Typical Ground Resistance Value

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garnerm

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Location
Montgomery, AL
:?What should the service entrance ground resistance for a school (any type) measure in ohms? Just a typical. Also, typical, how much would that increase per say ( i know there are alot of varaibles to consider) if you were to go downstream to a branch panel?

My situaltion: School 40+ years old, all original equipment. Replacing the Gym lighting. Feeding a new 277/480V panel from a panel in the same nearby elec. closet for all new lighting circuits. This existing panel has no ground bus. The panel is fed with 3 hots and a netural. The conduit (runs underground) is being used as the grounding electrode conductor. The panel that feeds this one is also the same way, which is fed from the Main Switchboard. This switchboard does have a service entrnce ground from the transformer. The netural and ground were not bonded together, I will be installing a mian bonding jumper at the main and seperating all the grounds from the neturals down stream. Problem: My measure ground resistance at the main switchboard is less than 1 ohm, my measure resistance at the furtherest panel (where my gym is), was measured at over 180 ohms. Is this normal/okay? I think I need to add some additional ground rods due to the age of the conduit feeders. Thoughts are much appreciated.:?
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I don't think it being a school matters any. The NEC rules for how "good" of a connection to earth are the same as any facility, as are what constitutes the required grounding electrodes.

I think a big problem you have is that if they really used a conduit as the grounding electrode conductor, that is not code legal, and AFAIK it never was. based on your description, I suspect the conduit is being used as an EGC and not a GEC.

Before I went in and messed around any I would be doing a survey of just what is actually there and finding out what all needs to be done to make it code compliant. It may turn out to be a lot more work than you at first imagined and you may need to get permission to spend the school district's money to fix it correctly.

I think the neutral bond probably needs to be made at the switchboard or at the xfmr. what you are calling the "service entrance ground", is probably really the GEC.

There is no reason you cannot add a ground bus to an existing panel.

Where are you measuring the 180 Ohms from and to what, and how? It is entirely possible that you have some current flow in your EGC and that may affect the reading of a standard VOM.

If the EGC that is the UG conduit really is no longer an effective EGC, adding ground rods will serve no purpose. You need an effective EGC back to the source in the event of a ground fault. If you really have 180 Ohms from one end of the UG conduit to the other, you have to deal with that issue. But, I would not trust the reading from a std VOM.
 
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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
You seem to be mixing up system grounding (earthing) with equipment grounding (bonding). They are two totally different things and the latter has nothing to do with dirt. Connecting the system to earth via a ground rod or whatever is done only at the service equipment and generally serves several comparatively minor purposes. IT is equipment bonding that you seem to have a potential issue with here - providing a low impedance path from those remote panelboards to the service equipment. What you want is to measure the resistance of the pipe run and check the tightness of the connections you have access to - if you have reason to believe there is some sort of issue with the ground fault path. As petersonra said, common to have to add a ground bar in older equipment where there were no EGC's pulled and the conduit system is the EGC
 
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garnerm

Member
Location
Montgomery, AL
petersonra said:
Where are you measuring the 180 Ohms from and to what, and how? It is entirely possible that you have some current flow in your EGC and that may affect the reading of a standard VOM.
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It could be possible, I did not measure this myself. An electrican measured this on the site. The 180 Ohms was supposedly the earth resistance of the conduit being used as the equipment ground which is bonded to the grounding electode system at the main switchboard.
 
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garnerm

Member
Location
Montgomery, AL
electrofelon said:
You seem to be mixing up system grounding (earthing) with equipment grounding (bonding). They are two totally different things and the latter has nothing to do with dirt. Connecting the system to earth via a ground rod or whatever is done only at the service equipment and generally serves several comparatively minor purposes. IT is equipment bonding that you seem to have a potential issue with here - providing a low impedance path from those remote panelboards to the service equipment. What you want is to measure the resistance of the pipe run and check the tightness of the connections you have access to - if you have reason to believe there is some sort of issue with the ground fault path. As petersonra said, common to have to add a ground bar in older equipment where there were no EGC's pulled and the conduit system is the EGC
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That is correct. We will do that. I think ultimately we are going to run another homerun to the panel to pull a seperate grounding electrode conductor to the main service entrance panel and terminate it to the ground bus so that we have a direct connection to the grounding electrode system. The problem is that the school board does not have the money to pay for the EC to repull new feeders with a ground. All these problems were discovered once the renovation of the Gym project was already underway so now we are just trying to come up with the most cost effictive solution to help them and correct some problems on the along the way.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
garnerm said:
That is correct. We will do that. I think ultimately we are going to run another homerun to the panel to pull a seperate grounding electrode conductor to the main service entrance panel and terminate it to the ground bus so that we have a direct connection to the grounding electrode system. The problem is that the school board does not have the money to pay for the EC to repull new feeders with a ground. All these problems were discovered once the renovation of the Gym project was already underway so now we are just trying to come up with the most cost effictive solution to help them and correct some problems on the along the way.
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Once again, I would point out that a GEC is NOT going to help you.

