When to use a ground grid

[eeprom]

Hello,
In designing the electrical system for a commercial building, such as a grocery store, how is it determined whether or not to use a ground grid. Is a grounding study required for all commercial buildings? Is it ever acceptable to use a ground rod?

thanks

 

[PHovnanian]

The use of a ground grid is an engineering decision made in order to comply with NEC Article 250 and others (assuming this is your governing code) together with state and local regulations.

If the AHJ wants to see a study and/or tests of the installation in order to verify compliance of the grounding system, a study probably should be done. You can just start pounding in additional ground rods to get electrode resistance below required values, but it doesn't look very professional.

 

[eeprom]

Thanks for your reply.

 

[waross]

Two issues here. Both depend on the local AHJ.
For smaller services many local authorities do not require a grounding study. Check your local codes, and check with the local AHJ.
If a ground study is not required, many jurisdictions have grounding rules that may add to or amend the National code.
Bottom line, check the local codes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[eeprom]

I've seen instances in new construction where the grounding bar is bonded to rebar within the building foundation, and to a water pipe, and to a ground rod, and to the building frame, as is consistent with article 250.50.

But I see nothing in the code which states how "effective" an effective ground path should be. It would be easy enough to set up a field test to determine the impedance of the ground path from any bonded point in a building. The question is, what impedance value would be acceptable? In mining applications, it is usually required that the resistance from a piece of equipment back to the ground bus is less than 2 ohms. What is a good effective path impedance for a commercial building?

 

[oldagpe]

IEEE STD 142 would be a good place to start.

One size does not fit all here.

Earth conductivity, lightning likelihood, frequencies and impedance of the ground system, electronics in the building, etc.

Method of building construction will affect the design decision.

Too many considerations for a quick answer.

 

[eeprom]

I'm sure there is no quick answer. And I have referenced IEEE 142. But as an engineer, if I design something I should be able to prove it works. Obviously creating a full fledged bolted short would not be a safe means of testing the system.

The effectiveness of the system comes down to the maximum voltage of conductive surfaces and fault clearing time. Aka...how much voltage can someone be exposed to and for how long. If I recall, IEEE std 80 states that 60 mA is enough to kill someone, especially if the duration of the exposure lasts longer than 1 heart beat.

I'm sure someone has come up with a good field test for checking various points of a building for expected voltage and duration during max fault current. If someone has a good means of checking, I'd like to know.

 

[waross]

Here is one instrument from Megger for testing ground electrode resistance.

http://www.megger.com/us/products/ProductDetails.php?ID=881&Description=

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

Grounding/bonding is a complex issue. The analysis methods and preferred solutions depend on which requirements must be met. These include: protection coordination, lightning/surge protection, personnel safety (step or touch potentials), electromagnetic interference mitigation.

Not every parameter can be reliably (or safely) field tested. Not every parameter needs to be in all cases. It is possible to test grounding electrode resistance without simulating a bolted fault and by analysis determine system behavior during such a fault.

One aspect of grounding to consider: In many cases, bonding is far more important for personnel safety and protection coordination than grounding (although the two are related). Grounding serves to tie a structure to some theoretical external ground reference while bonding serves to maintain all conductive surfaces within a structure at the same potential and to provide a return path to the system source for ground fault currents. I don't care if a building's structure jumps to a few kV during a fault so long as every object within and surrounding it (particularly me) reaches the same potential.

 

[oldagpe]

The standard of care is following generally accepted engineering practice and standards.

On a commercial property such as a grocery, the inspector will check compliance with NEC and any local amendments. Visual inspection and that is it.

If a commissioning test or periodic maintenance test is indicated, then InterNational Electrical Testing Association would be the place to start. They have both standards and background material.