cable from it running parallel to another cable and being disconnected from source voltage
Or a broken cable that is managing to pass a voltage through either water at the break or possibly carbon from some arcing.

If it is run from a different contact on the contractor then it could be a failed contractor.
Go back to basics and trace back to find a point where you do have a good voltage.

 

[Uffledust]

Thanks everybody who helped. Issue is still not fixed but believe there is a break somewhere. This is an Mod accommodation annex where the cables are run from a ground floor plant room up to a roof void above the first floor then across and out. Visually there is nothing obvious but need to go back and test. Had other tasks to get on with in the meantime. No junction boxes or anything so good 230v Coming out the contactor but not getting to final destination.


Not a very good picture however the setup is below

 

[Richard Burns]

This light is presumably the end of that block of lights as all the others work.
The voltage (60V) seen is spurious and meaningless, it just indicates it is near some power.
The switched line is disconnected between the previous light and this light, because the control gear have just been changed in this light I would suspect a poor termination of the switched live into the terminal for the light, e.g. catching the insulation or not going firmly into the terminal.
Though it could be further back than the actual fitting.

 

[Uffledust]

Richard. I believe you are correct. Although taking the light before it off I could only see 1 x 3 core an earth feeding that light. So as you suggest may be a connection just not in plain sight.

 

[Strima]

If it’s the newer style accommodation blocks they’re modular and prewired at the factory so there may be a junction box somewhere on the circuit.

 

[Charlie_]

Hi
Did you manage to sort the problem out?
Do any of those circuits need RCD protection?

 

[Andrew Hunter]

Hi
Did you manage to sort the problem out?
Do any of those circuits need RCD protection?

See where your trail of thought is going Charlie thought same thing myself instantly saw 60 volts.

 

[Uffledust]

Hi
Did you manage to sort the problem out?
Do any of those circuits need RCD protection?

Sorry I thought I had replied to you. No fault is still there. Requires further investigation. Waiting for the job to come back through. Should have RCD protection but it doesn't. All on mcbs. Very old military building.