Electrical earthing

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Clive Long,UK

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Apr 18, 2005, 1:38:20 PM4/18/05
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Hi,

I found at a friends house that some devices were "tingling". We
discovered the exposed metal surface of a PC was about 20V AC "above"
earth. Opening up the power socket we found the earth wire of the
spurred supply had been connected to the "earth screw" on the plastic
box. We reattached the earth from the supply to the "correct" point on
the socket and the AC voltage on the case disappeared.

Now

1. When is the "earthing screw" on the plastic box (sorry don't know
it's name) connected to "some" earth?

2. What has "happened" to the 20V AC that existed when the earth was
incorrectly attached? Is there some continually "leak to earth" on the
sockets where we have re-attached the earth wire?

Thanks


Clive

Alistair Riddell

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Apr 18, 2005, 2:00:49 PM4/18/05
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On Mon, 18 Apr 2005, Clive Long,UK wrote:

> Hi,
>
> I found at a friends house that some devices were "tingling". We
> discovered the exposed metal surface of a PC was about 20V AC "above"
> earth. Opening up the power socket we found the earth wire of the
> spurred supply had been connected to the "earth screw" on the plastic
> box. We reattached the earth from the supply to the "correct" point on
> the socket and the AC voltage on the case disappeared.
>
> Now
>
> 1. When is the "earthing screw" on the plastic box (sorry don't know
> it's name) connected to "some" earth?

Only when you connect it to earth by a piece of wire

> 2. What has "happened" to the 20V AC that existed when the earth was
> incorrectly attached? Is there some continually "leak to earth" on the
> sockets where we have re-attached the earth wire?

Electronic devices like computers do tend to leak to earth rather a lot
due to the design of their power supplies. This is quite safe and normal,
provided that the leakage to earth is within the limits set by relevant
standards, and provided that the earth connection is sound.

HTH

--
Alistair Riddell - BOFH
Microsoft - because god hates us

John Rumm

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Apr 18, 2005, 3:46:49 PM4/18/05
to
Clive Long,UK wrote:

> 1. When is the "earthing screw" on the plastic box (sorry don't know
> it's name) connected to "some" earth?

The earth terminal on a plastic box is typically designed to be used
when the box is on a lighting circuit and you need a place to "park" the
unused earth wire in the cable going to the switch.

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

Harry Bloomfield

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Apr 18, 2005, 4:43:40 PM4/18/05
to
Clive Long,UK laid this down on his screen :

> 1. When is the "earthing screw" on the plastic box (sorry don't know
> it's name) connected to "some" earth?

Sorry, don't quite follow?

>
> 2. What has "happened" to the 20V AC that existed when the earth was
> incorrectly attached? Is there some continually "leak to earth" on the
> sockets where we have re-attached the earth wire?

PC power supplies and similar have filters on the mains input designed
to prevent interference getting out of the box. The filter is connected
from both live and nuetral to ground. It is the voltage of connection
of the filter to ground which you are measuring and it is perfectly
normal.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org

John

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Apr 18, 2005, 5:25:11 PM4/18/05
to

"Clive Long,UK" <clive...@yahoo.com> wrote in message
news:2b77647f.05041...@posting.google.com...

> Hi,
>
> I found at a friends house that some devices were "tingling". We
> discovered the exposed metal surface of a PC was about 20V AC "above"
> earth. Opening up the power socket we found the earth wire of the
> spurred supply had been connected to the "earth screw" on the plastic
> box. We reattached the earth from the supply to the "correct" point on
> the socket and the AC voltage on the case disappeared.
>
> Now
>
> 1. When is the "earthing screw" on the plastic box (sorry don't know
> it's name) connected to "some" earth?

The terminal is provided to terminate an otherwise unused earth wire or
provide a point of connection between the cable earth wire and any
continuation of same. On its own it cannot provide a path to earth.

>
> 2. What has "happened" to the 20V AC that existed when the earth was
> incorrectly attached? Is there some continually "leak to earth" on the
> sockets where we have re-attached the earth wire?

It was probably due to leakage especially if the PC power supply had a
suppressor network on its input. A popular type has two capacitors in series
between live and neutral with the mid point earthed. This allows a small
current to leak to earth as it can act as a potential divider

Strictly speaking you ought to check your earth is "good" by an earth loop
impedance test at the socket but your effect in "losing" the tingle suggests
it should be OK.

I just know someone is going to shout part P any time now:-(


Christian McArdle

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Apr 19, 2005, 5:22:16 AM4/19/05
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> Opening up the power socket we found the earth wire of the
> spurred supply had been connected to the "earth screw" on
> the plastic box.

I would now test the entire electrical installation. One sign of utter
incompetence in its installation could a sign of many more.

Christian.


Mike

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Apr 19, 2005, 2:50:20 PM4/19/05
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"Christian McArdle" <cmcar...@nospam.yahooxxxx.co.uk> wrote in message
news:4264cdc9$0$287$ed9e...@reading.news.pipex.net...

But of course with Part P he isn't allowed to fix many of these
incompetances. Let's hope for a change on May 5th !!!!


Andrew McKay

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Apr 20, 2005, 3:00:10 AM4/20/05
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On Tue, 19 Apr 2005 19:50:20 +0100, "Mike" <mi...@nospam.com> wrote:

>But of course with Part P he isn't allowed to fix many of these
>incompetances. Let's hope for a change on May 5th !!!!

Getting rid of part P won't be high on any political parties agenda.
They would instantly be accused of reducing safety in homes etc, and
if there were just one fatality which might have been subscribed to no
part P inspection they will have the wrong sort of publicity.

Part P is here for the longer term I think. And I suspect that in the
fullness of time it will be beefed up still further so that no DIY
work will be acceptable and that the only people allowed to carry out
the work will be qualified tradespeople.

Andrew

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