Reground?

[russellroberts]

I was asked this question and I didn't have a ready answer other than I thought it would be a code violation.
The question was: "Why is it a problem to reground the grounded conductor downstream of the first disconnect"? Now this would be in a residential setting.
I know it's permitted in 250.32 at an outbuilding.
It's not a question of rebonding,but the examples were to take a recep,break the grounded conductor,take a wire outside and drive a ground rod,or to take a wire from a subpanel neutral buss to an outside ground rod,given that the subpanel has a 4 wire feeder and the neutral is isolated from the egc per code.

I would guess this would cause potential difference problems.

Code aside,what are the problems with such an arrangement?

And no, I'm not going to do this,I only want to have a good answer ready.

Thanks for any help. Russell

 

[bennie]

Re: Reground?

Russell: Excellent topic and questions.

I have synthetized every situation possible, to determine the hazards associated with parallel paths for neutral current.

The only answer I can find is the amount of EMF produced by asymmetrical current flow.

The amount of current flow in the earth will show as an unbalance on the feeder. This unbalance will produce EMF.

 

[russellroberts]

Re: Reground?

Thanks Bennie, I had an idea that would be the case,I just didn't quite know how to express it.

I heard a story of a guy who opened his metal gate one morning,and saw a spark. The problem was traced back to an outbuilding, which had a 3 wire feeder from his house,and landed in a 100a panel.The neutral bar was bonded and 2 ground rods driven per 250.32. There was found to be a recep which powered a small refrige with the grounded conductor nicked and touching the egc.

Is this an example of how the parellel path can manifest itself?

In the case of a similar situation with more current involved, it seems it could be deadly.

Am I on the right track?

Russell

[ May 25, 2003, 05:38 PM: Message edited by: russellroberts ]

 

[bennie]

Re: Reground?

The neutral conductor is multi-grounded outdoors, it should be single point grounded at the premises.

Multi-grounding causes common mode current flow with resultant EMF.

EMF is not concentrated in the earth, it is concentrated when common mode current is on interior conductors.

The powers that be, tell us that EMF is not a health concern. I think there's more known than we are being told.

 

[karl riley]

Re: Reground?

As I understand it, the separate ground rod situation you describe can result in a huge difference of potential in the plugged in device if there is a lightning strike or surge which sends current through the building. Because of the earth's impedance to lightning, the separate rod can have thousands of volts difference of potential from the service rod, depending on how far apart they are. Since this difference will be brought inside the appliance, an explosive arc can occur.

When CATV installations were first made, sometimes a rod would be driven for the cable ground, without bonding to the service electrode. I am told TVs have burst into flames and house fires resulted. As Bennie says, single point grounding must be aqdhered to.

And Bennie, your hunch about EMF and health has international backing. The cancer branch of the World Health Organization (IARC), though conservative, has declared 4mG (4 milligauss, = 0.4 microTesla - mT) and above as a possible (2b) carcinogen for childhood leukemia. Their conservatism lead them to leave out the other statistical links to other diseases.

This has been a fiercely fought controversy, with the big money on the side of those industries who would be most affected by stricter standards.

Luckily, by adhering to Code, almost all EMF generation in buildings is effectively eliminated. The exception is grounding to metal water pipes, but Code allows that to be remedied.
Kar4l

 

[bennie]

Re: Reground?

Karl: Very good article. I have printed it for my records.

I have a gauss meter. I am surprised with the readings I get in my own home.

I have some experience with grounding procedures, in unique geographical locations. I have a different perspective than some, due to observing some unusual events.

My continuing to question the interpretation of a separately derived system, is due to the EMF created on the equipment ground wire, when it is regrounded at the transformer. Regrounding the equipment/ground wire is the same as regrounding the neutral/ground wire.

I know the results from tests and measurements. My position is easily proven. There is a terrible and dangerous mistake being made in the present day interpretation of a separately derived system.

 

[russellroberts]

Re: Reground?

I'm on the job everyday,doing the work,passing inspections etc. but I sometimes wonder how much the average electrician actually knows and understands about this subject?

 

[tom baker]

Re: Reground?

Mike Holts says only 3% of electricans understand grounding and bonding. His lateset text on Grounding and Bonding goes into great detail on the dangers of objectionable current when the neutral is regrounded at a downstream panel. Do your self a favor and call Sarina at his office and order the video "City of Miami Open Neutrals" it will cost you about $20, it will explain the dangers of objectionable neutral current.

 

[russellroberts]

Re: Reground?

Thanks to Tom and Bennie. I've learned a lot from reading the posts on these type subjects and doing research on my own. But I've come to understand I don't know as much as I would like.

I have learned one thing though,you can spend a lifetime in this field and still be learning.

 

[bennie]

Re: Reground?

Russell: Most people only think of the current flow on it's way to the work. They don't think of the same amount having to return home after work.

When returning home the current gets sidetracked to a local bar. Then when it gets home there is a lot of noise.

The current should return home on the same conduit path as it used while going to work. Then you have a harmonic ideal situation.

 

[russellroberts]

Re: Reground?

Bennie, In my drinking days years ago,my wife told me the same thing. Only she wasn't near as nice. She probably wasn't talking about the flow of electricity,but it did have to do with harmonics.

[ May 26, 2003, 09:09 PM: Message edited by: russellroberts ]