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#129611 - 05/27/05 05:31 PM Earth as super all terminal-electrode?
gerry Offline
Junior Member
Registered: 05/24/05
Posts: 4
Please, good and knowledgeable people here, explain to me how an ac electrical 'presence' can discharge into the earth.

Let me give examples:

1. In order to protect oneself from a hot metal case as of an old washing machine, a wire is bolted to the case and fastend to a metal rod struck into the ground like on steel plumbing pipes, so that instead of electricity getting to you it will discharge to the earth.

2. To obtain half of a say 220 ac volts source (I live in a place where the house current is ac 220 volts), attach one wire to the 110 ac volts appliance and the other wire from this appliance to a steel pipe as in the ground plumbing struck to the ground.

3. An arc welding machine transformer uses only one wire, the other wire is attached to a piece of metal like in the iron gate of the property wall.

Any explanations for a simple layman?

Is the earth then some kind of a substitute for the other terminal of an ac current generator?

gerry
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#129612 - 05/27/05 09:58 PM Re: Earth as super all terminal-electrode?
winnie Offline
Member
Registered: 09/15/03
Posts: 649
Loc: boston, ma
Simple really; the Earth does _not_ in fact do the things that you suggest. _Grounding_ an energized case will leave that case still energized, an earth electrode will make a _very_ poor neutral, and a welder will work just fine without any connection to earth at all.

1) The 'grounded' case is made safe not by the connection to earth, but by the connection back to the electrical transformer neutral. In the event of a hot to equipment ground fault, the circuit breaker will trip, removing the hazard.

2) The midpoint of a 120/240V electrical supply is from the metallic connection to the transformer neutral, _not_ the connection to earth.

3) The 'ground' connection on a welder is simply one of the welding transformer terminals, the one that by convention is connected to the bulk material, not to the welding terminal. Since the bulk material is often in contact with earth, this becomes the 'grounded' transformer output.

Search this forum for the terms 'grounding' and 'bonding', and you will find many discussions of the topic.

-Jon
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#129613 - 06/12/05 06:15 PM Re: Earth as super all terminal-electrode?
Larry Fine Offline
Member
Registered: 02/18/05
Posts: 693
Loc: Richmond, VA
To add a bit, the earth is somewhat conductive in the local sense, and rather conductive in an overall sense, but never used as the only pathway for neutral or fault currents.

The purpose of bonding, and grounding this bond, is to minimize voltage differences (potential) between different conductive surfaces with which one might make contact.

The metal cabinets and the earth are at the same potectial (even if not zero volts) so someone standing on the earth and contacting an energized case doesn't conduct electricity.

[This message has been edited by Larry Fine (edited 06-12-2005).]
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com
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