TheRick

That all depends on your definition of effective, how low of a ground resistance are you looking for, or are you simply trying to make it a safe, code compliant install.

I will not argue the fact that further apart is better, but if you already have an effective ground with the first rod, and are simply installing the second to make the inspector happy what is the point of going further than required?

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Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

Zog

To be outside the sphere of infulence of the other rod.

TheRick

I understand why you would want to move out of the sphere of influence if your goal is to lessen the resistance to ground.......but what if the first rod is plenty low, and you are just adding the second to be code compliant?

__________________
Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

Zog

You dont know the 1st is low without testing it. and 99% of EC's dont have the equipment or training to properly test. If one is <25 ohms in most areas you dont need a 2nd rod.

TheRick

In my area the POCO requires 2 at least 8' apart regardless of how low a resistance reading is.

My question is........why is it so many here believe a ground rod resistance tester is such an amazing tool that only a select few can own, let alone know how to operate. They are readily available, and if you can use a clamp on ammeter, you can use one!

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Rick Boudreau
TASK FORCE SAFE INSPECTION TEAM LEADER
Tal Afar, Iraq
"fool-proof systems do not take into account the ingenuity of fools"

Zog

Then testing is a moot point.

You cant test a single ground rod with a clamp on tester. That is one of the many limitations of a clamp on group tester. There are only certian applications where they can be used and even then thier accuracy is questionable. They are misused more often than propery used and are not an acceptable means of groud testing by ANSI or NETA, not to mention most other testing specs for commisioning projects.

Most specs require a 3 or 4 point fall of potential test with at least 3 test points at 52%, 62%, and 72% of the distance to Z. Many times 10 or more points are required and plotted to prove the reading is accurate by showing a plauteu region.

waco

Well, as an old metrologist (calibration technician), I'd be hard pressed to come up with a repeatable,reliable method of measuring ground rod resistance (or conductivity), so I can sort of see why some jurisdictions just want something.

I routinely measure voltage between the GEC and the panel neutral before I hook up the GEC, which is an effective method measuring the "circuit" from PGN to the local earth ground and I can honestly say, I have never seen a panel ground bar have voltage on it once the GEC is hooked up.

Of course, like all science, I might be seeing what I want to see, but I doubt very much two ground rods are any improvement over one.

calimurray

we hardly ever put two ground rods in anymore, we did for awhile but now he just uses one 3/4" 10 foot ground rod and most times it passes inspection, it also I think has allot to do with the ground and how its sediments are!

In some places we are required to use two rods, and the ground wire has to be continuous through both rods, ground lugs, grounding bushings with no breaks in the wire.

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If you look past your nose you might find what your looking for!

Zog

Lots of testing has been done on this:

Total resistance with 2nd rod driven is 50-65% of single rod, depending on spacing

Total resistance with 3nd rod driven is 35%-50% of single rod, depending on spacing

Zog

Driving 2 rods is much more effective (See above post). A 10ft rod only reduces the resistance by about 5% or so, anything longer than 8 ft dosent do much at all. A 3/4" rod only decreases resistance by 5-10% depending on what test data you look at, either way, not much.

brian john

Jobs where we are involved in with ground rods typically we are driving anywhere from 10-300 rods. My feeling on a second rod for services and the proper spacing has to do with anything I do in my profession. If it is worth doing at all it is worth doing right and in this case BEING A PROFESSIONAL I want to try and achieve the lowest resistance with what is required.

On a 400 Amp service you could seat the conductor termination connector (lug) 1/2 on the bus and it would work just fine, but why would you? The correct way is to seat the connector fully on the bus ensuring a low resistance, professional installation, I see the 2nd rod as the same issue.

waco

My argument is that there is no solid science for the net effectiveness of multiple ground rods because there's really no solid science defining the matter of "earth ground" relative to power ground neutral (PGN). I suspect anything that pretty much eliminates any EMF between local "earth ground" and PGN at that location is adequate for both equipment, people, and animals.

I mean, if the conductivity between two ground rods eight feet apart is better than the conductivity between a water pipe or a well casing fifty feet from the ground rod, so what?

Point is, no EMF between the local earth ground and the local PGN.

This on top of my looking at another fire-trap house that should have been torn down fifty years ago. Family lives there that can no more afford an electrician than it can afford another half gallon of milk and I'm supposed to just walk away?

Man....