25-ohms ??

[fallssparky]

Re: 25-ohms ??

Bennie:

The equipment ground conductor system is important and should be circuited as carefully as the active line side. Design should not allow normal mode current or common mode current to flow.

How can this be ? If you put a clamp-on on a cold water pipe where the Water-meter is bonded and jumped, back to the Neutral buss, You will certainly read current going to the earth back to the transformer, the same is true for ground rods they will carry current in proportion to their resistance. To my accounting, thats three pathes back to the source, the Grounded(Neutral)conductor, the water pipe and the ground rods.
In our area Newton Falls, if there is a well all we need to do is to drive two ground rods.
Whether lightning picks the ground rod or not doesnt matter they have to be there, and tied back to the Neutral bus.
As far as your statement about the earth being a resistor this is true, from what I have been able to find out the average soil resistance in the U.S.A. is about 40 ohms, this is average.
As far as the earth goes we have basically turned it into a huge collective neutral. I dont see there not being anywhere where there is not some type of ground current.

Fallssparky

 

[bennie]

Re: 25-ohms ??

Fallsparky: I am sorry my response was not clear...I have stated before that common and normal current in the ground system, is no problem outside. It is a problem at the user end, inside the premises.

System routing should be employed to keep EMF from appearing inside.

This is one of the reasons for single point grounding, and not re-grounding the neutral after the main.

[ April 09, 2003, 02:15 PM: Message edited by: bennie ]

 

[tpwd]

Re: 25-ohms ??

it is apparent that most of the participants in this chat form are electricians and contractors, these discussions can only help each of us..but..some of you don't realize that inspectors work for cities, state and county goverments and engineers...and are required by law to enforce public adopted ordinances and/or documents. i agree that there are lots of "so called inspectors" that make up their own rules when they want to...but, not all inspectors are doing that...when you are in a court room, you learn the codes and laws real well...so, if a contract document says to not allow more than 25-ohms on a grounded service...keep trying...you can get it if you know what your are doing and how to get it.

 

[iwire]

Re: 25-ohms ??

Originally posted by tpwd:
yes...all the electrical engineers that i have talked with [over 25] say they want 25-ohms or less on services?....so we enforce them...

tpwd,

Most of us know that cites and towns can have ordinances that must be enforced but they must really be adopted by the town, not decided by the inspector who "talked to over 25 engineers" that want a certain value.

 

[roger]

Re: 25-ohms ??

Tpwd,

if a contract document says to not allow more than 25-ohms on a grounded service...keep trying...you can get it if you know what your are doing and how to get it

as a city inspector you have no business enforceing a spec. If the contract documents states all materials will be American made, do you inspect for this? If you did, you stand a good chance of getting burnt.

The specifications would be enforced by the engineers.


Roger

[ April 09, 2003, 06:19 PM: Message edited by: roger ]

 

[tpwd]

Re: 25-ohms ??

actually i am a state inspector...and engineers are designers, not inspectors...only a limited few know much about NEC requirements...the NEC requires the person responsible for enforcing the code is the AHJ...[inspector hired to enforce]..not the designer either engineer or architect.....

 

[roger]

Re: 25-ohms ??

Tpwd, your words.

the NEC requires the person responsible for enforcing the code is the AHJ...

This would mean you are only concerned with the NEC and any state amendments. If you are concerned with doing a punch list for the engineer or architect or enforceing their intended specifications, you should be employed by them.

As a government employee you are dancing on "conflict of interest" supporting their whims.

Roger

 

[iwire]

Re: 25-ohms ??

Yes that's right, you are to enforce the NEC
and any adopted ordinances.

Unless you State has an ordinance that says you are to enforce job specs I believe you are overstepping your authority.

[ April 09, 2003, 07:33 PM: Message edited by: iwire ]

 

[timbo]

Re: 25-ohms ??

I understand the 25 ohm rule (250.56) that applies to rod, pipe and plate electrodes. But what is the minimum resistance to ground for a ground ring around a Substation?

 

[dereckbc]

Re: 25-ohms ??

Timbo, I have never seen a ground ring in all the sub-stations I have workded in. I see ground grids. The grid forms a equipotential ground plane which minimizes step-rise potential during phase to ground faults. I don't know if the NESC has a target minimum impedance to earth or not. What is important is that all the structures within the sub-station fenced area and extending 7 feet beyond the fence line be bonded to the grid.

 

[timbo]

Re: 25-ohms ??

