Electrical earthing

[Clive Long,UK]

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?

1. 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]

Only when you connect it to earth by a piece of 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

[John Rumm]

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.

[Harry Bloomfield]

Clive Long,UK laid this down on his screen :

Sorry, don't quite follow?

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.

[John]

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.

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]

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

Christian.

[Mike]

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]

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