How to calculate resistance of grounding system?

T

tml1010

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Location
Texas
Occupation
EE
I have a customer that needs a ground bar with a resistance of less than 10 ohms to connect to for sensitive electronic devices. The building has a ground ring installed so my thought was to take an insulated wire from the ground bar to the ground ring. For low frequencies (60Hz) is the calculation just adding the resistance of the wire to the ground ring, or am I missing something? I am assuming they would act like resistors in series but haven't actually had to show a calculation before to prove it.

For reference I am using the tables in the NEC for the resistivity of the wire, and IEEE STD 142 (Green Book) to calculate/guess the resistance of the ground ring.
 
tml1010 said:
I have a customer that needs a ground bar with a resistance of less than 10 ohms to connect to for sensitive electronic devices
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What type of electronics?

Other than government funded facilities,because cost is no object, i don't think i have ever seen this general of a request. The few times it has been needed, the project specifications were very detailed.
 
The resistance of the wire does not play any part in this. You need to do ground resistance testing on the grounding electrode system.
 
mayanees said:
.....from the new ground bar, with a 4/0 copper bonded to the Grounding Electrode System from the ground bar. Me thinks!
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Every spec like this that I have seen was looking for 10 ohms or less of ground resistance. The resistance of the contact path between the grounding electrode system and the earth is what they would be looking for. The resistance of the wire would be 10s to 1000s of times less than the ground resistance of the grounding electrode.
 
tml1010 said:
I have a customer that needs a ground bar with a resistance of less than 10 ohms to connect to for sensitive electronic devices. The building has a ground ring installed so my thought was to take an insulated wire from the ground bar to the ground ring. For low frequencies (60Hz) is the calculation just adding the resistance of the wire to the ground ring, or am I missing something? I am assuming they would act like resistors in series but haven't actually had to show a calculation before to prove it.

For reference I am using the tables in the NEC for the resistivity of the wire, and IEEE STD 142 (Green Book) to calculate/guess the resistance of the ground ring.
Click to expand...
Can you post what the spec actually requires?
 
As others have mentioned, it is likely asking for 10 ohms or less from the ground bar to the earth, which you would perform a fall of potential test per IEEE 81. The ground bar conductor to the grounding electrode system is almost insignificant.
 
Agree its not rare here to see a spec that states 5 ohms ground resistance for the grounding system often they hire a independent soil test before and then fall of potential performance test after. Telcom, datacenter, CATV, broadcast radio, petroleum industry, distilling, water treatment...
 
Thank you all for your replies, I appreciate it and sorry for my slow response. IT stole my computer today to install several software packages since I am new at my company and I just got it back.

I am unsure how it will be measured, and there is no real spec. I agree testing the ground ring will be important, but knowing the customer it is most likely something they will not want to pay for. I will work on putting together a testing plan, but if not I'll let the owner know I am making some general assumptions in some calculations. This is equipment coming from China, I don't even have a cut sheet on it, but there is a language barrier and the vendor just keeps repeating 10 ohms. The facility itself is a large manufacturing site, over a million square feet and the wire running back to the ground ring could potentially be over 1000 feet so I am considering a #8 insulated wire which would be just under 1 ohm.

I had an epiphany that I could just bond to building steel if the ground ring is actually a grid and ties into the columns. I have to ask the customer but where I am it is about 1 out of 10 buildings have a full grid with the soil being more of a clay. They have not said anything about noise being the issue, so it seems like the easiest method if an isolated ground is not needed.

Side note - again thank you all for your responses. I have been a lurker on the forum since I started my career and have seen all of your names pop up on different threads consistently over the years. I appreciate the knowledge you guys share, and I know for a fact so do several of my peers. Thanks for taking the time to make the community better.
 
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