Ground Resistance Measurement question - Mike Holt's Forum

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! 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?) 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? Me smells a 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. 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 '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. 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... 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? When I worked for an electrical testing company, we tested grounds on new high rise hospitals and 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.)

When I worked for an electrical testing company, we tested grounds on new high rise hospitals and 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?

There is an AEMC model and , and also a Fluke model (?) that are clamp-on ground resistance measuring devices.* About $.* 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.

Originally Posted by michaellabeit I've got a customer who needs their grounding electrodes tested for an inspection (8 electrodes total, in a 4x4 configuration). We do grounding resistance tests but typically for electrodes outside or for setting up mobile power plants, where there's plenty of room for lengthy fall-of-potential tests. The building where the electrodes are located is cramped (see picture). I'm assuming that they are all connected via a grounding grid underneath, but I will verify that once I head out there obviously. Any thoughts on the best method of testing these?