Triad requirements for a Industrial Service entrance.

[tom baker]

We had it tested here a few years back.. we were losing analog input modules like crazy during lightning strikes and the grounding efficacy came into question. Found a few rods that were in the 30s of Ohms but that was likely never the issue anyway. Isn't the pass point pretty clearly 25 Ohms based on the NEC though, regardless if you think that value is actually any sort of meaningful threshold?

In the NEC if you are not able to get 25 ohms with one rod, you drive 2 and you are done.

 

[tom baker]

"Triad grounding is important in order to facilitate periodic testing of the effective grounding system. Three rods are buried and connected with insulated cable (probably 90 mm), of course the insulation of that part of the cable which is bolted to the rod is removed. Test pits allowing access to the top of each rod.
In order to effectively test the ground resistance the rest of the grounding system must be disconnected from the rod, if only a single rod was buried then for the duration of the test the system would be ungrounded which is extremely dangerous, the three rods and their interconnections allow this testing to occur and also eliminate the need for a test rod to be driven as 2 out of the 3 rods can be disconnected and the earth resistance test done while the third rod maintains an effective ground. The distance between the rods is usually equal to or a bit more than the length of the rod in order to avoid interference and thus provide a true measurement of the earth resistance at that point. This system also facilitates redundancy and increased availability of the grounding system and is used mainly for grounding of instrumentation and computerized systems and so is isolated (except for a single bond) from the plant (dirty) earthing network which may be buried bare copper, rods and building rebar etc."

The mention of 90 mm cable would make this a European standard. The grounding there is different as they do not have a grounded neutral and use a residual current detector. To use that grounding design for our system is not advised as we have pointed out.
And lightning protection grounding is mostly done with cad weld to minimize voltage drop at the connections.

 

[Hv&Lv]

The mention of 90 mm cable would make this a European standard. The grounding there is different as they do not have a grounded neutral and use a residual current detector. To use that grounding design for our system is not advised as we have pointed out.
and lightning protection grounding is mostly done with cad weld to minimize voltage drop at the connections

That quote came from a member of this forum in 2009
See my earlier link.