underfloor heating shocks

[ban-all-sheds]

What about getting hold of a suitable isolating transformer to power the heating from, and see what happens?

 

[fisherprice]

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.

Thank you for the feedback gents.

So that I am absolutely clear in my mind, I should take the earth reference point as either the MET, some bonded pipework nearby, or a socket screw etc?

And in terms of an adjustable resistive load across the two points, what is the most practical method of achieving this? Please excuse my lack of experience in this regard.

Presumably given the insulating properties of slate, one can assume that any earth path is via the grouting, and as such, the alu pad should not be allowed to touch the grout during the test?

 

[fisherprice]

What about getting hold of a suitable isolating transformer to power the heating from, and see what happens?

Thanks for this suggestion. I'd be grateful of you could confirm the theory to my uneducated mind. What might you expect to happen in terms of the physics?

 

[Taylortwocities]

The induced voltage from the floor (if that is what the problem is) is powered from a source that has its reference at earth (the supply substation).
If you put in an isolating transformer then you separate the earth reference to the UFH and the supply is then floating (the same as a shaver socket in a bathroom).

The theory is then that there could be no potential between the floor (floating supply) and anything else that has earth at its supply.

 

[fisherprice]

Thanks for clarifying that TTC. It's good to have another avenue to explore before resorting to lifting the tiles.

Would like to take this opportunity to thank everyone again for their input, it is greatly appreciated.

 

[fisherprice]

I am still very keen to try this experiment for measuring the induced voltage. Any recommendations on the best/most cost effective means of placing a reliable, easily adjustable resistance across the meter?

Any comments on my thoughts about the grout being the path to earth?

 

[fisherprice]

The induced voltage from the floor (if that is what the problem is) is powered from a source that has its reference at earth (the supply substation).
If you put in an isolating transformer then you separate the earth reference to the UFH and the supply is then floating (the same as a shaver socket in a bathroom).

The theory is then that there could be no potential between the floor (floating supply) and anything else that has earth at its supply.

If this were to solve the problem, where would that leave us with regard to the RCD protection stipulated by the regs? Presumably placing the RCD on the load side of the isolating transformer would be in compliance?

 

[electronicsuk]

If this were to solve the problem, where would that leave us with regard to the RCD protection stipulated by the regs? Presumably placing the RCD on the load side of the isolating transformer would be in compliance?

What use would it be? The supply to the UFH would be electrically separate from mother earth. That's not to say you can't have an earth connection on the secondary side of an isolation transformer in order to use an RCD, but you still need to connect the earth to something for it to be of any use. As you don't have an earthed grid or UFH cable with integrated screen, that doesn't leave you with a lot of options.

 

[fisherprice]

If this were to solve the problem, where would that leave us with regard to the RCD protection stipulated by the regs? Presumably placing the RCD on the load side of the isolating transformer would be in compliance?

What use would it be? The supply to the UFH would be electrically separate from mother earth. That's not to say you can't have an earth connection on the secondary side of an isolation transformer in order to use an RCD, but you still need to connect the earth to something for it to be of any use. As you don't have an earthed grid or UFH cable with integrated screen, that doesn't leave you with a lot of options.

I must have been half asleep. I was envisaging the rcd picking up an imbalance between line and neutral in the event of, say, insulation failure on one or other. But of course the whole point of being electrically separate is that a leak under fault conditions would not be trying to find it's way back to earth. However, if there was an insulation failure which inadvertantly connected one pole to earth, which could potentially happen in this case, then that failure references the secondary of the system to earth and protection is lost (although granted, someone being able to touch the live pole given that it's mechanically protected by the slate tiles is highly unlikely). Seemingly regular insulation testing would be necessary, but how can this be achieved given the installation conditions?

Would I be right in assuming that in the absence of RCD protection, electrical separation provides sufficient fault protection in order to comply with the regs? This is my understanding but to have it confirmed or otherwise would be very much appreciated.

 

[TicklyT]

I am still very keen to try this experiment for measuring the induced voltage. Any recommendations on the best/most cost effective means of placing a reliable, easily adjustable resistance across the meter?

Any comments on my thoughts about the grout being the path to earth?

I would dig around in the back of my garage for an old decade box I've got squirreled away, or rig up a potentiometer, possibly salvaged from an old radio / TV, where they are used for volume controls etc.

Once you've established the level as Bernard suggested, use your multimeter to establsh the resistance of the pot.

Here's one source....
http://www.maplin.co.uk/Module.aspx?ModuleNo=2205

 

[fisherprice]

Would I be right in assuming that in the absence of RCD protection, electrical separation provides sufficient fault protection in order to comply with the regs? This is my understanding but to have it confirmed or otherwise would be very much appreciated.

According to 17th ed. electrical separation is specifically not permitted for ufh (753.413), and by my understanding of 753.411, an earthed covering is now required irrespecitve of location if the heating unit has no exposed conductive parts.

So it seems there really are no alternatives other than to lift the tiles if the problem is to be rectified.

Thanks for the tips RE potentiometer, Tickly, although in light of the above it seems the test would sadly be redundant on the basis that ultimately my friend wants rid of the shocks, irrespective of my interest in the induced voltage.

 

[ban-all-sheds]

Like I said - thicker socks.