Difficult to say without more detail. Of the face of it, if it was a 'low energy' lamp (e.g. CFL or LED) and depending on quite where things were switched then capacitive coupling between the shower and lighting circuits might be another possibility.

Voltage drop (or rather the voltage rise on the N) might possibly account for it - there are strange switching systems around the world (e.g. 2-way systems  where each side of the lamp are independently switched between L & N rather simply opening or closing the circuit, and even split-phase systems where voltage drop on one side would pull the N voltage that way and so increase the apparent voltage seen by things on the other phase) - depends where the video has come from - such arrangements are rare this side of the Atlantic.

   - Andy.

I can say for sure it wasn't an led lamp from the look I would say it's an old style incandescent lamp. And was testing a ze of 0.45 ohms. 

From what your saying does that mean that when the voltage decreases on the line said it'll cause an increase on the neutral conductor? 

From what your saying does that mean that when the voltage decreases on the line said it'll cause an increase on the neutral conductor? 

Indeed. Say we had 230V at the supply and due to voltage drop we had 220V at the load - presuming equal impedance on the L & N conductors -  you'd loose 5V along L and 5V along N - so relative to the supply N (or Earth) - you'd see 225V at the load L and 5V at the load N. The load sees the difference between 5V and 225V.

(At least that's true for simple single phase systems - where 3-phase distribution is involved return currents can cancel out to a lesser or greater extent, so the voltage drop along N can be lower than L).

   - Andy.

Okay i believe I understand so the load neutral would then receive an extra 5 volts am I understand that right? 

What do you mean by, "glow" please? Is that the same as, "shine" (brightly)?

I have measured 50 v from L-N in a lighting circuit which was turned off, but only 1 mA was flowing. That may be enough to cause a CFL to flash. The one above my bed used to flash at about 1 Hz, but it was visible only once my eyes were properly dark-adapted. (There is now an LED lamp there and it does not flash.)

Petty much it has a constant dimmed light to it almost as if it's just been turned on and is beginning to warm up 

Okay i believe I understand so the load neutral would then receive an extra 5 volts am I understand that right? 

In a way, yes. A diagram might help:

If you like the relative voltages, in red, show the voltage differences across each part (cable and load), whereas the blue figures show the voltages relative to Earth.

But none of that directly explains why the lamp glows. If it's an incandescent, a substantial current must be available (much more than capacitive coupling would normally provide) and if the light were conventionally switched and the switch in the off position, there shouldn't be a path for such a current. There must be something odd with the setup or a  fault somewhere.

   - Andy.

Ah okay so across the line conductor where lossing 5v and then again across the load we lose a further 5 volts which goes down to earth and the neutral conductor. 

Currently I am only a 3rd year electrical apprentice I was wondering if there was any materials I could read into to understand this all better 

Thank you. 

AJJewsbury said:

There must be something odd with the setup or a  fault somewhere.

Given that it was on YouTube, it might have been faked.

Any chance of a link, Sam?

https://youtube.com/shorts/y08JoizQOgw?feature=share4

Heres a link to the video it's a YouTube short without much information but if you look at the first comment on the video it provides a little more detail personally I think it's wrong and there's something else causing the issue.