Ground Resistance Measurement question

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outpace

Member
Hello.Imagine a highrise building with an existing ground connection somewhere in the basement, in the middle of the building.* The building is surrounded by paved parking lots.How would I measure the 'ground resistance' of this ground connection?* I am reportedly to ensure it is '5 ohms or less'.* But without being able to sink a few ground testing probes nearby, I have no idea how to accomplish this...Ideas?Thanks!
 

benaround

Senior Member
Location
Arizona
outpace, This is just a WAG, but could you use something else in the area like a metal

sign pole, or anything in that order ??
 

outpace

Member
Something in the area would not assuredly represent the actual building's ground.* I must be able to state (as fact):* The ground resistance at your building ground is x ohms.* (I also thought, if it would be readily possible, do drive a couple rods 'in the open', measure THEIR resistance, bond to those, and then be able to state:* The building ground is (now) less than or equal to x ohms.* It is a nasty little problem!* challenge?)
 

brian john

Senior Member
Location
Leesburg, VA
Chain and water if you have sufficient room to go out a LONG way.

Assume you have driven electrodes then concrete footers, concrete slab and all the water pipe and underground utilities all of this is common to the made electrode. Measuring this is very hard to next to impossible. NOW you could try a clamp on but in my experience this will prove NOTHING due to distribution issues in most buildings..

NOW Why does a site in the middle of a building give a durn about what a metal stick in the dirt 5 floors away is doing?
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Me smells a telephone company or equipment manufacture quoting specs they have no idea why it is required as they just picked up in a Ma-Bell publication.

If you are brave you can locate the buildings service entrance and use a clamp-on resistance tester clamped around the service grounding conductor.

But to John’s point the actual ground impedance is meaningless, especially in a high rise building where you would use the building steel as a ground reference point on the same floor as the equipment forming a equipotential ground plane.
 
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brian john

Senior Member
Location
Leesburg, VA
To add to this have you every done any ground testing?
Do you have the equipment?
And as I said earlier, the distance you would have to go out to mean anything is 1000's of feet (Assuming you are taking into account the building footprint. If you can find the one or two meaningless driven electrodes for this facility it would simplify matters. do the chain and 150-200 feet out at 15% intervals.

Or explain that with a site in the middle of the building this is all like whizzing on a forest fire, DOING NOTHING.
 

outpace

Member
Yes, the smell of 'telco' is indeed in the air!* And yes, it's some spec that was pulled out of a hat (or other dark park place) and now the challenge is to satisfy that spec.* I am not familiar with the 'chain and water' testing method...
 

brian john

Senior Member
Location
Leesburg, VA
Can you locate a single electrode or electrode system?
Have you tried to use the clamp on method, while there are issues with this it MIGHT pacify the end user.

In lieu of driving test probes use chain on the pavement or concrete.

have you ever completed any ground testing in the past?
 

outpace

Member
There is an AEMC model 3711 and 3731, and also a Fluke model 1680(?) that are clamp-on ground resistance measuring devices.* About $2000.* Does anyone have experience with these?* Will they 'do the trick' re measuring 'ground resistance' at the main building ground location?
 

rcwilson

Senior Member
Location
Redmond, WA
When I worked for an electrical testing company, we tested grounds on new high rise hospitals and telephone buildings in downtown Seattle with no bare earth in sight.

The cast iron city water system with all of the fire hydrants and connections to earth was considered to be a massive grid with a resistance to remote earth of about 1.0 ohm. We tested the isolated ground rods for the new buidings to a few fire hydrants, using a two-point test and/or a ductor. This gave a reasonable estimate of the rods' earth resistance with some errors caused by proximity effects.

The rods were typically in the deepest basement of thehigh rise 50-100 feet below street level. (Biggest problem was keeping the test leads from getting kickked off the fire hydrants. I learned to hire the local homeless person from the nearest doorway to watch the leads for me. I'm pretty sure they were the ones kicking the leads.)
 

brian john

Senior Member
Location
Leesburg, VA
When I worked for an electrical testing company, we tested grounds on new high rise hospitals and telephone buildings in downtown Seattle with no bare earth in sight.
To my knowledge this is not an approved method.

The cast iron city water system with all of the fire hydrants and connections to earth was considered to be a massive grid with a resistance to remote earth of about 1.0 ohm. We tested the isolated ground rods for the new buidings to a few fire hydrants, using a two-point test and/or a ductor. This gave a reasonable estimate of the rods' earth resistance with some errors caused by proximity effects.
You have proof of this?

The rods were typically in the deepest basement of thehigh rise 50-100 feet below street level. (Biggest problem was keeping the test leads from getting kickked off the fire hydrants. I learned to hire the local homeless person from the nearest doorway to watch the leads for me. I'm pretty sure they were the ones kicking the leads.)
Were the driven electrodes connected to the building steel? water piping? Rebar?
 

jghrist

Senior Member
There is an AEMC model 3711 and 3731, and also a Fluke model 1680(?) that are clamp-on ground resistance measuring devices.* About $2000.* Does anyone have experience with these?* Will they 'do the trick' re measuring 'ground resistance' at the main building ground location?
These will provide accurate resistance readings if the resistance is large in relation to the resistance of the multigrounded utility neutral system. Unless you can disconnect the building ground from the utility system, the clamp-on grounds are the only practical method. With the building ground connected to the utility system, the "zone of influence" is the size of the utility distribution system. A three-point fall-of-potential measurement requires the current probe to be outside the zone of influence.

I'd review the manufacturer's application requirements and if you can reasonably argue that you meet them, use the clamp-ons.
 

dereckbc

Moderator
Staff member
Location
Plano, TX
If you use the one of the clamp on resistance testers as I mentioned in post # 5 it will tell you if you meet 5 ohms or less requirement. They are not accurate, but can confirm 5 or less. If you clamp the Service Grounded Circuit Conductor or MBJ and it read say 4 ohms, what that tells you is your GES is less than 4 ohm's because what it it reading in the facility GES impedance in series with the utility MGN.
 
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