25-ohms ??

[don_resqcapt19]

Re: 25-ohms ??

arney,

2. The less resistance the grounding path has,the faster and steeper the voltage dissipation into the Earth, and the less chances that somebody would be hit by "pace potential" - a difference in potentials between two points on ground, about a normal step apart.

I assume that you are talking about lightning voltage in this statement.
Don

 

[brian john]

Re: 25-ohms ??

Actually,this statement applies to electrical distribution as well as lightning. In substations there are design standards/codes that limit the touch and step potential. The grounding grids must meet the design standards/codes and are tested to these standards/codes.

 

[harryg]

Re: 25-ohms ??

http://lightningeliminators.com/Tech_Papers/Green%20Grounding%20Manual.pdf web page

 

[don_resqcapt19]

Re: 25-ohms ??

Brian,
The only point of my question is to point out that the grounding electrode system provides very little change in the step/touch potential on a building system fault. I don't even think that the grounding electrode system, itself, is that big of a part in reducing the substation touch potential. The conductive grid just under the surface of the gravel in the substation is what actually limits the step potential within the substation.
Don

 

[bennie]

Re: 25-ohms ??

NESC/IEEE Standard 80, a 368 page manual, gives all the procedures for grounding substations.

When anyone addresses ground impedance, it is usually assumed to mean the resistance between the rod and the earth.

Lightning charges like a variety of impedance values. They are not particular.

Using the earth as a ground plane is to provide protection during inadvertant faults, and overvoltages.

The laterial impedance is the important consideration. The earth is not a bond(short)a ground grid is a bond(short). Multi-point grounding connections, to a grid, will not produce high differences in potential as the earth.

The ideal ground grid is a sheet metal plate over the entire area needing an equi-potential plane.
This is not feasable, but high voltage gradients will not be present during current flow.

 

[arneykaner]

Re: 25-ohms ??

Originally posted by don_resqcapt19:
arney,

2. The less resistance the grounding path has,the faster and steeper the voltage dissipation into the Earth, and the less chances that somebody would be hit by "pace potential" - a difference in potentials between two points on ground, about a normal step apart.

I assume that you are talking about lightning voltage in this statement.
Don

Don,

Yes, and also a high voltage (especially above 35 KV) hitting the ground - that is why highvoltage substation design always includes a burried grounding grid calculations.

 

[bphgravity]

Re: 25-ohms ??

Relating to the 25-ohms requirement; this proposal has been rejected.

Proposals Report of CMP-5 ROP 5-128 250.52(A)(5) Rod and Pipe Electrodes ROP 5-133 250.(A)(6) Plate Electrodes ROP 5-144 250.56 Resistance of ? Proposals to require an minimum of two electrodes and remove the 25 ohm resistance requirement were rejected.

Does this imply that the intent of the code is indeed to require 25-ohms even after two electrodes are installed?

[ May 28, 2003, 05:17 PM: Message edited by: bphgravity ]

 

[arneykaner]

Re: 25-ohms ??

Originally posted by don_resqcapt19:
Brian,
The only point of my question is to point out that the grounding electrode system provides very little change in the step/touch potential on a building system fault. I don't even think that the grounding electrode system, itself, is that big of a part in reducing the substation touch potential. The conductive grid just under the surface of the gravel in the substation is what actually limits the step potential within the substation.
Don

Don,

I agree with you that the effect is not decisive
with 480V system. Probably, my preference for driven electrodes vs. building steel, water pipe, etc. has been originated way back in my engineering school and my experience back in Russia. In general, in Russia and Europe driven electrodes and underground grid are preffered method of grounding - it is more expensive, but been argued to be more reliable and having creating less stray current through the building elements - concrete reenforsment steel rods and piping.

Also, as an engineer I am not comfortable with 25 ohms - I strongly prefer not to exceed 5. But this is my personal( and, probably, not impartial) opinion.

 

[pierre]

Re: 25-ohms ??

I just joined this forum recently, so I am reading this a little late, but I have something to add if anybody might come back to this post.

Sometimes we all get so caught up in our thoughts that we loose thought of what is really going on.
The Supplemental Electrode (most often a ground rod, because a ground rod is the most economical solution)is only required if the primary grounding electrode is the waterpipe. If we do not use the waterpipe for the primary grounding electrode, the supplemental electrode is not necessary or required. The reason for supplementing the cold water is
1. sometimes when the waterpipe is being repaired, it is replaced with plastic.
2. How many times have we seen the cold waterpipe with the grounding electrode conductor disconnected, usually after plumber has performed his work and either forgets to or just does not replace it.
By the way, the supplemental bonding connection is NOT PERMITTED TO BE CONNECTED TO THE COLD WATERPIPE !!! 250.53(D)(2) the last sentence in that paragraph dictates where the connection can be made. If the grounding electrode conductor should be disconnected from the cold waterpipe, at least the bond would be connected ahead of the disconnected GEC connection to the cold waterpipe. Also this is why the supplemental electrode is permitted to be installed into the panel independently of the cold water electrode, and be bonded to the same bus.

Not trying to be a pain in the butt,
Pierre

 

[bennie]

Re: 25-ohms ??

Pierre: The supplemental electrode is a made(artificial) electrode. When no other electrodes are available the made electrode is required, by default.

When a water pipe is available, the conductor to the pipe, is the only one that can be connected to the neutral bus of the service. The neutral bus is not a ground electrode bond.

Electrodes must be bonded(shorted)at the electrodes, not at the supply point(neutral bus).

Read 250.53(E). Where does it state "the supplemental electrode can not connect to the water pipe. This section appears to require the bond(short) be made at the water pipe, there's no other place for a bond.

You are not a pain Difference of opinion creates interest.

 

[bennie]

Re: 25-ohms ??

A bond conductor is a short. A ground electrode conductor is an inductor.

Two ground electrodes with individual conductors(inductors), constitute two ground(earth) connections, therefore two ground electrode systems.

Two ground electrodes, with one conductor from the neutral bus, and bonded(shorted) to the second electrode, constitute single point grounding and one ground electrode system.

This is from Engineering Standards, and Federal Specs. Anyone can argue against this opinion, but please give substantiating documentation.