ground rod testing

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Redhorse01

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when is the best time for this????? i mean if its raining will it give a bad reading or does it really matter??????? if anyone know's please lend a hand thanks :confused: :confused: :confused:
 
Redhorse01 said:
when is the best time for this????? i mean if its raining will it give a bad reading or does it really matter??????? if anyone know's please lend a hand thanks :confused: :confused: :confused:
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Unless there is some reason to test them, I would not waste my time. It serves no useful purpose in most cases.

Now if someone else wants to waste THEIR money testing ground rods, they can spend as much as they want on it. But it is not something that a basic electrician is likely to have the equipment or knowhow to do.
 
To answer your question, we, like most companies require 48 hours without rainfall prior to conducting ground rod testing. However, testing under drought conditions may give you unrealistic results also.

There will also be seasonal variotions of earth resistance, depending on where you live. In the midwest we typically see 2x the soil resistance in the winter months compared to summer months.

The IEEE std 142 requires 5 Ohms or less for large commerical or industrial systems and 1 ohm or less for generating or transmission station grounds. The NFPA 70B recommends that ground systems are electrically tested every 3 years.
 
The resistance of a ground rod will vary greatly depending on soil condition, temperature, moisture content, etc. The fact that a ground rod has 25-ohms resistance or less today does not mean it will have the same resistance tomorrow. Why are you testing the ground rod? If you are trying to meet a "spec" test it when the ground is warm and wet. If you are trying to sell additional grounding, test it when it is cold and dry!
If you are using a "fall of potential" meter the rod will have to be disconnected from your electrical system for the test which may make the testing impractical. Mike Holt has a short video on ground Resistance testing which is very interesting.
 
You've discovered the greatest flaw in section 250.56 of the NEC. The earth resistance today will not be the same as it is tomorrow or week from today. There is NO reasonable time to test grounde rod earth resistance.
 
I have tested numerous electrodes and do see seasonal changes, drought to flood changes obviously, but little difference from normal rain to 48 hours later though most specs call for a 48-72 hour dry period. You are in theory testing the depth of the electrode not just the surface conditions. A little rain is not going to sink down a 8-10 feet. But what do I know.
 
im testing all the ground rods on my military base its something that must be done according to military regs but i was asking because i was getting some weird readings while it was raining so i was just wondering becasue it was raining if it would have a possiblity of giving bad reading. also im useing a clamp on ohmeter witch is nice but at the same time its kind of a hit and miss meter because it will say for example 950 ohms on a ground rod but then if i use the vibroground it reads like 6.3 ohms so.... but then again i'll use it on an other ground rod and will have a good reading so i dunno
 
You are using a clamp on? There are many things that can give poor readings and only certian applications when a clamp on can be used. Your best bet is a fall of potential test, when done properly can verify your readings are accurate and not being influenced by the spheres of influence from the rod under test and your current probe (Z).
 
The fall-of-potential meter requires the installation of two additional electrodes a pre-determined distance from the ground under test. This may not be possible or practical in many areas, such as a building surrounded by a paved parking lot. It also requires that the grounding electrode be disconnected from the electrical system for the test, which may not be advisable for an occupied building.
 
haskindm said:
The fall-of-potential meter requires the installation of two additional electrodes a pre-determined distance from the ground under test. This may not be possible or practical in many areas, such as a building surrounded by a paved parking lot. It also requires that the grounding electrode be disconnected from the electrical system for the test, which may not be advisable for an occupied building.
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this is why i have to use a clamp on becasue of the paved areas around the bldg's.
 
zog said:
The IEEE std 142 requires 5 Ohms or less for large commerical or industrial systems
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For what possible reason would this matter at all?
zog said:
and 1 ohm or less for generating or transmission station grounds.
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maybe understandable.
zog said:
The NFPA 70B recommends that ground systems are electrically tested every 3 years.
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If it matters little what the ground resistance is in the first place, why test every three years?

I would buy doing a visual inspection of the GEC and bonding wires once a year for obvious damage, but anything past that seems overkill, absent some specific need to do it.
 
I worked with an old fire alarm tech for a while in 1980. The municipal fire alarm system was 125V DC two wire, but if one wire failed it would go to ground for the return. He had to test the ground rods, if it failed he would pee on it.
One neat thing about the 125 VDC it would go miles and miles on 19 ga copper.
 
petersonra said:
For what possible reason would this matter at all?

maybe understandable.

If it matters little what the ground resistance is in the first place, why test every three years?

I would buy doing a visual inspection of the GEC and bonding wires once a year for obvious damage, but anything past that seems overkill, absent some specific need to do it.
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Earth resistance of less than 5 ohms is critical to eliminate "step" and "touch" potential. Higher resistance on the ground system can effect OCD's due to excessive impedence.
 
If you are relying on grounding to reduce step and touch potential, you will need a resistance of much less than 5 ohms. At 120-volts a 5-ohm resistance will carry 24-amps. How long will it take to trip a 20-amp breaker carrying 24-amps? In the meantime your Grounding electrode conductor, and your grounding electrode will be energized to 120-volts. Sounds like step and touch potential to me. Hopefully your grounding system is also connected back to source, which will provide the low impedance ground fault return path required in article 250. This low impedance path will develop enough current to trip the overcurrent device and eliminate the step and touch potential. At system voltages, grounding provides little in the way of personnel safety, regardless of how low we can get the resistance. We need to accept the fact that grounding is effective only for lightning and high voltage occurrences. At the voltages that are developed in a lightning strike, there is little difference between a 5-ohm and a 50-ohm resistance.
 
I have never seen a ground grid at a industrial plant or utillity at 120V, the application we are discussing here is 15kV-345kV (or higher). I agree that a low ground to earth resistance for a house or a Taco Bell will not matter.
 
zog said:
...the application we are discussing here is 15kV-345kV (or higher)...
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Oh? The only details I've seen from the OP were "ground rods around buildings on a military base". :-?

One of those buildings could be a Taco Bell. ;) :D
 
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