underfloor heating shocks

[fisherprice]

sorry to highjack this thread but looking for assistance and this is the closest related issue I can find.

My friend has ribbon style UFH installed under slate tiles. RCD protected, and RCD is working fine. UFH cables all giving resistance readings within manufacturers range. Earth mesh was not installed as apparently not necessary when not in bathroom (cue 17th edition arguments, but the installation was 3 years ago). There are 5 separate UFH mats covering the floor area in question, and all run off same point.

When the UFH is turned on, and he is wearing socks, but no shoes, and touches any item of plugged in electrical equipment (TV, stereo amp, laptop etc) in the room he gets a small shock (comparable to a static discharge when getting out your car for eg). He has also experienced the same thing when touching another person (his missus says the sex is electric ).

When the UFH is off he cannot replicate the shocks, no matter how much he shuffles around in socks.

Apparently the problem has persisted since installation but it seems that now he has finally had enough- hence he's asked me if I can help.

I guess what's written in this thread probably explains it but I'm confused as to why he's getting the shocks when touching class II equipment.

Would be very grateful for an education in this regard.

Thanks in advance.

 

[TicklyT]

Just thinking..... A neutral fault would make the whole system behave like a capacitor, with the heating ribbon as one 'plate', nicely charged to 240 Volts, the flooring as the dielectric, and a pair of feet as the other 'plate'..... the shock you'd get from the floor would then be proportional to your shoe size.

Edit 25/4/09 - My reply makes less sense here since the thread has been split.
The original thread I posted to referred to an identified neutral fault, which is not the case in this thread.

However, it still may be worth investigating capacitive coupling.

 

[fisherprice]

Short of lifting the tiles (obviously he's not keen to take this option until all others have been exhausted), how would one establish and locate a potential neutral fault?

BTW I contacted the manufacturer's support line operator who, after jumping to the conclusion it must be an earth fault, then denying any expertise in the area whatsoever, claimed they had never experienced this issue in any of their products and effectively washed their hands of it claiming that it is an issue relating to the installation (I don't necessarily argue the latter point, but what an attitude to take!).

Through further online searches and looking at different manufacturers pages I have become aware of the existence of equipment which facilitates the electrical equivalent of a plumber's pressure test on the cables/ufh system in order to establish if/where current is leaking, but that the average spark is not going to have ready access to such a thing. Anyone with any experience in this regard? If so what are the cost implications of getting someone in?

Still keen to learn the theory with regards to the shocks he's getting aswell.

 

[Taylortwocities]

I hate to tell you this, but....

I was called in to investigate exactly this problem with a Carbon Film UFH in a kitchen (wooden floor).
Shocks were encountered if on person was on the wooden floor and touched another person standing on an adjacent (slate) floor. Also if you touched the taps etc.

The supplier (Flexel/Ecofilm) at first denied this ever happening before. After much prodding they said that this was known by the maker (Scandinavia, edit: Scotland) and they could supply a conductive mesh that needed to be earthed.

Floor lifted, earth film installed, floor relaid, problem solved.

PS they are still arguing about who is gonna pay for it!

 

[electronicsuk]

I'm not so sure about a neutral fault. I assume what you're proposing is a break somewhere in the neutral, leaving the entire UFH system at 230v. However, if that were the case, you wouldn't expect the resistance readings to match the manufacturers specifications. If it were something happening only when the system gets hot, the neutral break would cause the floor to stop heating, and this cycling would presumably lead to a noticeably lower than usual heating output.

Have to agree with TTC that an earthed grid of some sort should have been installed, or used a heating tape with an integrated earth braid/foil!

 

[fisherprice]

Thanks gents.

So if my understanding of TTC's account is correct, this isn't really a fault as such; rather that these type of systems require the earth covering in order to prevent what is an unwanted but otherwise consequential side-effect of this type of system. If this is indeed the case, then to my simple mind it seems the burden should rest with the manufacturer who knew of, but didn't raise the issue at the time of purchase. If the manufacturer had maintained their initial denial (as they have done with my enquiry) then it becomes less clear who is responsible.

 

[fisherprice]

Matthew, according to my friend, the heating works fine, no cut outs or reduced power. Having said that, the problem has persisted since installation so I guess he has no point of reference to compare.

Am I correct in thinking it must surely be EM induction, on the basis that slate was at one time used to insulate old fuse boxes in wales? Presumably the earthed covering 'soaks this up'?

 

[ban-all-sheds]

When the UFH is turned on, and he is wearing socks, but no shoes, and touches any item of plugged in electrical equipment (TV, stereo amp, laptop etc) in the room he gets a small shock (comparable to a static discharge when getting out your car for eg).

Tell him to wear thicker socks - that will definitely fix the problem.

 

[fisherprice]

just to clarify, I didn't install the ufh. I didn't even know the bloke 3 years ago. Come to think of it I don't actually like him that much anyway

 

[Taylortwocities]

Same with my little problem. I didn't install it.

It was suggested that it might be an earth or neutral fault on this one too, but it is surely EM induction.
This is how I explained it to the customer:

Untitled

• Taylortwocities
• 24 Apr 2009
• 1

 

[fisherprice]

Thanks for that TTC, but to complicate things- when he got a shock from touching another person, that person was standing on the same tiled surface (with the ufh below), albeit possibly with shoes on.

Furthermore, he gets shocks from touching class II equipment such as TV which is also on the same surface (but on a stand- don't know the material of the stand), and should not provide a path to earth, no? Or have I got confused somewhere?

 

[electronicsuk]

It's still entirely plausible for there to be a difference in potential between two different areas on the floor, if not extremely likely when you consider that the voltage on the heating element will become lower along its length until you reach the neutral cold tail.

 

[fisherprice]

good point, well made.

what about the apparent earth path via the class II?

Or am I being dumb and is it that a greater pd is being induced in the tv/audio equipment, and by touching it he is becoming the path to earth?

Come to think of it, he mentioned the stereo amp to be 'buzzing' albeit barely perceptibly.

I guess ultimately the practical solution is unavoidable, I'd just like to have a clear picture of the physics in order to help the bloke rationalise the impending doom and gloom. If there are any experts willing to explain I'd be very grateful.

 

[TicklyT]

Do you think laying a bit of aluminium foil, say 300mm square, on the floor, and measuring any induced voltage between the foil and earth with the UFH turned off, then again with the heating turned on would be a valid test for capacitive coupling?

 

[bernardgreen]

That is the way to measure it.

Use a high impedance meter measuring volts located next to the earth reference point and a long lead to the test pad will show the induced voltage. The voltage should reduce if a resistive load connected between the test pad and earth.

If you want a thorough test result then move the pad until you find the locations with highest voltages per mat and use those as the test sites.

Note the voltage without a load across the meter. Then connect a load across the metter and adjust the resistive value of the load across the meter until the voltage drops to be half the un-loaded voltage. Then the resistive load is approximately equal to the impedance of the capacitive coupling between heating mat and the area of the test pad. From that you can then work out if the capacitive coupling is sufficient to permit a dangerous amount of energy to be capacitively coupled into a body that is in contact with an earth or other route for that energy to pass through the body to cause injury.