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#14721 - 09/30/02 11:34 AM Too much grounding?
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Hi all!

I started thinking about the different systems for grounding and stumbled across a potential problem.

In the US and much of Europe the standard way of grounding a building is to connect the ground wire to the neutral at the panel, meter or main fuse. (I won't use the terms grounded and grounding wire, since I only end up confusing myself. I'll also ignore the ground rods in this case.)

At the panel you have single pole breakers, which trip if there is too much current in the hot wire. There can never be a dangerous short between neutral and ground, since they are tied together.

Now: If I add a subpanel, using a cable with a separate ground wire I will no longer have neutral and ground tied together next to the breakers. When there is load on the circuits at the subpanel, there will be a voltage drop in the neutral in the cable from the main panel. Hence, there is a potential between the ground and neutral in all circuits on the subpanel. This includes those without load. If I short circuit neutral and ground at e.g. a socket, there will be current flowing due to this. This current never goes trough the breaker. Isn't this a danger? Have I missed something somewhere?

If we assume the voltage drop is 10V in the cable from the main panel, there vill be a 5V difference between ground and neutral. Suppose there is a 2.5 mm2 cable (13 AWG) cable with a neutral-ground short circuit 3 meters (10 feet) from the subpanel. The current flowing in this will then be:
I=V/R=5/(2*3*0.017/2.5)= 123 Amps (!)

A GFI will of course eliminate the problem, but most circuits don't have GFI:s.

[This message has been edited by C-H (edited 09-30-2002).]

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#14722 - 09/30/02 01:29 PM Re: Too much grounding?
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
If you short the neutral to the grounding conductor, all you have done is create a parallel path for the neutral current. To find the current that will flow in the grounding conductor, you have to do a parallel circuit calculation based on the current flow in the neutral prior to the short and the impedance's of the neutral and grounding conductors.
Don
_________________________
Don(resqcapt19)

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#14723 - 09/30/02 02:05 PM Re: Too much grounding?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
C-H,
As Don has said, shorting the neutral and ground at the sub-panel creates a parallel path for the existing neutral current. It doesn't increase that current.

The flaw in your logic is that the 5V potential difference between neutral and ground that you have assumed is actually no longer present once the neutral & ground are shorted.

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#14724 - 09/30/02 02:23 PM Re: Too much grounding?
tsolanto Offline
Member

Registered: 09/03/02
Posts: 131
Loc: Long Island NY
The reason for isolating the ground and neutral in a sub panel is so the ground wire doesn't carry any neutral current. Imagine if the neutral that fed the subpanel were to break, the full load of the sub panel would be going through the ground wire. If someone were to disconnect the ground while it was under load they might not live through it. This happens to plumbers when they replace the water meter, now we jump it to prevent that from happening.

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#14725 - 10/01/02 09:38 AM Re: Too much grounding?
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
 Quote:

The flaw in your logic is that the 5V potential difference between neutral and ground that you have assumed is actually no longer present once the neutral & ground are shorted.


Perhaps I should have taken an example:

A 25 mm2 (3 AWG) 3-wire cable going to a panel, carrying 100A. The voltage drop is 10V. Then I connect a 2.5 mm2 (13 AWG) wire between neutral and ground at the load side. If we assume that this wire is very short, the resistance can be neglected.

If the ground and neutral conductors has the same area, the current will then be shared evenly between them, right? The current going through the 2.5 mm2 wire is now 50A, right?

But, indeed there is no longer a potential difference.

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#14726 - 10/01/02 01:16 PM Re: Too much grounding?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
If the ground and neutral conductors has the same area, the current will then be shared evenly between them, right? The current going through the 2.5 mm2 wire is now 50A, right?


Yes, if the wires are the same length, the same uniform cross-sectional area and made of the same material, then the current will divide equally (assuming also that the connections at both ends also offer the same resistance to each path).

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#14727 - 10/02/02 08:49 AM Re: Too much grounding?
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
Tsolanto:
 Quote:

The reason for isolating the ground and neutral in a sub panel is so the ground wire doesn't carry any neutral current. Imagine if the neutral that fed the subpanel were to break, the full load of the sub panel would be going through the ground wire. If someone were to disconnect the ground while it was under load they might not live through it.


Yes, separating ground and neutral is usually assumed to be a good thing. Among the reasons for doing so are the ones you mention.

People now talk of using hot, neutral and ground (TN-S) rather than just hot and neutral (TN-C) to supply houses, since a separate ground wire eliminates the dangers of a loose or broken neutral, just like you have pointed out. This means that the potential difference between ground and neutral at a receptable will be half of the total voltage drop from transformer to receptable.

Pauluk:
 Quote:

Yes, if the wires are the same length, the same uniform cross-sectional area and made of the same material, then the current will divide equally (assuming also that the connections at both ends also offer the same resistance to each path).


From your answer above I take it that you agree that the small conductor beween ground and neutral in the previously described case will carry a current which is much higher than it can handle.

If the load is in fact a panel and the small wire is a short cable to a receptable next to the panel, does this change anything? The resistance in the cable is no longer neglible, but nevertheless very low.

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#14728 - 10/02/02 07:56 PM Re: Too much grounding?
Mike_Riverside Offline
Member

Registered: 08/18/02
Posts: 34
sorry to drift off topic here but...

aside from codes...can we assume that in a real situation with proper wire size, etc. that it would be safe to supply a sub panel, that is in close proximity, lets say 5-10 ft for example, where voltage drop is not an issue, with 3 wires?

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#14729 - 10/03/02 01:08 PM Re: Too much grounding?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
 Quote:
I take it that you agree that the small conductor beween ground and neutral in the previously described case will carry a current which is much higher than it can handle.

Definitely! If the 100A neutral current were divided equally between neutral and ground paths then the 2.5 sq. mm link wire would get warm very quickly.

 Quote:
If the load is in fact a panel and the small wire is a short cable to a receptable next to the panel, does this change anything?

Not sure I'm quite with you here.....

Do you mean what would happen if the neutral-ground short were at the receptacle end of a short cable run from a sub-panel?

Mike,
If you run a 3-wire connection to a sub-panel, however short, you still have the problem that if the neutral connection goes bad at any point then the panel and all grounding conductors connected to it could rise to full line potential.

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#14730 - 10/04/02 07:09 AM Re: Too much grounding?
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
 Quote:

Definitely! If the 100A neutral current were divided equally between neutral and ground paths then the 2.5 sq. mm link wire would get warm very quickly.


Thanks. Then I'm not entirely off track. I did start to doubt my sanity, since everyone including me have considered shorts between ground and neutral to be harmless.

 Quote:

Not sure I'm quite with you here.....
Do you mean what would happen if the neutral-ground short were at the receptacle end of a short cable run from a sub-panel?


Yes, exactly that.

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