Thanks Dereck for your quick response. And yes I am searching for ?THE? maximum allowable resistance to ground specification for a Ground Grid. Believe it or not, we have a customer that ?wants .5 ohms or less?.

 

[dereckbc]

Re: 25-ohms ??

Code requirement is one thing, specification is another. If the customer specified requirements exceeding code or industry and is paying for it, then that is what they should get.

It would be wise to get survey or testing done from someone that specializes in ground systems like XIT to obtain what is going to be needed to obtain .5 ohm's or less. It would be embarressing and expensive to build a grid to come to find out it measured 10 ohm's after completion.

The survey can be as simple as soil samples sent into the agency or a in depth site survey.

[ May 21, 2003, 07:03 PM: Message edited by: dereckbc ]

 

[brian john]

Re: 25-ohms ??

I do numerous earth ground resistance test in clay, loam, sand, and ect…………..Day to day measurements seldom change season to season changes due occur as do year to year. Depending on weather. The 25-ohm is a starting point and was set for that only.
In our area with mostly clay type soil, we almost always get under 25 ohms. Why care,
Because lighting does occur, and primary to secondary faults do occur. AS for testing your own grounds owning an earth resistance tester is only a start one has to understand the principles of testing electrodes, there is in this test more to the test than randomly driving test probes and taking a measurement.

 

[michael nye]

Re: 25-ohms ??

25 ohm to what what type of tester and how do you test this, sorry for the beginner question?

 

[karl riley]

Re: 25-ohms ??

Brian, have you ever included a way to verify or test the resistance of the earth itself, since you are testing the impedance from the rod/earth connection? Have you moved the second electrode out different distances to see what effect this has? I would be interested in hearing your results.
Karl

 

[bennie]

Re: 25-ohms ??

I am curious as to how the impedance changes with depth of the rods. Also how it changes after being in use for a period of time.

 

[arneykaner]

Re: 25-ohms ??

I would like to adress three issues:
1. 25 ohms hard to achieve
2. Why should we care?
3. Why engineers insist on 25 or even less.

1. 25 Ohms is not hard to achieve: I am native form Russia, worked in Ural Mountain area, wich is a rock and hard rock only, and we always achieved the Russian national standard of not
exceeding 5 ohms. Just follow a Russian standard
of driving electrodes at least 10 M (30 FT) apart
and put an underground greed between them - of course, it should be in the contractor's budget.

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.

3. The engineers are liable under the State licensing laws for due diligence on public safety.
I would rather pay for a change ordrer, than risking the State licensing detartment invetigation ( I am licensed in IL and WI).

 

[brian john]

Re: 25-ohms ??

Generally customers wait to after the site is built and request a three-point earth resistance test, in lieu of performing a 4-point earth resistance test prior to building the site. The advantage of the four point test is one can make a reasonable determination of the earth resistance over the area of the proposed grounding electrode and the surrounding area.

Any testing requires knowledge of the limits of the meter utilized. A common mistake is trying to measure the complete electrode system, including the neutral/grounded conductor back to utility transformer. The customer generally wants the resistance of the separate electrodes, i.e. driven rods, water pipe ect….. Proper testing specs from the customer help spell out what the customer requires.

A specialized meter often referred as an Earth Megger, three point earth tester or just earth resistance tester, Biddle and AEMC both make a good tester. Lately several companies have developed a clamp on tester, this tester is a specialized meter that is only good in certain applications.

From the day the ground rods are driven, the grounding system starts to deteriorate, on going testing can verify the condition of the ground electrodes. Lightning hits add to the deterioration, different soil type is more prone to damaging some types of electrodes. This is a good reason why the type of electrodes utilized should not be just a given but spec’d for the soil type there are a wide variety to choose from, galvanized, copper clad, stainless steel, or a chemical rod.

Deeper electrodes are generally better electrodes, more soil contact, below the freeze line, into permanent moisture level, below possible fill dirt ect……………..

 

[paul renshaw]

Re: 25-ohms ??

Quote "it only wants to get to the earth (it's opposite pole) and will go through anything to get there." Hurk27, Lightning is not trying to reach the earth , it goes after items that have become charged to an opposite potential. Earth potential is what you are putting at the rods, masts, etc for protection, not attraction. Lightning has a lot to do with proximity.

 

[paul renshaw]

Re: 25-ohms ??

As far as the rods go in our area, the inspectors let 1 rod go all the time. I have tried to address this situation, and was told it has always been this way and it is okay. I use ufer grounds if possible, but if not, drive two rods.