You need a low impedance bonding path back to the source. And code requires it to be run with the circuit conductors. You can't get past that requirement by running a seperate green wire anywhere.

If the UG conduit is messed up enough that it is no longer an effective ground fault clearing path, you have to fix it right. Trying to put a bandaid on it is not the answer. The only way you fix that is to replace the UG conduit or somehow run a green wire through it and that is likely to be a huge problem.

You really need to tell the school district the truth and ask for money to investigate just what is there. Given your apparrent confusion over what is required, and how to fix it, it might be a good idea to get someone who is less confused over the actual requirements involved.

You really need to first determine with some certainty whether or not the UG conduit is still an effective bonding means or not. 180 Ohms sounds like a lot but not knowing how it was measured, it is hard to say with any certainty.

I would suggest some weekend when you can shut the power off that you shut off the building power, install bonding locknuts on each end of the UG conduit, disconnect whatever N-G bond(s) you might have and measure the resistance in the UG conduit with a ground integrity tester between the two bonding lock nuts. It is sort of a megger type system.

You cannot do this for free no matter how broke the school district is. You need to be upfront with them about how expensive a proper repair could be and get them to front the money to figure out just what really needs to be done.

If I had to guess, there is probably nothing wrong with the UG conduit that prevents it from being an effective ground fault path. 180 Ohms is not much, and while it is a lot more impedance than an effective clearing path would have, it does not seem like an open circuit. More like a bad connection somewhere or a measuring error.
 
zog

zog

Senior Member
Location
Charlotte, NC
garnerm said:
It could be possible, I did not measure this myself. An electrican measured this on the site. The 180 Ohms was supposedly the earth resistance of the conduit being used as the equipment ground which is bonded to the grounding electode system at the main switchboard.
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180 ohms using what method? What test equipment?
 
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garnerm

Member
Location
Montgomery, AL
petersonra said:
Once again, I would point out that a GEC is NOT going to help you.

You need a low impedance bonding path back to the source. And code requires it to be run with the circuit conductors. You can't get past that requirement by running a seperate green wire anywhere.

If the UG conduit is messed up enough that it is no longer an effective ground fault clearing path, you have to fix it right. Trying to put a bandaid on it is not the answer. The only way you fix that is to replace the UG conduit or somehow run a green wire through it and that is likely to be a huge problem.

You really need to tell the school district the truth and ask for money to investigate just what is there. Given your apparrent confusion over what is required, and how to fix it, it might be a good idea to get someone who is less confused over the actual requirements involved.

You really need to first determine with some certainty whether or not the UG conduit is still an effective bonding means or not. 180 Ohms sounds like a lot but not knowing how it was measured, it is hard to say with any certainty.

I would suggest some weekend when you can shut the power off that you shut off the building power, install bonding locknuts on each end of the UG conduit, disconnect whatever N-G bond(s) you might have and measure the resistance in the UG conduit with a ground integrity tester between the two bonding lock nuts. It is sort of a megger type system.

You cannot do this for free no matter how broke the school district is. You need to be upfront with them about how expensive a proper repair could be and get them to front the money to figure out just what really needs to be done.

If I had to guess, there is probably nothing wrong with the UG conduit that prevents it from being an effective ground fault path. 180 Ohms is not much, and while it is a lot more impedance than an effective clearing path would have, it does not seem like an open circuit. More like a bad connection somewhere or a measuring error.
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Thank you everyone for you responses. I have study this issue with your responses and now have a tens better understanding of the issue. I totally agree with you petersonra, I am the one who will handle this because i do understand this now. I will not allow the seperate ground to be run back to the main switchboard. We will test the integrity of the conduit feeder to be used as the EGC as detailed in your response. Once verified that this conduit is in good shape I will feel better that we have an effective path of grounding back to the grounding electrode system at the main switchboard. We we seperate the grounds from the neutral bus bar and install a new ground bus bar. The conduit does have a bonding bushing on it which we will bond to the ground bus bar. Again, thank you all, thats why i love this site!!!
 
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garnerm

Member
Location
Montgomery, AL
zog said:
180 ohms using what method? What test equipment?
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He used a earth Ground Clamp meter. i now understand how this meter cannot be used for this application and was effictive used in the wrong method, therefore the reasoning for a wired number. We will use the correct meter to test the reistance of the conduit itself.
 
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