In article , Ian_m
writes
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 ?

In the uninstalled state it does appear to be a box ticking exercise, in your
place I would test between live, neutral and screen. The test should really
be done at working voltage or above to so that means a megger rather than
a multimeter, many things are insulators at low voltage but less so at
mains levels.

I think they are trying to avoid claims for failure after fitting, perhaps under
a screed, by requiring you to test before pouring.

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?)

Not my area of expertise but with plastic insulation I'd be concerned about
anything much below 20M.
--
fred
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