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Discussion Starter #1
Hi,

Last night I held one probe of my digital multimeter with licked fingers while touching the other end on some things that should be earthed:

1. a screw holding on a light switch face plate with a metal back box
2. pipes in my airing cupboard

I was quite surprised to find I measured 10.5 VAC between me and the screw and 9.5 VAC between me and the plumbing.

Is this normal? Could someone else with a DIGITAL multimeter try this? Remember to set it to read AC :)
 

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WHOA !!!!!!!

DANGEROUS, VERY VERY DANGEROUS, DO NOT TRY THIS.


Kingsley,
What would you do if you had had a dodgy appliance plugged in somewhere, with substantial voltage leaking to the earth circuit. You lick your finger and hold one probe, and complete the circuit with the other probe......your toast mate if your tickers not up to the job.


A few years ago, I had this very situation arise in my radio room. My antenna mast (at the time) had a substantial earthing system put in place below it's concrete base (several hundred meters of heavy gauage copper wire and five 8ft long 2" diameter copper earth spikes), the conductivity to earth via my antenna system offered considerably less resistance than the conductivity to earth through my mains systems earth terminals.

The mains supply filter which I was using at the time was rather old, and it was leaking voltage to earth via it's capacitor / diode filtering system. Problem was, the best earth it could see was via the ground of my antenna system, and I found out when disconnecting one of the many antennas from the back of a radio one evening. RF burns are one thing but I was bloody suprised, after picking myself up, to find 175V potential between the filter earth line and my antennas true earth.

I now have my antenna earth (true earth), and mains earth fully isolated from each other.

The reason for my warning, you are potentialy a better route to earth than your dodgy mains earth system, and no domestic ELCB will protect you when your bypassing it in this way !
 

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Discussion Starter #3
Notwithstanding your comments Jason, that's why I said a DIGITAL multimeter. Clearly, if anyone's not sure what they're doing, don't do it. The internal resistance of a digital multimeter is so great that as long as you set it to read voltage (NOT CURRENT) you'd be fine.

But yes, if you're not clear about what you're doing, I agree with Jason 100%.
 

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Discussion Starter #4
Obviously if I'd said lick your finger and touch the earth directly that would be a different matter because then you're connecting yourself directly to the house electrical system ... but I didn't suggest that.
 

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Discussion Starter #6
JasonO said:
What would you do if you had had a dodgy appliance plugged in somewhere, with substantial voltage leaking to the earth circuit. You lick your finger and hold one probe, and complete the circuit with the other probe......your toast mate if your tickers not up to the job.
For what it's worth (and for God's sake nobody try this themselves), I've just tried touching the other probe onto the live ring main and didn't feel a thing ... the meter read 110 VAC but would probably have read higher if I'd had greater capacitance or was actually earthed myself (which I wasn't because I was standing on a wooden floor).
 

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Kingsley,
I don't get the relevance of your third reply to my post, and I certainly don't think I deserve that from you. :(

Having a bad day, or just peeved with me for pointing out that what you are encouraging people to do is dangerous ?
 

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kingsley said:
For what it's worth (and for God's sake nobody try this themselves), I've just tried touching the other probe onto the live ring main and didn't feel a thing ... the meter read 110 VAC but would probably have read higher if I'd had greater capacitance or was actually earthed myself (which I wasn't because I was standing on a wooden floor).
You had one probe of the MM held in the wet fingers of one hand, the other probe connected to the Live of the ring main. Have I got this right Kingsley ?

This indicates that the potential difference between the ring main and what exactly is 110VAC ?
 

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Discussion Starter #9
Hi Jason, I'm not peeved at all, you must have misread me. I'm not sure I understand how you thought I was having a dig? Certainly not the case! I'm actually glad you posted that in case people who don't have an understanding of the likes of Ohm's law did something daft - I would defintely not want people to do this if they weren't sure of what they were doing. In the heat of the moment in my original post, I had overlooked that.

You had one probe of the MM held in the wet fingers of one hand, the other probe connected to the Live of the ring main. Have I got this right Kingsley ?
Yes, absolutely. Clearly if I'd had the thing switched to measure current rather than voltage it would have been an altogether more unpleasant experience.

As you know, an ideal ammeter has zero electrical resistance and an ideal voltmeter has infinite electrical resistance. Therefore an ammeter would have given me a nasty shock but the voltmeter insulates me.

