Electrically testing underfloor heating

[Ian_m]

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?)

[fred]

In article , Ian_m writes

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.

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

[meow2222]

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

[Ian Middleton]

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.

[Clive Mitchell]

In message , Ian Middleton writes

If the earth wasn't in use before it might be worth checking it's connected at the other end. In the hands of idiots I've seen the earth core used as feed in a two way lighting arrangement. Never assume anything. That's how you get electrocuted.

[Lurch]

On Tue, 6 Mar 2007 15:57:05 -0000, "Ian_m" mused:

You check before installation so you have a reference measurement.

[Ian_m]

I have now have both a before and after reference measurement, both > 20M.as the test equipment I have has 0-20M and greater than 20M readings @ 500V.