On 6 Mar, 19:23, wrote:
On 6 Mar, 15:57, "Ian_m" wrote:

I am in the process of installing electric underfloor heating mat. The
installation checklist has fill in boxes box for heating element resistance,
probe resistance and insulation resistance both before and after
installation.

However the heating mat is double insulated (has only live and neutral
wires, though is screened) so how am I suppose to test the insulation
resistance. I can undertstand after fitting, testing between live/neutral
and earth as a check against accidental insulation damage, but before
installation ?

The form looks like a generic form and I suspect is wrong, the inulsation
resistance being for the mains wiring as opposed to the heating mat itself.

Any thoughts on what values I am looking for after installtion ? (n G Ohm?)

I hope youre using paralleled elements, as individual elements are a
bit prone to failure. With parallel elements its a minor thing if one
dies.

I'm not sure theres any practical point testing L-E before pour, since
the insulation between L&E will be different once laid. As Fred said,
perhaps a deny-your-claims exercise. A visual or tactile check for
damage would be more useful.

Your min R will be in megs, not gigohms.

NT

Ah ha. The earth wire has been found, it just started about 1-2"
further down the wire than the live and neutral. Interestingly the
wiring diagrams show a single black line connected to the thermostat,
not showing it is really 3 wires. Anyway L/N - earth measured at
greater than 50M both before and after instalation so all OK.