Underfloor Heating - blowing RCD

[NigelCollins]

I've had electrical underfloor heating in the bathroom for 4 years and its been working perfectly. We went away for a couple of days and I set the Devireg touch to holiday mode so that it would come on when we got back. When we returned the RCD that supplies the underfloor heating (and the freezer!!) had tripped. Once I'd established that it was the UFH that was the problem, I did some simple testing with a Fluke 117:

Resistance of mat 69.6 Ω
Resistance of one leg of mat to earth 67.1Ω
Resistance of second leg of mat to earth 6.4Ω

Measurement taken from the controller - wires disconnected and using earth fed from mains and connected to earthing terminal on back box. There is no earthing connection on controller.

I assume the measurement to earth should be virtually ∞ (Infinity) - or certainly thousands of Mega-ohms.

Am I missing something or does this mean there is a short to earth somewhere on the mat close to the controller.

If so then any idea how to locate it to a single tile - forever hopeful.......

 

[JohnW2]

I assume the measurement to earth should be virtually ∞ (Infinity) - or certainly thousands of Mega-ohms. Am I missing something or does this mean there is a short to earth somewhere on the mat close to the controller.

I would say that your assumption is correct and that you're not missing anything.

If so then any idea how to locate it to a single tile - forever hopeful.......

Maybe others will have something more 'hopeful' to say, but I doubt that you could repair it even if you located the fault.

Your figures suggest that the short to earth is close to one end (the end which gave the lower resistance measurement) - probably about 10% of the total element length from that end.

Kind Regards, John

 

[NigelCollins]

Thanks John - what surprises me is why the mat would suddenly short to earth rather than maybe exhibit an HR fault?

 

[EFLImpudence]

what surprises me is why the mat would suddenly short to earth rather than maybe exhibit an HR fault?

That's what a 'short' is.

 

[NigelCollins]

Yes I understand that but with a mat lying under tiles with very little stress I am trying to understand why it would suddenly short rather than degrade over time. Oh well I have found a company that will locate fault and repair mat to within a few millimetres- not cheap though at £420 + vat.....

 

[JohnW2]

Thanks John - what surprises me is why the mat would suddenly short to earth rather than maybe exhibit an HR fault?

They can do either (or both). What you are experiencing is presumably the breakdown of insulation (between the 'element' and an outer earthed covering), without the conductor of the 'element' having broken (which, alone, would result in a HR fault - although the broken end may well touch the earthed covering, thereby also resulting in a fault to earth).

Kind Regards, John

 

[NigelCollins]

Yes I understand that but with a mat lying under tiles with very little stress I am trying to understand why it would suddenly short rather than degrade over time. Oh well I have found a company that will locate fault and repair mat to within a few millimetres- not cheap though at £420 + vat.....

 

[flameport]

I am trying to understand why it would suddenly short rather than degrade over time.

Most likely corrosion from water ingress under the tiles.

Most heating cables contain a fine powder such as magnesium oxide as the electrical insulation - a chemical which also absorbs moisture very easily and is then conductive.
Once the outer covering of the heating cable is compromised, the wires and the insulation material are directly exposed to whatever surrounds them - there is no halfway house of things being slightly damaged.

not cheap though at £420 + vat.....

A cheaper option is to accept the reality that the heating is destroyed, and cannot be used any more.

Even if they fix this individual fault without damaging the tile(s), other faults may occur elsewhere. There may already be multiple failures.

 

[NigelCollins]

supplementary question - If i replaced the fused spur feeding the UFH with a 13A 30mA Double Pole passive RCD connection unit, then would that trip before the consumer unit RCD thus protecting all of the other stuff fed by the consumer unit RCD?

 

[ban-all-sheds]

1) Not with any degree of certainty. Either or both would operate.

2) Even if it did it would trip as soon as you applied power to the mat, so that would never work, so you'd be no better off than if you just left it switched off.

 

[jj4091]

Oh well I have found a company that will locate fault and repair mat to within a few millimetres- not cheap though at £420 + vat.....

I would have thought this would be covered by your buildings insurance if you have it.

 

[JohnW2]

I would have thought this would be covered by your buildings insurance if you have it.

I have never come across any buildings insurance, per se, that would cover failure of heating equipment - unless that failure were due to some sort of 'accidental damage' (very unlikely). Even if one had purchased an 'emergency cover' (or whatever) add-on to the policy, I doubt that would cover such things.

Kind Regards, John

 

[SimonH2]

If you know the routing of the cable (unlikely*) then it's possible to calculate where the fault is. You'd then have to uncover the cable - ie lift the appropriate tile(s). Then you can probably repair the wire if the manufacturer provides repair kits - without a proper repair kit, it would be a bit hit and miss whether it could be done safely and reliably.

* When I was helping a mate put his in, we took photos (with a tape measure in the shots for scale) so it would be known in future exactly where the heating wire went. Of course, whether that ever got passed onto a future owner is a different matter.