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Topic Title: Touch voltage definition help
Topic Summary: you feeling clever?
Created On: 13 July 2013 12:14 PM
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 13 July 2013 12:14 PM
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Zs

Posts: 2858
Joined: 20 July 2006

hello,

I've been writing some guidance for a client on the subject of earthing and bonding. Apropos of 20V flowing in the earth in one area of the building which happens to be the only area where the metal work of the desks is not connected to any form of earth. I am encouraging them to connect the power bars to the metal work of the desks via a short flying lead in the cable dump trays. As you know, this is not my specialist subject and I usually explain things with lots of hand waving and news stories about dead race horses/surviving trainers (the Newbury incident).

I've covered the difference between earthing and bonding. I've explained safety earthing, high protective conductor currents, am just about to cover electromagnetic.

But, I have mentioned touch voltage and now I must explain it in basic terms. I find myself really struggling to produce a couple of sentences to explain touch voltage which I have mentioned under 'bonding'.

The difference in the voltage flowing in something conductive between the first part of a body touching it and the second part of the body touching it. That difference is what makes it dangerous because if it was the same voltage it wouldn't be as serious...kind of thing, but less 'down the pub speak'.

Can you help? I'm trying to avoid getting into the realms of the milliamp.

In return, I'll let you have the whole page when it is done.

Thank you

Zs
 13 July 2013 02:07 PM
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Zs

Posts: 2858
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Ha, obviously not feeling clever then?

I've done it now. There is some rubbish on the web about it especially something on the Screwfix forum. But I found a few useful sentences elsewhere.

Why do engineers talk in such pompous, user-unfriendly language?

Zs
 13 July 2013 02:23 PM
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John Peckham

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Nah not that clever hence I am putting off opening the box and trying to assemble a barbecue purchased this morning from Homebase!

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John Peckham

http://www.astutetechnicalservices.co.uk/
 13 July 2013 02:54 PM
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davezawadi

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You going to bond the barbie the JP?
I wouldn't encourage earthing desks (or kitchen tables or anything similar) as all that you are doing is generating a conductive earthed environment, which means that any potential earth fault on an appliance may give a nice low resistance earth to hang onto with the other hand.

Oh the touch voltage is that which you get in the above scenario. Take care in the English as voltages do not flow in things, they are simply potential differences to other things!!

See you on linked in.
Regards
David

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David
CEng etc, don't ask, its a result not a question!
 13 July 2013 03:34 PM
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potential

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20V flowing in the earth in one area

Has me completely baffled.
 13 July 2013 04:31 PM
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Zs

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I'm reading an Electrical Safety Council Technical Document right now.

The F word has been used .

Zs
 13 July 2013 05:09 PM
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John Peckham

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"The F word has been used ."

I have been using that word a lot assembling the barbecue. Anyway I must be clever as all I have to do now is get someone to take a digital picture of the assembled barbecue with me holding a screwdriver next to it, then email the photo to a private training company with my credit card details and they will send me my NVQ3 in mechanical engineering.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 13 July 2013 06:13 PM
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Legh

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Touch voltage
A voltage appearing on metalwork that can be touched.

It might be caused by:

1/ An earth fault on a circuit causing the voltage to suddenly appear on nearby metalwork
2/ Equipment with a natural earth leakage current caused by filters without an earth connection
3/ An imbalance of loading on a PME supply causing a voltage to be present in the supply neutral and hence exported to the earthed metalwork.
4/ A cable or equipment with a natural capacitance unearthed.
5/ JPs BBQ after he's connected the electronic ignition.

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 13 July 2013 07:45 PM
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davezawadi

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I wouldn't read it if I were you, most of it may well be dubious or simply wrong! The F word comes out when someone s trying to get one over on you...... and is a definite warning that continuation is a bad idea.

Regards

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David
CEng etc, don't ask, its a result not a question!
 13 July 2013 09:05 PM
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Zs

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Originally posted by: Legh


A voltage appearing on metalwork that can be touched.


Legh


Nice. Simple. Stolen.

