Grounding electrode impedance

[karl riley]

Since there is so much attention given to lowering grounding electrode impedance, it is interesting to consider what would be the results of routinely obtaining very low ohms.

Suppose a new system came on the market to get the ohms down to 1 ohm. (Actually, an EMF expert I know has suggested that should be our goal, now that water pipes are becoming plastic).

If this were achieved, every building would dump a percentage of its neutral current into the ground as a parallel path back to the transformer. There would be current flowing in the earth throughout the neighborhood. For livestock a potential would develop between front and rear hoofs. "Stray current" would be a common problem.

Any other ideas about what the results would be?

Is it possible that 25 ohms is actually a good gate-keeper for our system? (except in electronic environments where lightning is a problem).

Karl

 

[dereckbc]

Re: Grounding electrode impedance

How about a system that does not use a grounded circuit conductor?

[ October 15, 2003, 10:48 PM: Message edited by: dereckbc ]

 

[hurk27]

Re: Grounding electrode impedance

Dereck:
Bingo!

I think this way too.
Remove the bond between the MGN and the XO on the transformer,only use grounding electrodes at the service or at the pole not both. if at the pole keep neutrals and Earthing ground separate. let the POCO's use there own earthing grounding electrodes for there MGN only. and Keep the two systems separate.

[ October 15, 2003, 10:54 PM: Message edited by: hurk27 ]

 

[Guest]

Re: Grounding electrode impedance

When there was talk of laying the BART (Bay Area Rapid Transit) tracks many years ago there was much over the fence debate about whether the tracks would go AC or DC. There was much discussion about how one or the other would corrode the neighboring pipes, etc.

I would be an advocate of more double insulation; no grounding electrode; "whole house" GFCI protection; etc.

 

[nvcape]

Re: Grounding electrode impedance

At an older industrial installation, the system had 12KV primary and 480V secondary without grounds, except for a fairly high resistance ground detection circuit. When a accidental ground occurred, it had to be cleared immediately as a second one then conducted. If you are grounded and the system isn't how much current flows if you touch a conductor?

As far as 1 ohm grounds, have seen grounds that were virtually impossible to read with a test set; the "sphere of influence" still holds and you don't read it too far away.

 

[dereckbc]

Re: Grounding electrode impedance

Hurk, I do not think the POCO would set still for being responsible for the customers protection, but the same principle could be used if the customer owned the transformer. Just bond the XO at the transformer, then run your 3 or 4 wire curcuit plus ground. Sounds like industrial.

 

[hurk27]

Re: Grounding electrode impedance

The thing I would like to see removed is the MGN to XO bond at the transformer, and keeping the electrode conductor/neutral bond separate from the meter on. This would eliminate the water pipe problem, and the danger of primary current on the house grounding if there is a loss of the MGN at the transformer.

 

[don_resqcapt19]

Re: Grounding electrode impedance

If you separate the grounded conductor and grounding conductor at the utility transformer, the impedance of the fault clearing path will go up unless you make the grounded and grounding conductors the same size.
Don

 

[brian john]

Re: Grounding electrode impedance

Hurk, I do not think the POCO would set still for being responsible for the customers protection, but the same principle could be used if the customer owned the transformer. Just bond the XO at the transformer, then run your 3 or 4 wire circuit plus ground. Sounds like industrial.


PEPCO, one of many local utilities offered this to small commercial customers as a cost saving means, this worked for 10-15 years till the equipment started to fail. Although PEPCO required regular maintenance, few complied and PEPCO did not track the customers service.

As for ungrounded systems I have given this some thought for years, especially as applied to 480 vac systems, this installation would minimize arcing ground faults in 4 wire 480 vac systems (for distribution only) at the main switch gear and bus duct. Lighting and outlets would be fed through transformers. Draw backs the extra transformers, owners ignoring alarms, lack of maintenance, inexperienced electricians working on the system.

 

[c-h]

Re: Grounding electrode impedance

There have been investigations as to the effect of connecting the neutral to the grounding electrode. Have a look a this page http://www.emfs.info/Source_distribution.asp
maintained by the British National Grid.

Since the voltage is twice that of the US and more customers are served per transformer, the result is similar to lowering the impedance. On the other hand, there is no requirement in the UK to have a grounding electrode, unless it's the sole means of grounding. (I.e. neutral not connected to ground at consumer)

Anyway, they estimate that the net current (the current outside the cable) is on average 15% of the neutral current.

 

[karl riley]

Re: Grounding electrode impedance

Interesting discussion. In some European systems neutral and ground are only bonded at the Tformer. In the UK, GFCIs are used extensively in their "ring" circuits.

But noone responded to the scenario of 1 ohm grounding electrodes. After all, isn't this what some segments of the electrical industry are assuming is best, lower and lower impedances?

I am starting to come to the conclusion that "no less than 25 ohms" might be a good idea to prevent too much current in the earth, with the net current conditions on the service drops and distribution lines. Check that link from the UK site concerning net current on distribution lines.

Karl

 

[hurk27]

Re: Grounding electrode impedance

Don:

If you separate the grounded conductor and grounding conductor at the utility transformer, the impedance of the fault clearing path will go up unless you make the grounded and grounding conductors the same size.
Don

Not the grounding conductor the MGN (primary neutral) to the secondary XO (neutral)

the fault path will still be at the meter base bonding and or the main disconect.

 

[don_resqcapt19]

Re: Grounding electrode impedance

hurk,

Not the grounding conductor the MGN (primary neutral) to the secondary XO (neutral)

Then there would be no fault clearing path for a winding to ground fault on the primary windings.

This would eliminate the water pipe problem

How would this change the "water pipe" problem. If I bond my grounded conductor to a common underground water pipe at my house and you bond it next door the water pipe is still in parallel with the grounded conductor.
Don

 

[bennie]

Re: Grounding electrode impedance

I will offer a different impression on this topic. I will agree with Don, on one part of his suggestion to call grounding , bonding.

The ground electrode conductor is essentially a short to earth, fitting the definition of a bond conductor. The lower the impedance makes a better short to ground.

To equalize electrostatic charges on above ground objects, the lower the impedance means a lower potential difference.

A substantial charge can remain when the path is over 25 ohms.

A lightning, or high voltage charge can remain on metallic material for a long time after the strike.

The higher the resistance makes the earth and the material a capacitor. Discharge this capacitor across a PC board, and the board is history.

After my class on Electrostatics, I am convinced it is important to establish a low resistance(bond) to the earth.

 

[pierre]

Re: Grounding electrode impedance

Bennie
What happened to that "stick in the mud' attitude?

How often is lightning really a factor? Except in places such as Florida of course.
If capacitor charging was/is an issue, would not a fault condition cause a similar condition?

Pierre

 

[hurk27]

Re: Grounding electrode impedance

Don what I was trying to say is if you were to remove the bond from the primary neutral to the secondary neutral at the transformer, you will still have the electrode at the pole for the primary fault and you will have a electrode at the service to protect the secondary from higher voltage. but this would isolate these services from other services fed from other transformers down the street as you said it would not isolate them from services fed from the same transformer. but would cut down on how many services that could back feed the grounding from a lost neutral. as it could only could come from the services that are fed from the same transformer. but as it is now a service with a lost neutral on the next block could back feed the grounding with current as all the services are tied together across the primary neutral to secondary neutral bond.

[ October 20, 2003, 01:40 AM: Message edited by: hurk27 ]