Little mystery that's bugging me

timhoward · Dec 19, 2023

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 · Dec 19, 2023

Just a thought is the boiler Earthed?

timhoward · Dec 19, 2023

Mike Johnson said:
Just a thought is the boiler Earthed?

Yes. Removing the earth stops the strange readings.

timhoward · Dec 19, 2023

[ElectriciansForums.net] Little mystery that's bugging me

James · Dec 20, 2023

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

Avo Mk8 · Dec 20, 2023

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.