Electric Underfloor Heating - Poss Leak to Earth

[cbee55]

Hello all

I am in the process of installing electric underfloor heating (the film type for laminates) into a newly built conservatory, and all appeared to be going fine. I layed the mats, made the connections, and it appears that the mats are warming up.

However, when I was putting some bulbs into the wall lights, I was getting a tingling felling from the metal work on the light fitting. I put my meter on the metal work, and it was registering anything from around 1vac to 36vac. This only happens when the heating mats are on, when the thermostat turns off it disappears.

I have done a resistance check on the mats, and they are roughly what they should be, and today I am going to try connecting the mats up separately (4 mats in total). I did a continuity check from Neutral to Earth and get a tone, but I understand this could be normal. I assume there is some kind of leak, as if it was a short the RCD would of tripped out.

The lighting in the conservatory is using a fused spur on the ring in the conservatory, and not the normal lighting circuit for the house.

Anyone have any ideas, or any other tests I could try.

 

[bernardgreen]

First check that the metal parts of the lights are connected to earth. Use you meter on the ohms range and measure between the lights metal work and a known good earth. The earth pin of a socket should be a good earth. If it is less than a couple of ohms then the lights are earthed ( yes it should be less but for the purpose of this investigation a 10 ohms or less is OK ). But if you can measure between lights and the earth at the consumer unit ( use a long lead ) that will be more definitive as it is not impossible that the socket may not be earthed

If they are not earthed fix that first.

If the lights are earthed and you are getting a tingle from them them it is you that is "live". ( Similar to static electricity that gives a short sharp shock and/or spark but continuous ).

You are "live" because the live wires in the heating mats are capacitively coupling voltage into any one walking on them.

Putting an earthed metal grid over the heating mats will ( should ) solve this problem but check with the manufacturer of the mats about the type of grid that can be used with their mats.

 

[cbee55]

Hi Bernard

Thanks for your quick reply. I think you may have solved the problem. At the moment I only have the mats layed, and have been walking on them in my stocking feet. I have powered the system up this morning and touched the metal work and still getting the tingling feeling (it's a constant tingle rather than a quick static shock). However, I have put a piece of underlay on top of the matting, and don't appear to get the tingling feeling any more. So, as you mentioned it's probably me who is live.

I have put my meter on the metal work and still getting fluctuating readings as before, but would this be normal ?

 

[rebuke]

You say you've put your meter on the metal work - what have you put the other end to?

 

[cbee55]

I've just put the probes across the metal of the light and the plug sockets (brushed chrome type). It appears that I am no longer getting the readings I was previously, just small residual amounts.

I have since done some more testing and it would seem that Bernard was spot on. Stand directly onto the mats, get a tingle, put some insulation between me and the mat, no tingle. I have a plastic vapour barrier and laminate floor to go on top of the heating mats, so they should obviously solve the issue.

 

[rebuke]

In theory you should get 0V across those, as they should both be connected to earth. Does the metal light switch definitely have an earth (CPC) wire connected?

 

[cbee55]

I've checked the earth between the switch, lights and the sockets and all is fine. I have just realised that the part of the light I am trying to take a reading off, has some type of coating on it. Have took readings from bare metal, and the screws on all the sockets, and I am getting back a reading of between 0.00 - 0.01vac on my meter.

So looking at it, I'm sorted. Thanks to you both for your quick and helpful replies.