pre drilling for ground rods in hard soil
- Thread starter cesoj
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- Placerville, CA, USA
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- Retired PV System Designer
Since it is part of a POCO production/transmission infrastructure rather than a simple office, the installation would be covered by the NESC rather than by the NEC. At a commercial site covered by the NEC, the standard for ground rods is driven into undisturbed dirt.
If that is not possible, some standard alternatives like burying a rod in a horizontal trench.
I believe that a listed ground enhancement kit (typically chemicals which absorb moisture and create a conductive interface layer) can be used in connection with an oversized drilled hole.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
I don't recall that the code actually says anything about undisturbed dirt. In most cases ground rounds are going to be driven into the dirt next to the foundation which is pretty much fill in most cases.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
Bentonite or Marconite are two of the materials that can be used to fill the bore hole.
Fulthrotl
~Autocorrect is My Worst Enema.~
- Occupation
- E
and, it's most likely a chemical ground.
a 2" copper pipe that is encased into poured material.
erico makes them.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
How do you drive a rod into such a hole, sounds like you just drop it in. Chances are they didn't put the same dirt back into the hole but rather something prescribed for the situatioun. 8 inch diameter has much more surface to soil contact then a 5/8 rod would have had.
Bottom line this is probably not a "driven rod" system.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
I doubt that very much. You don’t go to the trouble of drilling 10+Ft and then sling soil in as the infill.
A local substation to me the supply company had to drill 50Ft bore holes in a triad formation to get a decent reading.
Yeah, I bet they wanted 1 ohm or less.
Do you remember how far apart the holes were drilled from one another?
rlundsrud
Senior Member
- Location
- chicago, il, USA
Couldn't you just use concrete for the backfill?
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
<1Ω is what they aim for as it means the MV/LV earths (or whatever you want to call it) can be linked. >1Ω → <20Ω two separate earth nests are required spaced a minimum of 8 metres apart. >20Ω and they have to start drilling.
Tony S
Senior Member
- Location
- Resting under the Major Oak UK
Something I should have added, for urban distribution our two systems have major differences.
Whereas you in the US tend to have multiple pole mounted transformers each with its own earth, earths that are linked together forming a large earth nest.
We in the UK use larger transformers feeding a fair sized area at LV. There are four 1000kVA transformers within 750 metres of my front door but there again I do live in student land where demand is high. The power companies aren’t allowed to use the MV interconnections to transfer earthing between substations so each has to comply with The Electricity Supply Regulations 1988 and UK Power Networks COP.
Whereas you in the US tend to have multiple pole mounted transformers each with its own earth, earths that are linked together forming a large earth nest.
We in the UK use larger transformers feeding a fair sized area at LV. There are four 1000kVA transformers within 750 metres of my front door but there again I do live in student land where demand is high. The power companies aren’t allowed to use the MV interconnections to transfer earthing between substations so each has to comply with The Electricity Supply Regulations 1988 and UK Power Networks COP.
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