Unused wires showings live - why?

[Peter Mc]

I am replacing my bathroom extractor (+timer) fan. The wiring comprises of one single switched live wire (connected back to the lights switch) plus another cable (live and neutral) connected back to the mains light ring.

1. The switched live wire/cable comprises of the red wire only connected back to the lights switch. The neutral wire has been cut short and the earth wire simply terminated at both ends (no earthing required or possible on the fan) - PROBLEM: when the switch is ON and I test with a pen tester both the neutral and earth wires show a positive (pen lights). They are not touching, indeed no where near, any terminals or casings.

2. In a similar way the permanent live/neutral wires are properly connected, the unused earth wire simply wrapped around the wire casing for tidiness (no earthing point on fan) again well away from any terminals. Yet this earth wire also shows as LIVE, and is always on.
The fan and lights all work by the way.

I considered that the cables themselves may be faulty, so swapped them for a short length of new cable to use as a test. Same problem occurs.

Is this something serious or just some stupid way in which a pen tester can show a false positive (possibly through some sort of conductance across the wires?). I am a DIY-er, no electrical qualifications.

Help please as I'm worried!

 

[securespark]

Yes, your volt stick is deceiving you.

Double check it by putting a multimeter across the "live" wire and the neutral, you will most likely not find mains voltage.

 

[winston1]

Lights are not in a ring.

1. The earth wire should be connected to earth at the source end.

Pen testers are unreliable. It does not show positive as it it AC. You think it shows live, it is actually showing capacitive pick up from the adjacent live wires.

2. As above the earth wire should be connected to earth at the source end.

 

[Taylortwocities]

Rub your pen tester on your nightie

Does it light up? Yes!

Are you dangerously live? No!

Suggest you invest in a proper 2-probe main voltage tester, before embarking on DIY electrical work.

 

[Peter Mc]

Thanks for the comments. Given the setup I agree it can only be some sort of 'pick up' from the other wires and so giving a false reading. Made sure everything was safely tidied away though and will look at getting a multimeter (and learning what it is and how to use it...)

 

[cjard]

Yep, a wire connected to nothing that runs in parallel with a conductor carrying live AC is effectively a capacitor, and capacitors conduct AC. The rising and falling magnetic field created by the alternating current in the live conductor induces a current in the adjacent conductor. It will go away if that conductor is connected to something, like ground

 

[JBR]

Yep, a wire connected to nothing that runs in parallel with a conductor carrying live AC is effectively a capacitor, and capacitors conduct AC. The rising and falling magnetic field created by the alternating current in the live conductor induces a current in the adjacent conductor. It will go away if that conductor is connected to something, like ground

Yes, just like our steel standard lamp (double insulated). It's interesting, because I can feel the potential (my wife can't) - almost like a slight 'roughness' when I slide my finger along it.
I understand it's quite safe - at least I hope so!

 

[endecotp]

No, not like your steel standard lamp I hope! ... let's do the maths:

Imagine your standard lamp is a metal tube, 2m tall and 1cm in radius with a live conductor of radius 0.5mm running up the middle. The capacitance between the live conductor and the metal would be

C = l 2 pi epsilon / ln (r2/r1)
= 2 x 2 x 3.14 x 8.8e-12 / ln(1e-2/0.5e-3)
= 3.7e-11 F

The impedance of that at 50Hz is 1 / ( 2 pi f C ) = 86 Mohm

If the resistance to ground of your body were zero, the current that would flow would be I = V/R = 2.7 uA.

According to the introduction to:
https://mn.uio.no/fysikk/english/research/projects/bioimpedance/publications/papers/dc.pdf

The threshold for detection of 60Hz AC on a fingertip has been measured at around 1 mA, and on the tongue at around 45 uA. So the current you should be getting from your lamp is one or two orders of magnitude below that.

Feel free to re-do that with real dimensions.

if I were you, I'd be doing some insulation resistance measurements on that lamp.

Or wondering what else about it you're sensing, like actual roughness.

 

[cjard]

I know what you're referring to, as I've had a few electrical products over the time that have felt fuzzy.. Most recent one I recall was some cheap and nasty charger for the iPad that when plugged I made e iPad feel furry on metal parts. It also sent the capacitive screen crackers and made it unusable, so I binned it. I too have had difficulty relating my sensations to the female unit, she didn't get it, and I gave up.. But I thought that was par for the course in a relationship

 

[bernardgreen]

Yep, a wire connected to nothing that runs in parallel with a conductor carrying live AC is effectively a capacitor, and capacitors conduct AC. The rising and falling magnetic field created by the alternating current in the live conductor induces a current in the adjacent conductor. It will go away if that conductor is connected to something, like ground

The capacitive coupling does not involve magnetic effects, it is electrostatic, electrons n one wire repelling electrons in the other wire.

Voltages are induced even if theres is no current in flowing in the wire, no current, no magnetic field