Our brilliant State leadership has taken a stand in regard to 250.56 that it is the inspectors responsibility to show 25 ohms resistance or less before requiring an additional electrode. I understand that this is more a "policy" question, than a Code question, but is it an inspectors responsibility in any of your areas ?
25 ohms or less
- Thread starter augie47
- Start date
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roger3829
Senior Member
- Location
- Torrington, CT
At a training session our state electrical inspector told us he has checked many areas in the state and was unable to get the 25 ohms with one rod. Even with 2 rods most of the state can not achieve the 25 ohms. So it's automatic that we install 2 rods. WE would have to prove it was 25 ohms with one rod if we didn't want to drive 2.
- Location
- Logan, Utah
The code is not there just to satisfy the inspector. How does the electrician know whether or not the single rod meets the requirements of 250.56 if they don't do the test themselves?
In my area if ground rods are used (very rarely) then 2 rods are installed becasue it is the cheapest and easiest method to meet the requirement of 250.56.
Chris
In my area if ground rods are used (very rarely) then 2 rods are installed becasue it is the cheapest and easiest method to meet the requirement of 250.56.
Chris
No problem, just buy you a $2000 clamp on ground resistance meter and you are good to go.
barbeer
Senior Member
- Location
- Pinellas County, FL
No problem, just buy you a $2000 clamp on ground resistance meter and you are good to go.
With what budget?
That is a crappy policy your state leadership has taken. IMHO that is.:smile:
zog
Senior Member
- Location
- Charlotte, NC
No problem, just buy you a $2000 clamp on ground resistance meter and you are good to go.
Clamp on measurements are limited in thier applications and are done incorrectly more often than correctly, best to do a 3 point fall of potential if you need to test your ground resistance, but in most cases it is not worth the time and effort (Unless you have a specific spec that needs to be met) and drive a 2nd rod.
Most EC's dont have the equipment, training, or experince to properly measure ground resistance so I think this policy is a joke.
zog
Senior Member
- Location
- Charlotte, NC
Clamp on ground readings
Clamp on ground readings
Before everyone gets on my case, here is why:
1. How it works: It sets up a current in the ground circuit by magnetic induction. It then measures the current and voltage drop to determine the resistance. Ideally the ground being measured is a ground rod in a system of many other ground rods.
2. Pitfalls and Tips: Like any other instrument it has limitations. Please read the manual. There are a few real important items to notice like...
a. There MUST BE AT LEAST TWO EARTH GROUNDS (ground rods) in the ground circuit for it to work properly. If you try to measure one ground rod, it will read OL (out of range). The meter's limit is 1200 OHMS, after that it will read OL. OL means that the ground circuit is open or the ground resistance is over 1200 ohms. A tactical ground should be far less than 1200 ohms, more like 25 ohms.
b. Also, the fewer ground rods in the circuit, the less accurate it is. This is because a clamp on ground tester uses other ground rods in the system (added in parallel) as a reference. If there are only two ground rods, the reading you are getting is the SUM of the two resistances. It is not possible to tell (with this instrument) if one ground rod has a greater resistance than the other when there are only two ground rods in the circuit.
c. There is a warning on the meter that reads R<1 OHM. If this is on, the reading is inaccurate and you are measuring a closed circuit conductive loop.
d. There is a warning on the meter that reads "NOISE". If this is on, there is over 50V or over 5A on the ground circuit. There is probably an electrical fault condition if this is true - power down the system and render it safe until the problem can be corrected.
e. Another pitfall is that the conductive contacts in the jaws get dirty. Wipe them frequently with a SOFT lint-free cloth. If the contacts are dirty, the meter will indicate "jaws open" or possibly read OL.
Clamp on ground readings
Before everyone gets on my case, here is why:
1. How it works: It sets up a current in the ground circuit by magnetic induction. It then measures the current and voltage drop to determine the resistance. Ideally the ground being measured is a ground rod in a system of many other ground rods.
