Huge voltage drop between switch and fitting

[Niscars]

I am attempting to replace our kitchen extractor fan with a very similar model but have found that there is 240V at the fused switch but I only measure 163V at the other end of the wire in the fan outer casing. The distance between the two is less than 2 metres. Four core cable with just live and neutral connected.
The old fan was u/s so I have never seen this set up working.
NB: There is no change in voltage when the kitchen light is turned on or off.

I just can't understand this. Has anyone got any thoughts please before I have to call out a sparky.
Thanks, Oscar

 

[JohnW2]

I am attempting to replace our kitchen extractor fan with a very similar model but have found that there is 240V at the fused switch but I only measure 163V at the other end of the wire in the fan outer casing. The distance between the two is less than 2 metres. Four core cable with just live and neutral connected. The old fan was u/s so I have never seen this set up working. .... I just can't understand this.

Nor can I, really. Even if there were very poor connections and/or a seriously damaged cable, there would not be any voltage drop in the absence of a load - so you should get either 240V or (if connections/cable were totally 'broken') nothing at the fan end.

What are you measuring with?

Kind Regards, John

 

[Niscars]

Hi, it's a Clarke digital tester. I thought that maybe there was something else connected inline but I can't find anything else that stops working when the fan power is off.

 

[ban-all-sheds]

there would not be any voltage drop in the absence of a load

But there could be an induced voltage

 

[JohnW2]

there would not be any voltage drop in the absence of a load

But there could be an induced voltage

What sort of 'induced voltage' could result in their being 240V (derived from from a negligible impedance source) between conductors of a cable at one end, but only 163V between them at the other end? Confusion due to induced voltages really only arise in cables which are not energised.

Kind Regards, John

 

[JohnW2]

I thought that maybe there was something else connected inline but I can't find anything else that stops working when the fan power is off.

Even if they were, nothing else should produce anything like as much voltage drop as you are seemingly observing in a couple of metres of cable - that would require hundreds of amps, which would show itself in other, probably dramatic, ways!!

Kind Regards, John

 

[Niscars]

Got it. The neutral wasn't properly connected at the switch. 240V at both ends now.
Sorry to waste your time, but thanks for your help.

 

[JohnW2]

Got it. The neutral wasn't properly connected at the switch. 240V at both ends now.

I'm glad it's sorted out. I'm still surprised that (in teh absence of a load) you got the measurement you did - as I said, I would have expected either 240V or nothing!

Kind Regards, John

 

[ban-all-sheds]

What sort of 'induced voltage' could result in their being 240V (derived from from a negligible impedance source) between conductors of a cable at one end, but only 163V between them at the other end?

One that happens when a/the conductor(s) is/are not continuous, and the 240V at one end is not getting to the other.

 

[JohnW2]

One that happens when a/the conductor(s) is/are not continuous, and the 240V at one end is not getting to the other.

Maybe, but I'm still extremely surprised by the magnitude of this 'induced voltage' (induced from where, I wonder?).

Kind Regards, John

 

[ban-all-sheds]

Who knows what other cables run closely parallel to this one?

 

[JohnW2]

Who knows what other cables run closely parallel to this one?

As you say, who knows - but we are told that it is "less than 2m" in length, so I remain rather surprised.

Kind Regards, John

 

[endecotp]

My guess is that the earth was connected. So you have a potential divider between live and earth, with one impedance being the input impedance of the meter and the other being the capacitance from neutral to earth.

Very approximately, say that both conductors (E and N) are 2mm in diameter and are 0.5mm apart, and 2m long. C = eps A / d. eps = 1e-11, A = 4e-3 x 1, d = 0.5e-3 so C = about 100 pF. Impedance of a capacitor Z = 1 / ( 2 pi f C ). f = 50 Hz so Z = about 30 MOhm.

My multimeter has an input impedence of 10 MOhm, so I would expect it to show 1/4 of mains voltage in this case. A meter with a 100 MOhm input impedence could show 187 volts.

I note that some meters now have modes where they present a lower input impedence. Having personally observed the confusion that these sorts of stray-capacitance-related readings cause to even experienced electricians, this sounds like a very worthwhile feature to have.

 

[JohnW2]

My guess is that the earth was connected. So you have a potential divider between live and earth, with one impedance being the input impedance of the meter and the other being the capacitance from neutral to earth. ... Very approximately, say that both conductors (E and N) are 2mm in diameter and are 0.5mm apart, and 2m long. C = eps A / d. eps = 1e-11, A = 4e-3 x 1, d = 0.5e-3 so C = about 100 pF. Impedance of a capacitor Z = 1 / ( 2 pi f C ). f = 50 Hz so Z = about 30 MOhm.

Yes, roughly. FWIW, I actually get about 80 pF (ignoring the insulation), hence about 40 MΩ.

My multimeter has an input impedence of 10 MOhm, so I would expect it to show 1/4 of mains voltage in this case.

Again, roughly. With my 80 pF figure, that would obviously be about 1/5 of mains voltage (i.e. about 48V in the OP's case) - which is the sort of figure I would have expected.

A meter with a 100 MOhm input impedance could show 187 volts.

That's true (again, a bit lower with my calcs), but I think that it would probably be very unusual for the sort of multimeter the OP is likely to have to have an input impedance greater than 10 MΩ (quite possibly less) - so, again, I would not really have expected him to get a reading much over 50V.

I note that some meters now have modes where they present a lower input impedence. Having personally observed the confusion that these sorts of stray-capacitance-related readings cause to even experienced electricians, this sounds like a very worthwhile feature to have.

Indeed, that is a valuable feature. Some meters have a 'switchable shunt' one can put across the input. Personally, I usually reach for the cheapest analogue meter I can find in a dusty corner of my toolbox! - at, typically, 1kΩ/V, that's an input impedance of only about 0.6 MΩ on a 600V range - low enough to minimise most of these confusing 'induced' voltages (not that 'induction' has usually got much to do with it!)!

Kind Regards, John

 

[sparkwright]

Got it. The neutral wasn't properly connected at the switch. 240V at both ends now.
Sorry to waste your time, but thanks for your help.

Is this the neutral badly connected, or connected to the wrong terminal?