Ground resistice test for ground ring and ground rods

[transman2]

When performing a ground resistence test on mulitipile ground rods, ring and the triad should they be connected and permorm ground resistence test at one of the triad points or should each rod be done before connecting ring to rods?

Thanks,

 

[iceworm]

When performing a ground resistence test on mulitipile ground rods, ring and the triad should they be connected and permorm ground resistence test at one of the triad points or should each rod be done before connecting ring to rods?

Thanks,

Questions:

• What do you want to find out about the grounding system?
• Is there a spec to meet? What is the spec?
• Do you have a mandated test method? Are you using a fall-of-potentional? Are you using a clamp-on ground tester?

Answers to these Q's should tell you what you want to know.

ice

 

[transman2]

I think I'm asking a simple question about a general method. Do you test each ground rod or do you test after you have cad-welded your ring to your rods.

 

[roger]

I think I'm asking a simple question about a general method. Do you test each ground rod or do you test after you have cad-welded your ring to your rods.

You test the rods as an assembly.

Roger

 

[dereckbc]

I can help you out, but to be honest it is almost a futile measurement especially in an urban environment. There are two methods primarily used.

The most accurate method (sort of) is what is called 3-Point Dead Fall of Potential test using a Meggar type high voltage resistance meter. . It involves driving two short ground rod into the dirt at specified distances from the unit under test. However there are three requirements for it to be semi-accurate which is almost impossible to meet.

The length must be 10 times the length of the GES UUT. So if that building has say a 100 foot diameter ground ring you are looking at 1000 feet of cable for P1, and 620 feet for P2.
There cannot be any metallic structure under the ground crossing the path of test field like water and gas pipes.
The test must be performed multiple time from different directions to get a weighted average.

So as you can see it is almost impossible for such a large system. Doable for say a 2-rod system, but a ring, not worth the effort.

OK I do a lot of this for a living in the Telecom sector and I can tell you how to do it in 5 minutes or less. Get your hands on a Clamp-On Ground Rod Tester like a AEMC Model 3731 Ground Tester. I will even sell you one for $1200

Only trick to using it is clamping it around the correct cable so you do not measure a Wire LOOP. Secondly if there is significant return current flowing in the GEC will require turning-off the main breaker, but the Service Neutral must remain connected. If the facility is wired up correctly and has a Grounded Circuit Conductor service you clamp it around either MBJ or GEC that leaves the service disconnect point going to the first electrode. It is not highly accurate but more than good enough to verify if a Ground System meets a spec. It always reads higher than actual impedance. So if the spec calls for 5-Ohms and the meter reads 5 or less Ohms it passes. If you use it be sure to follow the instructions and understand what it tells you.

 

[GoldDigger]

I think I'm asking a simple question about a general method. Do you test each ground rod or do you test after you have cad-welded your ring to your rods.

A lot depends on what the performance of a single rod is, and whether you are required to meet a certain overall resistance or just have a specific type of ground electrode in the GES.

If any one of the rods measures a lower resistance than your specification, you are home free. If you are required to have two or more rods, each with less than a certain resistance, you can still test them separately.
But if you are counting on the number of electrodes to reduce the overall GES to earth resistance, you MUST measure the system as whole. Because each ground electrode may be within the zone of influence of the others, you cannot apply a simple resistors-in-parallel calculation.

Either buy a new or used clamp-on tester as detailed by dereckbc or rent one for the purpose if this is a one-off requirement.

 

[roger]

I can help you out, but to be honest it is almost a futile measurement especially in an urban environment.

I agree Dereck so this leads to a question. As an engineer, why do you think 99% of design enginerers include "Ground Testing" of the GES in their specs when the system includes building steel, rings, triads, metalic water entrances, CEE's, and a kitchen sink?

This is even the case in installations where performace grounding is not really an issue.



Roger

 

[dereckbc]

I agree Dereck so this leads to a question. As an engineer, why do you think 99% of design enginerers include "Ground Testing" of the GES in their specs when the system includes building steel, rings, triads, metalic water entrances, CEE's, and a kitchen sink?

You looking for trouble mister?

I know the answer to that. Because that is the way they have always done it. I know that sounds sarcastic, but it is truthful. It is no secret I am from the Telecom industry and we are real jerks about meeting the "5-Ohm " spec. Honestly in my youth I supported it. Why? Because that is the way it has always been done and was in company engineering standards documents. As I became more wise, educated, and experienced I learned a 5-Ohm impedance is meaningless and useless.