The real world isn't quite so ideal, though. Voltmeters still do have a very high resistance though. Digital ones have a far higher resistance than analogue ones, hence my original post insisting on digital.
This indicates that the potential difference between the ring main and what exactly is 110VAC ?
My fingers.

If mains was DC, then I would expect it to have read zero as my body would have been charged up, through the multimeter, to mains voltage. There would have been a small initial current while charge passed to me because my body has some capacitance.

However, as mains is AC, my body is always playing catchup to a moving target and the difference between the ring main live and me was therefore about 110V. Before I licked my fingers it only read about 80V, showing that there's a lot of contact resistance on dry hands.
 

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Kingsley,
Your mad ! you gladly hook yourself up to the ring main with some dodgy old multimeter you found in the bottom of an old uncles toolbox, yet you won't have a poop in someone elses loo. ;) :D

Nice experiments here, I just don't have the faith in my own multimeter to try it.



Sorry for taking your comment the wrong way, usually when people say 'feel free to delete my thread if you see fit' they're objecting to my intervention, and I'm feeling a bit sensitive these days. :D
 

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Discussion Starter #12
JasonO said:
Kingsley,
Your mad ! you gladly hook yourself up to the ring main with some dodgy old multimeter you found in the bottom of an old uncles toolbox, yet you won't have a poop in someone elses loo. ;) :D
LOL, I'm quite particular about my botty :D

In my defence it's a Maplin White Gold multimeter I bought new a few years ago; it's quite a nice unit :)

I certainly wouldn't try it with an analogue one - not enough internal resistance for me to feel safe.

JasonO said:
Nice experiments here, I just don't have the faith in my own multimeter to try it.
Fair enough, can't argue with that ...

JasonO said:
Sorry for taking your comment the wrong way, usually when people say 'feel free to delete my thread if you see fit' they're objecting to my intervention, and I'm feeling a bit sensitive these days. :D
Yeah I've noticed a few people having a pop at you for your moderating activities. Rest assured that I'm not one of them - I'm sorry if I've accidentally caused any offence. Don't worry, I still see you as friendly and cuddly :)
 

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Discussion Starter #13
Mookistar said:
out of interested, what made you think to do this in the first place?
My wife got an electric shock off the bedroom light switch last night. I have however concluded that she probably built up a charge walking across the carpet and inadvertently earthed herself on the face plate screws. So, the shock was 'going the other way' if you get my drift.

Incidentally I just tried it at the neighbour's house and got very similar results so I'm guessing it's probably normal after all. Downstairs the light fittings only give me about 3 volts so I'm guessing there may be some sort of 'being a receiving antenna' type action in the wiring. It would be very high impedance so any load to speak of would short it out easily with very little current passing (just a few microvolts, I expect).
 

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Before I start here, I will say that I only have a basic understanding of household electrics although I do understand the workings of a household ring main & lighting circuits etc.

As some of you know I am moving to Sweden in a few weeks time and in my many visits over there I am amazed and the lack of safety in relation to appliances and sockets being earthed in Scandanavia ! 99.9 % of the places I have been in over there and our new house included do not have any earthing in standard plug sockets with exception to sockets in the kitchen, outside (if any) and the bathroom (yes I did say bathroom !), bathrooms over there have a normal style switch to turn the lights on and off (they dont see the need for a pull cord switch !) & normally one or two sockets for hairdryers etc.

I am taking over my appliances (TV, DVD, Home Cinema Amp, Washing Mackine etc. etc.) when we move and I really don't know whats going to be best when using them as everything here is earthed whereas over there hardly anything is at all :confused:

I know we are mega cautious with safety over here but away from here it's like the dark ages with the way the 220v supply works.

They even have a 400v supply to power radiators etc. which I understand is 3 phase and very deadly if incorrectly worked on ?

Does anyone have any knowledge of the european system for domestic electrical supply ??
 

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Discussion Starter #17
Not sure but not all countries earth their neutral wires (in the UK I believe neutral is earthed back at the substation).

If neutral isn't earthed, earth won't form part of the circuit. Consequently, it stands to reason that connecting something across live and earth wouldn't really draw much current. It would make sense that you'd only get a shock if touching both terminals at the same time.

Not sure whether that applies to Sweden though.
 

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Discussion Starter #19
Actually I think I may have contradicted myself a bit. I'm sure capacitance must come into it even if neutral isn't earthed back at the substation. Plus, you can't guarantee that your neighbour hasn't done something to earth one of the cables anyway ... so yes, I'd be a little concerned.
 
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