petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
something posted on the automation email list
From: Hugo Ahrens
To: xxxxxxxx @ CONTROL.COM
Subject: Re: ENGR: Resistance to ground measurement
It's been a long time (wrote a paper on this in 1975). If you don't have, or
don't want to buy the specialized ground resistance tester here is what you
need to do what is called the "Fall of Potential" method:
1. Identify the ground you want to test. Measure with a clamp on ammeter if
there is any current flowing (if yes reduce that to below what you can
measure - this may require a total shutdown).
2. Prepare a long cable (#14 is fine) some 200 to 500 feet away from any
metal structures or underground metal, and connect it to a spike pounded
into the ground (usually a foot will suffice). (I think I remember this
correct, that you need to be at least 5 times the greatest distance
measurement of the ground system you are trying to measure). Connect the
wire to the distant ground and leave the other end of the wire isolated,
treat it as if it is hot. With a voltmeter at highest range check the
voltage between the ground to test and the distant ground. If there is any
more than about 2 Volt you will need to shut the site down or find some
other way to reduce that.
3. Connect a current generator from the ground under test and the distant
ground and inject, say 1 to 5 Amp (that must be held steady throughout the
test).
4. With a sensitive voltmeter connected to the ground under test and a short
"roving ground rod" (connected with another long #14 wire), pound the roving
rod in about every 1/10th of the distance between the ground under test and
the distant ground. Measure and record the voltage at each point.
5. Plot a distance vs voltage graph with distance on the x axis. You should
see more or less of an S shape. The inflection point from the middle flat
part toward the higher curve is taken to be the characteristic resistance
(obtained by Ohms law).
Did I say be careful with distant electrode wiring?
From: Hugo Ahrens
To: xxxxxxxx @ CONTROL.COM
Subject: Re: ENGR: Resistance to ground measurement
It's been a long time (wrote a paper on this in 1975). If you don't have, or
don't want to buy the specialized ground resistance tester here is what you
need to do what is called the "Fall of Potential" method:
1. Identify the ground you want to test. Measure with a clamp on ammeter if
there is any current flowing (if yes reduce that to below what you can
measure - this may require a total shutdown).
2. Prepare a long cable (#14 is fine) some 200 to 500 feet away from any
metal structures or underground metal, and connect it to a spike pounded
into the ground (usually a foot will suffice). (I think I remember this
correct, that you need to be at least 5 times the greatest distance
measurement of the ground system you are trying to measure). Connect the
wire to the distant ground and leave the other end of the wire isolated,
treat it as if it is hot. With a voltmeter at highest range check the
voltage between the ground to test and the distant ground. If there is any
more than about 2 Volt you will need to shut the site down or find some
other way to reduce that.
3. Connect a current generator from the ground under test and the distant
ground and inject, say 1 to 5 Amp (that must be held steady throughout the
test).
4. With a sensitive voltmeter connected to the ground under test and a short
"roving ground rod" (connected with another long #14 wire), pound the roving
rod in about every 1/10th of the distance between the ground under test and
the distant ground. Measure and record the voltage at each point.
5. Plot a distance vs voltage graph with distance on the x axis. You should
see more or less of an S shape. The inflection point from the middle flat
part toward the higher curve is taken to be the characteristic resistance
(obtained by Ohms law).
Did I say be careful with distant electrode wiring?
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