I have busted a lot of telecom manufactures myths about 5-Ohm ground. I ask them what is the purpose of such a low impedance. Guess what answer they give. Because that is the way it has always been done. It took me a couple of decades to find the moron who got that started. Well one day at a conference I met a long time Ma-Bell Power Protection Engineer and he revealed the answer. Remember the days or rotary dial telephones and party lines? Back in those days Telephone Companies used earth as a conductor for Ring Voltage. Ring voltages were applied to either Ring-to-Ground, Tip-to-Ground, and Tip-to Ring. They also used different ring voltage frequencies. So on a Party Line you could ring up to 12 different houses on a single party line. Inside your phone when the telephone company actually installed phones in your home would wire your phone for the correct pair and install frequency selective bandpass filters for your ringer. Mystery solved.

Today that legacy still exist. Except today when a company build a building call out for a 5-Ohm ground so the EC does not just install 2 rods and call it a day.

 

[GoldDigger]

.. Remember the days or rotary dial telephones and party lines? Back in those days Telephone Companies used earth as a conductor for Ring Voltage. Ring voltages were applied to either Ring-to-Ground, Tip-to-Ground, and Tip-to Ring. ... Mystery solved.

Today that legacy still exist. Except today when a company build a building call out for a 5-Ohm ground so the EC does not just install 2 rods and call it a day.

Bravo! I have always wondered myself where that came from, and your story sounds good.

I imagine if you go far enough back in time, the phone installer was the one who drove the ground rod too, if there was none present.
(It could have gone to a rural phone with no POCO AC on the property at all, hence no ground rod.)
Any idea what the maximum ground electrode resistance was to allow proper ringer operation? I would expect that it could have been into the hundreds of ohms, as long as the line resistance component of the ring loop did not get too large.

 

[dereckbc]

Bravo! I have always wondered myself where that came from, and your story sounds good.

Well I have heard it a couple of times now, so I am confident that is where it comes from. Another tid bit of history is where the NEC 25 Ohm requirement comes from the Civil War Telegraph lines. Telegraph was a single wire and they used earth as a return conductor.

Any idea what the maximum ground electrode resistance was to allow proper ringer operation? I would expect that it could have been into the hundreds of ohms, as long as the line resistance component of the ring loop did not get too large.

Not exactly but pretty high as 26 AWG twisted copper has a DCc resistance of about 130 Ohms/Kilometer and they can run circuits 10,000 feet or about 3 Kilometers before they have to use Repeat Coils. So if i had to guess 500 Ohms max. Long before my time.

Funny there are still some party lines around in extreme rural areas, but all the equipment has changed and they just use selective frequency ringing now of 15 to 50 Hz, or ring coding that rings everone on the line and you have to learn to recognize your ring sequence. That would be rare today as most party lines today is no more than 4 numbers per line.

 

[petersonra]

Well I have heard it a couple of times now, so I am confident that is where it comes from. Another tid bit of history is where the NEC 25 Ohm requirement comes from the Civil War Telegraph lines. Telegraph was a single wire and they used earth as a return conductor.

I seem to recall reading somewhere that originally that single wire was made of iron.

 

[zbang]

I must be old, I thought grounded and frequency-selective ringers were common knowledge . There was also the half-wave variation, put a diode in series with the ringer. I think I still have the BSPs that detail all this.

 

[dereckbc]

I must be old, I thought grounded and frequency-selective ringers were common knowledge .

Don't feel alone, I am mid 50's and know about them. I remeber as a young kid growing up on the outskirts of Tulsa we had a party line with coded ringing. Ours was to short and one long ring or Morse Code U ..-.

 

[GoldDigger]

Don't feel alone, I am mid 50's and know about them. I remeber as a young kid growing up on the outskirts of Tulsa we had a party line with coded ringing. Ours was to short and one long ring or Morse Code U ..-.

And now people only think of selective ring tones as identifying the person who is calling.

At some point, with three wire patterns, two or more frequencies and up to six? coded rings, it is not long before the phone line is busy too much of the time. Although it was not used nearly as much "back then".

 

[zbang]

Don't feel alone, I am mid 50's and know about them.

Early 50's here but a voracious reader of old technology books, including all the BSP's I could get my hands on pre-divestiture and the AT&T "green book", which I have in several additions .

 

[GoldDigger]

Early 50's here but a voracious reader of old technology books, including all the BSP's I could get my hands on pre-divestiture and the AT&T "green book", which I have in several additions .

My favorite, although I never got hold of a copy of my own, was the list of acronyms used. It was more than 40 pages, softbound, and was titled:

Bell Environment Acronym Report - Human User Guide (BEAR HUG)

PS: Possibly "Reference" rather than "Report" ?

 

[transman2]

Thanks guys I think I have a good understading now!