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#34424 - 02/11/04 04:35 AM Why a continuous ground?
Big A Offline

Registered: 01/22/03
Posts: 47
Loc: Lynchburg, VA
Does anybody know why a system has to have a continuous ground? I understand why you would need it in things that were close together but I don't understand why if I have a feed on one side of the street, and a metal cabinet on the other side of the street, these must have a continuous ground. Seperate grounds are not allowed.

Any reasons why?

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#34425 - 02/11/04 04:50 AM Re: Why a continuous ground?
Attic Rat Offline

Registered: 12/14/03
Posts: 530
Loc: Bergen Co.,N.J. USA
...So that it can never be disconnected,or the lugs,bugs,etc..become loose and create a high resistance to ground.
.."if it ain't fixed,don't break a Licensed Electrician"

#34426 - 02/11/04 05:22 AM Re: Why a continuous ground?
earlydean Offline

Registered: 12/22/03
Posts: 749
Loc: Griswold, CT, USA
To facilitate the operation of the OC device you need a low impedance fault path back to the OC device.

#34427 - 02/11/04 11:08 AM Re: Why a continuous ground?
Big A Offline

Registered: 01/22/03
Posts: 47
Loc: Lynchburg, VA
So the OCP will not function with a seperate ground? I guess that makes sense, but wouldn't the difference of potential between the two grounds have to be some huge (say 1 meg ohm) difference?

#34428 - 02/11/04 11:25 AM Re: Why a continuous ground?
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Lets say, for example, that the grounding electrode on that remote box had a resistance of 25 ohms. Let us further stipulate that the grounding electrode system on the main panel has a resistance of 15 ohms. You get a dead 'hot to ground' short on the remote box, but you don't have any equipment grounding conductor in the feed to the box. Finally, presume that this is a single phase 120V to ground circuit.

Net result: 3A of current flowing through the ground. The remote box is sitting at 75V to ground, and the circuit breaker won't trip. All you've done is created a shock hazard and wasted electricity heating the earth.

Note: 'Resistance to ground' of a grounding electrode is something of a made up number. You presume that the earth as a whole is a perfect conductor, and then attribute any resistance to current flow to the grounding electrode. But in reality the earth is a real conductor with real resistance, and the distance between your electrodes will matter. Imagine that your two electrodes were driven into the ground parallel to each other and 1cm apart. Clearly the resistance between the electrodes will be lower than the overall resistance to earth. But the approximation is good for electrodes spaced significantly greater than their size.



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