Discuss Electric UFH tripping RCD in the Electric Underfloor Heating Wiring area at ElectriciansForums.net

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cpharvey

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Hi all, i'm a home owner just looking for some advice.

Over Christmas the RCD on a split panel fuse box kept tripping. We narrowed this down to the master bathroom underfloor heating. The buildout for the loft including the master bathroom is about two years old, the UFH is under tiles. I think it's a ProWarm matt.

We had an electrician come out and while looking at it he also identified our fuse box was not in great shape. We've had all kinds of electrical issues since we bought the house a few years back. So I decided to replace the entire fuse board which he did last week. We had a very vague hope the new board may resolve the RCD issues of the heated floor, but alas my luck isn't that good. I'm still happy with the new fuse panel and glad we made that change, but now I have to figure out what to do with the floor.

The electrician bought a thermal camera and we looked at it. There's two hotspots and one of them he believes two wires are too close and he's thinking they have bonded and shorted. Of course this means lifting tiles, repairing wiring and then tiles. Undesirable for sure, but not the end of the world for one of the hotspots, but MUCH harder for the second one.

In reading other posts, I realized testing the resistance was probably a good idea. Bear in mind i'm not an electrician, just an interested homeowner who has tools. However, reading the resistances didn't make a lot of sense to me when comparing to other posts i've seen where the resistances were very different. Hence my post here to see if I can understand. So go gently please.

With the circuit power switched off, and with the floor matt live, neutral and earth disconnected from the thermostat, these readings come from the wires that would connect to the thermostat.

OHM Meter is set to 200.
Live <--> Neutral = 16.6 ohms
Live <--> Earth = 185
Neutral <--> Earth = 185

Oddly, when I measured L<->N earlier today before finding this forum, I could have sworn it was more like 35 ohms. Forgive me, but when measuring resistance does it matter which order you connect the red/black measuring leads to the wires? I think not with resistance.

So with the above measurements, is it possible to say if there's a short in the matt?

The room is about 12SqMeters but I don't know if the ProWarm matt is 100,150 or 200W because I cannot find the paperwork.

Important to note that with earth disconnected the matt warms up (that's how we used the IR gun), but as soon as the Earth is connected the RCD trips when the unit activates. This certainly suggests a short to me (sadly).
 
You would normally measure L+N (connected together) to E, done using a meter that measures M-Ohm, and at 250V DC.

The readings of 185 look a bit odd, if these are 185 Ohm.

But it sounds like you have a short to E somewhere, and no amount of adjusting hot spots is going to fix that. You can't use it (other than dangerously) with the E disconnected.
 
Hi - a couple of hundred Ohms to E indicates a fault and likely means the UFH cable has a short to its protective sheath. This most likely has been caused by rough handling during installation or renovation works. This insulation resistance test is a standard test for an Electrician to perform and would confirm the existence of a fault in the mat.

Did the mat ever work without tripping the RCD?
 
if it were 200w / sqM
12 x 200 =2400w

2400w is aproximatley 10A
to draw 10A the resistance between L and N should be in the region of 24 ohms.

150w / sqM
12 x 150 = 1800w

or 7.5A therefore restance should be about 32 Ohms

It may be a coincidence that you think earlier you measured 35 Ohms but it is more likely that when you measured it, the fault was not there. It may well short when a slight change in temperature happens.

In any case, you should not be measuring any resistance between live and earth or between N and earth with the test equipment that you have.

I have therefore NO GOOD NEWS for you.
it would appear that there is a short in the mat that will have to be repaired or replaced.
 
The room might be 12sq m but doubt the heat mat is, as space for bath/shower/WC/Vanity unit should be allowed for (matting free), probably more like 9sqm!
Not sure how relevant the size/rating is to the issue you've described. You'll know the current draw (clamp meter) but little else without original floor plan.

Repair kits are available for UFH and the hotspot is a good place to start as resistance readings without a floor plan do not help.
Unfortunately the tiles will be damaged, so hope you have some spares, in accessing the heat mat but be careful not to damage the matting further in removal of the tiles otherwise you may as well pull it all up and start again.
 
Thanks for the quick replies all, even if not good news. I'll try to answer the questions.

I tried connecting L+N and then E, but I got odd results. It seemed to start at about 135 and climbed constantly until the meter read 1. Perhaps I was part of the circuit or I was simply doing this incorrectly with my multi-meter.

Yes the matt has worked solidly for about 2 years. We have always had problems with the split panel fuse box tripping the loan (single) RCD and specific MCB's, hence the reason to change out to a newer box. I think years of home ownership changes (house is about 20 years old) have caused some circuit issues in the house which we are trying to fix, and hopefully have now. There's some really odd circuitry, like the sockets in the old master are part of the kitchen wiring. Really odd but probably done when the kitchen was refitted a few years back. Anyway I digress.

Even with the new fuse box (Consumer Unit?), it trips the RCD (this panel is surge protected with each trip switch being an individual RCD).

The thinking of my electrician is that the cables were laid too close in one specific spot (and perhaps a second more painful spot), and the heat has caused them to bond and corrupt the sheath and therefore earth is touching live. So the repair we're considering is lifting a couple of tiles, fixing and separating the offending wires, and keeping our fingers crossed it's not the other hotspot which has a tile going under the glass shower enclosure. (BTW, I won't be doing this repair, I'll leave that to the professionals.)

Below is how I had my MM set for the resistance testing btw.


Electric UFH tripping RCD 20210109_163933 - EletriciansForums.net
 
A reading 'rising' and stopping at 1 usually indicates 'over range' and is likely to be higher than 200ohms but will cause the RCD to trip.
Further testing is not going to correct or locate it.
Ideally you need an insulation resistance meter to give more accurate results.
The RCD is doing what it is supposed to do in the event of an earth fault and as previously said operating it without earth connected is dangerous and foolhardy, except perhaps under the guidance of an electrician for testing as previously described.
I think you know what to do and hope the fault is located at hotspot No1.
 
Option #1: run it with the earth disconnected... risk of electric shock
Option#2: dig up tiles and hope it can be repaired without ripping up the whole floor to replace.
Option#3: leave it disconnected and buy a pair of woolly socks.
 
Thanks all. Since you all concur with our thinking that it's an earthing problem, we'll of course pursue having it fixed by professionals. Wooly socks are not the direction I want to go in. :)
 
No its not an earthing problem. The integrity of the earthing conductor (cpc) of the matting is obviously intact and doing its job!
The problem is a break in insulation between a live conductor and earth causing a current leakage to earth resulting in the RCD tripping.
 
maybe OP could have rephrased it better as "a fault to earth" as opposed to "an earthing problem".
 

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