Little mystery that's bugging me

[timhoward]

A BG fused connection unit, with neon.
A boiler.
Fuse out. Spur switched off.

I was checking polarity after a colleague had 2nd fixed it and without thinking chose the load N not the supply N.
This led to discovering:
120v between Supply Live and Load N
120v between Supply Live and Load Live

I tried his 2 pole tester, same except that one said 50v.

Look at boiler end, all fine.
The strangeness stops when I disconnect earth at the boiler.

IR tests L-E and N-E at boiler (at 50v) were >99M

Should I conclude that there's are very very high resistance L to E and N to E paths inside the boiler and yet the impedance is low enough to light up a tester, or am I missing something embarrassingly basic?!
Thanks

 

[Mike Johnson]

Just a thought is the boiler Earthed?

 

[timhoward]

Just a thought is the boiler Earthed?

Yes. Removing the earth stops the strange readings.

 

[timhoward]

 

[James]

Often the flame detector is a single wire sensor using boiler earth as one pole.
I suspect you are reading through it.

 

[Avo Mk8]

Another possible explanation is that if the boiler has any sort of traditional interference filter in the mains input, this will lead to the 120V effect, as there is a capacitative divider across the mains, with the centre point earthed. This has cropped up on previous posts here with other appliances. It depends a bit on the impedance of the measuring device as to what voltage you see, but with high impedance you will get half mains voltage across each CY capacitor.

 

[timhoward]

Thanks, both of you!
The filter concept was vaguely in my mind but I’ve never been told the reason or seen a diagram before.

 

[westward10]

What @Avo Mk8 suggests crossed my mind. Try an IR test at 500v.