2. Pitfalls and Tips: Like any other instrument it has limitations. Please read the manual. There are a few real important items to notice like...
a. There MUST BE AT LEAST TWO EARTH GROUNDS (ground rods) in the ground circuit for it to work properly. If you try to measure one ground rod, it will read OL (out of range). The meter's limit is 1200 OHMS, after that it will read OL. OL means that the ground circuit is open or the ground resistance is over 1200 ohms. A tactical ground should be far less than 1200 ohms, more like 25 ohms.
b. Also, the fewer ground rods in the circuit, the less accurate it is. This is because a clamp on ground tester uses other ground rods in the system (added in parallel) as a reference. If there are only two ground rods, the reading you are getting is the SUM of the two resistances. It is not possible to tell (with this instrument) if one ground rod has a greater resistance than the other when there are only two ground rods in the circuit.
c. There is a warning on the meter that reads R<1 OHM. If this is on, the reading is inaccurate and you are measuring a closed circuit conductive loop.
d. There is a warning on the meter that reads "NOISE". If this is on, there is over 50V or over 5A on the ground circuit. There is probably an electrical fault condition if this is true - power down the system and render it safe until the problem can be corrected.
e. Another pitfall is that the conductive contacts in the jaws get dirty. Wipe them frequently with a SOFT lint-free cloth. If the contacts are dirty, the meter will indicate "jaws open" or possibly read OL.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Washington State puts the responsibility on the EC. The rule says that if a test is not performed to ensure the resistance is under 25, then a second ground rod must be installed.
I think that is how we should do it too. I was trieng to be funny.
charlie
Senior Member
- Location
- Indianapolis
I think the 2011 code will just go ahead and require the installation of two ground rods and end the confusion. :smile:
IMO, it is the EC's responsibility to provide all required testing results.
In the case of the 25 ohms or two rods, it cost us more money to break out the test set and do a three point fall of potential test than to drive the second rod.
When it comes to testing medical installations, we have to provide all test reports/results (with the exception of medical equipment itself) for the final inspection, if we were to tell the AHJ that this is their responsibility we would be waiting a long time for a final. I don't see other required testing being any different.
Roger
In the case of the 25 ohms or two rods, it cost us more money to break out the test set and do a three point fall of potential test than to drive the second rod.
When it comes to testing medical installations, we have to provide all test reports/results (with the exception of medical equipment itself) for the final inspection, if we were to tell the AHJ that this is their responsibility we would be waiting a long time for a final. I don't see other required testing being any different.
Roger
zog
Senior Member
- Location
- Charlotte, NC
That will be great
zog
Senior Member
- Location
- Charlotte, NC
How about require 3 rods, and they need to be made in the USA by a US steel producer.
Gus, I don't think the State should be the competion of the private sector's business, also,
I don't see why the State would want the liability to end up on their lap?
An Inspector is a person who enforces the codes, etc., etc., and is a witiness to such
tests, not the person to perform such tests. What's next, emg. lights, generators, -----
I don't see why the State would want the liability to end up on their lap?
An Inspector is a person who enforces the codes, etc., etc., and is a witiness to such
tests, not the person to perform such tests. What's next, emg. lights, generators, -----
brian john
Senior Member
- Location
- Leesburg, VA
I am pushing for 3 rods, I recently bought stock in a company and...
As for Augie's post seems you state needs to get REAL. Who is the governor an EC?
Actually it seems they should have just said one rod and go.
Or the inspectors pull out a multimeter (wrong tool for the job) and say reading above 25 ohms 2nd rod please.
I don't see any method specified in NEC 250 to indicate how you must come up with the resistance value. A fall of potential test is certainly a recognized method. But what about just connecting your ungrounded conductor (via an overcurrent device) to the rod and measuring the current that flows (but don't stand too close to the rod!)? Ohms law and you're done.
This would come up with an easy to measure resistance. And it is probably just as good of an answer since the 25 ohms seems to be an arbitrary value (I'm in the Benny camp there).
This would come up with an easy to measure resistance. And it is probably just as good of an answer since the 25 ohms seems to be an arbitrary value (I'm in the Benny camp there).
stickboy1375
Senior Member
- Location
- Litchfield, CT
Personally, just do a Ufer and forget ground rods all together.
brian john
Senior Member
- Location
- Leesburg, VA
And in reality there is no proof that any of the electrodes are any good unless you have tested them. But testing them makes little or no sense for many applications as there is no requirement for a minimum acceptable resistance.
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