Light fixing testing at 1.6v when switched on

[shreeman]

I am currently replacing an old ceiling pendant light with a new double-insulated metallic pendant light. The previous light was not double-insulated so I moved all the wiring in the old plastic ceiling rose into a junction box to be pushed into the hole away from the new metal rose.

The two wires of the light were connected, the light was hung and all worked as expected. Having not come across a double insulated light before, and being nervous of a metal light, I tested the wire shade with a non contact detector pen and saw that it indicated the shade was live.

Thinking the light must be dangerously faulty, I took it down and powered it up via a plug (with appropriate precautions). This time the shade was not live.

I then tested the light fixing (see pic) and saw that *this* was the cause of the live signal despite being screwed into wood. This led me to scan the surrounding ceiling and I saw that quite a large area was flagged as live by the detector (50cm2). Using an endoscope I inspected inside the hole and, whilst there were wires visible, they were not significantly close to the ceiling plaster, nor had I screwed through any of them. Measuring the voltage of the fixing using a multimeter measured 1.6v.

There was no obvious signs of damp in the plaster. but does this mean there is definitely a short somewhere causing this? (I'm guessing it's an obvious yes!) Are there any techniques for diagnosing where the problem may originate without making more holes in the ceiling or lifting the floor above?

 

[EFLImpudence]

I am currently replacing an old ceiling pendant light with a new double-insulated metallic pendant light. The previous light was not double-insulated so I moved all the wiring in the old plastic ceiling rose into a junction box to be pushed into the hole away from the new metal rose.

Is the light 240V or is there a 'transformer' to which the wires are connected?

The two wires of the light were connected, the light was hung and all worked as expected. Having not come across a double insulated light before, and being nervous of a metal light, I tested the wire shade with a non contact detector pen and saw that it indicated the shade was live.
Thinking the light must be dangerously faulty, I took it down and powered it up via a plug (with appropriate precautions). This time the shade was not live.

OR - this time the detector did not give you a false reading.
Why did you not test it with the multimeter?

I then tested the light fixing (see pic) and saw that *this* was the cause of the live signal despite being screwed into wood.

I don't understand - or is there supposed to be another picture?

This led me to scan the surrounding ceiling and I saw that quite a large area was flagged as live by the detector (50cm2).

Scanned with what?

Using an endoscope I inspected inside the hole and, whilst there were wires visible, they were not significantly close to the ceiling plaster, nor had I screwed through any of them. Measuring the voltage of the fixing using a multimeter measured 1.6v.

So, nothing to worry about - but how did you measure it?

There was no obvious signs of damp in the plaster. but does this mean there is definitely a short somewhere causing this? (I'm guessing it's an obvious yes!) Are there any techniques for diagnosing where the problem may originate without making more holes in the ceiling or lifting the floor above?

Depends on how you are measuring.

Your description is not detailed enough.

You should measure an item by measuring the voltage with a two-probe device, one probe on the item and the other to a known good earth.

 

[shreeman]

- The light is 240V. No transformer.
- When the light shade was tested on the ground in isolation away from the ceiling fixing, the pen detector did not indicate it was live (no beeping). Multimeter connected to neutral supply and touching the metal shade of the light read 0v.
- The light fixing I'm talking about is the metal strip onto which the light is hung. This strip is screwed into the ceiling (as per pic).
- By scanning, I meant just moving the pen detector over an area of the ceiling.

When using a multi-meter connected to neutral supply, touching the metal strip fixing in the ceiling, shows the 1.6v I previously mentioned. Connecting the multimeter between ground and the light fixing gives 1.2V.
The screws of the fixing are screwed into plaster/wood. The screws and the metal fixing strip are not touching any wires.

 

[EFLImpudence]

- The light is 240V. No transformer.

Ok.

- When the light shade was tested on the ground in isolation away from the ceiling fixing, the pen detector did not indicate it was live (no beeping). Multimeter connected to neutral supply and touching the metal shade of the light read 0v.

Ok. That's all good then.

- The light fixing I'm talking about is the metal strip onto which the light is hung. This strip is screwed into the ceiling (as per pic).
- By scanning, I meant just moving the pen detector over an area of the ceiling.

When using a multi-meter connected to neutral supply, touching the metal strip fixing in the ceiling, shows the 1.6v I previously mentioned. Connecting the multimeter between ground and the light fixing gives 1.2V.

Do you get a measurement when testing the surrounding plaster to neutral or earth?

I don't really have an answer to the 1.6 and 1.2 Volts.

The screws of the fixing are screwed into plaster/wood. The screws and the metal fixing strip are not touching any wires.

Ok.


While the light fitting itself might be double insulated the ceiling bracket with the cable passing it and the connections underneath it is not.
I would recommend earthing the bracket by connecting the earth wire to one of the fixing screws.
This would then likely remove the suspect voltage.

 

[winston1]

It is just harmless induction from nearby live wires.

It just goes to show why pen type detectors are rubbish. Throw it away.

 

[endecotp]

I tested the wire shade with a non contact detector pen and saw that it indicated the shade was live.

Measuring the voltage of the fixing using a multimeter measured 1.6v.

Believe the multimeter, not the pen.

1.6 V can be attributed to either induction and the high impedence of the multimeter, or a true voltage difference between earth and neutral. Neither is a problem.

 

[shreeman]

Thanks all.
Wiring the bracket to earth got rid of the ghost voltage.

And yes, I would never fully trust the pen so would always back up with probes, and/or multimeter (unfortunately I'm currently without the LoZ setting).