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Old 01-17-2013, 12:44 PM   #1
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Towel rail


Hi Guys,

Not an electrician as such but an ex TV engineer, so not totally stupid (not yet anyway) and I have just wired in a new chromed towel rail in our bathroom and as always these things make me nervous in such a location.... so some advice please?

It's all wired in and seems to be working and warms up OK but on checking for voltage with my digital meter from the surface of the heater across to one of the taps I'm reading 105v AC, why is this?

I can touch the rail without getting a shock but, call me cautious but if feels somewhat strange and best described as sort of a vibration. Knock of the mains and if feels fine so what the heck can it be.... or am I just being over careful with electrical items in a bathroom?

Thanks guys.

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Old 01-17-2013, 12:53 PM   #2
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Towel rail


It could be voltage induced in the towel bar surface (electricians say phantom voltage). But a finite number of milliamperes will flow if you "short" the metal towel bar to the nearby metal faucet. It is not unusual to feel a power line frequency vibration when rubbing your hand on the bar while your bare feet are flat on the floor.

High impedance voltmeters (most digital models) are very apt to show a noticeable voltage reading from an object to ground from induced voltage.

Now if the number of ma that would flow if you shorted the bar to the tap is high, say, more then ten, we have excessive leakage, enough to count as a genuine fault from a live part inside to the metal bar. That is a defect.

Connect a small wattage, say, 7 watt, incandescent lamp between the bar and the tap. If it glows, not necessarily brightly, or trips a ground fault current interrupter then the current leakage is great enough to be a fault.

If the lamp does not glow then you can use the in-line amperes function of a multimeter to measure the number of milliamperes. The subject subcircuit is between the hot feed to the towel bar and the faucet and the load is the dielectric (insulating material between the energized part(s) inside and the exposed bar.

When you do just the meter measurement, you create a subcircuit with two loads in series: the dielectric, and the meter itself. The larger the impedance of the meter compared with the impedance of the dielectric (at power line frequency), the larger the voltage reading on the meter, up to the supply voltage.

(If you did not do the lamp test and there was a real fault, the resulting current could blow out the meter during an in line amperes test.)

More than about 5 ma should trip a GFCI (RCD).

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Last edited by AllanJ; 01-17-2013 at 01:10 PM.
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Old 01-17-2013, 08:21 PM   #3
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Towel rail


As a tv engineer you should be familure with terms like
" Capacitive coupling" & " Inductive coupling "
These would explain the voltages.
Usually it is low current,
But still disconcerting for some people.
Use of an isolating transformer would stop it !
How ever I am not sure what code would allow
in your area.
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Old 01-18-2013, 12:00 AM   #4
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Towel rail


Quote:
Originally Posted by Bannon View Post
Hi Guys,

Not an electrician as such but an ex TV engineer, so not totally stupid (not yet anyway) and I have just wired in a new chromed towel rail in our bathroom and as always these things make me nervous in such a location.... so some advice please?

It's all wired in and seems to be working and warms up OK but on checking for voltage with my digital meter from the surface of the heater across to one of the taps I'm reading 105v AC, why is this?

I can touch the rail without getting a shock but, call me cautious but if feels somewhat strange and best described as sort of a vibration. Knock of the mains and if feels fine so what the heck can it be.... or am I just being over careful with electrical items in a bathroom?

Thanks guys.
If you are in UK the towel warmer bar it have to be on RCD protected circuit.

I know in France it depending on what " zone " it is located in the bathroom but most case the towel bar which I installed useally end up hooked up on the RCD.

Merci,
Marc

Readers : note the RCD is the same as GFCI
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Old 01-18-2013, 03:45 AM   #5
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Towel rail


Thanks for your replies.

I have rechecked the wiring and all seems in order.

I'll try the lamp test later and see what result I get. Checked it again this morning and depending on what power setting it is on I am getting a definite 'tingle' when touching the rad which is rather disconcerting. I also had this with a microwave oven we used to have.

Would fitting a an RCD cure the problem, or just remove the worry?

Regarding France, I had the same thing happen with an electric radiator (I lived there for a few years) and was told my a local electrician it was because the mains were reversed!

Last edited by Bannon; 01-18-2013 at 05:06 AM.
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Old 01-18-2013, 08:05 AM   #6
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Towel rail


Installing the RCD (GFCI) removes the worry. This protects against electrocution since if the leakage is too great, the difference between outgoing and return current via the mains trips the RCD. Smaller induced currents will still flow if a circuit to ground, such as from your hand touching the bar to your bare foot on a not perfectly dry floor, is completed.

Yes, if the power plug (cord cap) is reversed in the receptacle (wall plug) you could sometimes feel more of a vibration under some circumstances. When the largest live parts inside or those live parts closest to the external parts are conneccted to the neutral side of the line (mains) then their voltage relative to ground is held close to zero and any induced current when you touch the bar will be less. The switch on the appliance properly cuts the "hot" side and when in the off position prevents voltage from being applied to the components inside and then no voltage is induced in nearby exposed parts. Most equipment in the U.S. has "polarized" plugs; one prong (the neutral) is larger than the other and the receptacle slots are sized to match. This prevents reversing the plug.

Is the circuit you hooked up the towel bar to grounded (has a ground wire aka equipment grounding conductor accompanying the circuit conductors to the breaker panel)? The ground wire should be connected to the frame (the bar itself).
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Last edited by AllanJ; 01-18-2013 at 08:16 AM.
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Old 01-18-2013, 09:12 AM   #7
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I have now installed an RCD and the problems of the buzz, the slight tingle.... and my worrying, has all gone!
The circuit is grounded by the way Allan, and I have backtracked to the main circuit board and all trips or functioning as they should as is the RCD of course.

Bathrooms and electrics eh? Always a worry (to me anyway) so 'belt n braces' are best to be safe :-)

Thanks for your advice guys, much appreciated.

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