Ground Ring Burial Depth Reasoning?

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JKR

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Does anyone know the reasoning (technical or practical) behind the burial depth of a ground ring be 30 inches or greater, 250.53(F) in the 2008 handbook. Thx
 
My guess would be that since it must encircle the entire structure this would limit the possibility of it being damaged at some point in the future. 30" is pretty deep even if you were planting shrubs or installing a sprinkler system at a later date. And it might even function better at that depth.
 
I doubt it makes it function and better, since it has almost no function. Even the NFPA 780 isn't so strict. Section 4.13.4 only requires ground rings for LPS's to be 18 inches deep when in direct contact with the earth.

If 18 is good enough for lightning protection, it should also be good enough for a grounding electrode for a service.
 
In the northern states, the depth could prevent damage from frost heave. Also, the soil is more likely to have a higher water content at that depth.

Jeff
 
bphgravity said:
If 18 is good enough for lightning protection, it should also be good enough for a grounding electrode for a service.
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My only thought is maybe Rob is onto the reason - an NEC grounding electrode would be more strictly protected (by burial depth) than an optional grounding system...right?
 
georgestolz said:
My only thought is maybe Rob is onto the reason - an NEC grounding electrode would be more strictly protected (by burial depth) than an optional grounding system...right?
Click to expand...

I was just taking an edjucated guess. If you ever looked into a +30" trench, it's rather deep and anything within it is kind of out of the way.
 
A ground ring is always an optional grounding method. When used under the NEC, it serves no real purpose. When used under the NFPA 780, it is expected to carry the full surge current of a direct lightning strike.

Which one do you think needs to be more restrictive?
 
bphgravity said:
A ground ring is always an optional grounding method. When used under the NEC, it serves no real purpose. When used under the NFPA 780, it is expected to carry the full surge current of a direct lightning strike.

Which one do you think needs to be more restrictive?
Click to expand...
A ground ring can protect against touch voltages to a grounded building in the event that a primary fault on the utility system causes a high Ground Potential Rise (GPR) on the common primary/secondary ground. A ground fault at the primary of a service transformer can cause a very high GPR and depends a lot on how much current flows back to the substation source through the neutral and how much through the earth.

For this purpose, however, 18 inch depth is better than 30 inch depth. The value of the ground ring for touch voltages is to get the ground surface voltage at a persons feet close to the voltage of the grounded building that the person might be touching. A wire buried closer to the surface will make the voltage at the surface closer to the ground electrode voltage. The grounded building is at the ground electrode voltage. The 30 inch depth must be for physical protection.
 
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