Sorry,I missed that when you posted Legh. Been away for a couple of hours assembling a triple pulley system as an aid for lifting some heavy kit over a cantilevered structure that I knocked up last weekend. How's that BBQ coming along JP....need a hand with all the bits you've got left over

Zs
 13 July 2013 10:25 PM
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sparkingchip

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Touch voltage- if it hurts, don't!
 14 July 2013 08:24 PM
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MrOther

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Originally posted by: Zs

I've covered the difference between earthing and bonding. I've explained safety earthing, high protective conductor currents, am just about to cover electromagnetic.


Zs


Earth - Ok, Bonding - Ok, High Protective Currents - Ok, what's safety earthing? Got a feeling this is another word for something else. Thanks

MrOther
 14 July 2013 08:29 PM
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MrOther

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Originally posted by: davezawadi

You going to bond the barbie the JP?

I wouldn't encourage earthing desks (or kitchen tables or anything similar) as all that you are doing is generating a conductive earthed environment, which means that any potential earth fault on an appliance may give a nice low resistance earth to hang onto with the other hand.

David


Surely if you start bonding something in high metallic environment like this, then you got to the full hog and treat it like a special environ and bond everything to avoid this happening, i.e. like a bathroom.
 14 July 2013 09:15 PM
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weirdbeard

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Originally posted by: Zs

Apropos of 20V flowing in the earth in one area of the building which happens to be the only area where the metal work of the desks is not connected to any form of earth. I am encouraging them to connect the power bars to the metal work of the desks via a short flying lead in the cable dump trays.


Sorry to be blunt but I am unsure here... what is meant by there is 20V flowing in the earth and how is connecting a desk to earth with a flying lead going to improve the situation?
 14 July 2013 09:26 PM
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UKPN

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touch voltages are liable to occur between simultaneously accessible exposed-conductive-parts and extraneous-conductive-parts in an installation, particularly under conditions of fault within the installation.
in this context "extraneous-conductive-parts" may include not only "earthy"
metalwork,such as metallic services pipework, but also a surface on which a person is standing which presents no appreciable resistance to the general mass of earth.
class 1 equipment can have a earth fault caused by a breakdown of the insulation between the line conductor of the load and the earthed enclosure of the equipment. the earth fault is assumed to be accompanied by an open circuit in the load.
the earthed enclosure and the metal pipe which is connected to the MET of the installation by a main bond are simultaneously accessible to a person.
an earth fault (If) flows, giving rise to a touch voltage between the accessible enclosure and the metal pipe.
the magnitude of the touch voltage (in volts) will be approximately equal to If (in amps) multiplied by the impedance of the cpc (in ohms).

Regards
 15 July 2013 09:51 AM
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AJJewsbury

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I'm a little puzzled too. Any ideas why this 20V difference is appearing? (and between what? - it's just that I've got a nagging feeling that bonding metalwork that probably isn't of itself extraneous is necessarily a solution).
- Andy.
 15 July 2013 10:00 AM
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Parsley

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A difference in voltage potential being experienced by a person who
makes contact simultaneously with more than one conductive part, which is not normally energised.

Protective bonding lowers the magnitude of the touch voltages, the benefits are more noticeable for TT earthing systems than TN systems.

Regards
 15 July 2013 11:59 AM
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redtoblackblewtopieces

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I think if you go back to a thread about ZS and R1+R2 the post from Zs on the 27 May talks about 20volts from ceiling grid in an office to a metal desk which if reading correctly is now going to be bonded, so unless the bond goes directly from desk to ceiling grid don't touch both at the same time ?
Kevin

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Compliance by Approved Documents
 15 July 2013 12:53 PM
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John Peckham

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As Dave Z above says bonding the desks may introduce a hazard. If the desk is bonded to earth and a tradesman, or office worker pinning up the Christmas decorations, stands on the desk and touches the ceiling grid then it could be an early bath depending on footwear.

If there is a 20V potential now between ceiling grid and desk then this could rise to 230V if the desk is bonded.

It may be useful if "electricians" stopped using ceiling grids as containment systems in the first place.

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John Peckham

http://www.astutetechnicalservices.co.uk/
 15 July 2013 01:07 PM
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Parsley

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It may be useful if "electricians" stopped using ceiling grids as containment systems in the first place.


That would be too easy John, I remember highlighting unsupported cables laying on a suspended ceiling grid to a fast food chains surveyor last year as well as approximately 50 other observations. Only to be shown another contractors satisfactory EICR done 3 months before ours. The surveyor's response was all the stores are done like that what's the problem?

Regards
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