Continuity where you would not expect continuity!

[HappyHippyDad]

I am wiring up a new kitchen and conservatory.

I'm not sure why I checked continuity between line and the metal back box of the light switch, but I did. I haven't even finished the first fix and the metal back box does not have an earth connected to it yet. The voltage tester (on resistance setting) beeped to show continuity. I then tested with the megger (IR test) and got 0.02Mohms. I can't fathom how there is 20000ohms of resistance between line and an unearthed metal back box?

This was at the end of today, I was (am) tired so I don't think my brain is in good detective mode. All I can think of is somewhere in that wall a screw has gone through a cable and is making contact with the line? Any other ideas?

 

[westward10]

You don't have continuity.

 

[timhoward]

I suspect you are IR testing the mass of earth through a damp wall to a metal box, rather like a poor earth electrode.
I'm guessing that even though line is not energised there is a path somewhere through a load to a N bar and back to real earth whether at cut out or further afield.
Is that plausible?

 

[HappyHippyDad]

Apologies @westward10 , I did think that when I wrote it, but thought it was a catchy title!

Thanks @timhoward , that does indeed sound plausible.