Parking lot light pole ground rod

[ronaldrc]

Re: Parking lot light pole ground rod

I have no idea how resistive a concrete base is all the bases I ever worked around might have had 3 or 4" of the renieforcement rod in the dirt at the bottom the steel workers would make them and we would insert them in the hole like you said Bennie they where anywhere between three and five foot long.

And I have no idea of the resistance of dry concrete is it more or less than dirt?

Here is something Don posted on this a while back.

http://www.mikeholt.com/cgi-bin/codeforum/ultimatebb.cgi?ubb=get_topic;f=18;t=002562#000000


Ronald

 

[bennie]

Re: Parking lot light pole ground rod

Ronald: According to the calculations on http://www.cpccorp.com/deep.htm

Deep earth grounding. The impedance is half at 10 feet deep, than at the 5 foot level.

[ July 26, 2003, 03:16 PM: Message edited by: bennie ]

 

[ronaldrc]

Re: Parking lot light pole ground rod

Thats a good paper I didn't read it all.

I don't no about it being economical to drive a 10ft rod. I've spent hours trying to get 8 footers down in some places.I'd been about as well off digging a ditch and buring them.

Ronald

 

[bennie]

Re: Parking lot light pole ground rod

Ronald: I've written this before, you have no problem. Try driving a ground rod at the South Pole on the 2 mile thick ice cap.

When dirt is available it is called grounding. When only ice is available I called it icing.

 

[karl riley]

Re: Parking lot light pole ground rod

And whatever happened in that case where a kid was electrocuted touching a light pole? It was going to be "thoroughly investigated". Any news?

Karl

 

[flightline]

Re: Parking lot light pole ground rod

I'm curious about how you gentlemen [and ladies of course] feel a manufactured driiled in pole foundation would change things.

Our town has an local addendum for the addition of a local ground rod at every pole base. This is a vestage from when the building inspectors did ALL the inspections. Since the electrical inspectors [us] work evenings, the building inspectors do the pre-pour inspections. In discussions with my partner, we both agree what they have been passing is useless in that many have just stuck the rod in a couple of feet with the top sticking out the top of foundation. If we keep this at all, it will be our suggestion that the contractors install a sectional road, or keep it out of the pour altogether so that the minimum deiven depth set forth in 250 may be met. I'd like to hear what you think.

 

[don_resqcapt19]

Re: Parking lot light pole ground rod

The diameter of the grounding electrode also has a large effect on the ground resistance of the electrode. Look at this grounding nomograph. In the example shown on the nomograph, an 10' deep, 8" diameter electrode would have the same resistance as the 20' deep 5?8" rod. An electrode with a diameter of 8" that is 5' deep will have the same resistance as a 5/8" diameter rod that is 10' deep. The rebar cage in the concrete pole base would be much larger than 8" in diameter, so the normal 4'+ deep base that is 18"+ in diameter should provide a lower ground resistance than a standard 10' ground rod. This, of course, assumes that the ground is not frozen, but we have, for the most part, agreed that the grounding elecrode at a light pole is only for lighting protection and lightning strikes when the ground is frozen are somewhat rare.
Don

 

[karl riley]

Re: Parking lot light pole ground rod

I keep getting the feeling that in discussions like this one problem with agreeing is that some are under the impression that the ground rod is there to to provide sme kind of safety in case of a fault to the lamp post, whereas others know it won't do that; it's just there for lighning dispersal (I won't e3ven say protection).

But little by little, answers such as Don's may get the message across.

Karl

 

[bennie]

Re: Parking lot light pole ground rod

The first 5 feet of surface material has a higher impedance than the 5 to 10 foot level.

I am referring to fault path impedance. If the ground electrode path to the transformer ground only carries one amp during a fault, this is a positive. The one amp may be the needed last straw, in the time trip curve.

The total impedance of the neutral conductor when in parallel with the earth, is less than the lowest impedance. Multiple paths only improve the fault clearing ability, when one path may not clear.

The rods should be installed to 10 feet deep. Because the rods alone will not permit the overload device to activate, is not a logical reason to ignore them.
There is more of a chance of a fault occurring than a lightning strike.

 

[don_resqcapt19]

Re: Parking lot light pole ground rod

Bennie,
Any installation where the fault return path through the earth plays any real part in opening the OCPD, is not installed in a code compliant manner. The job of carrying the fault current back to the grounded conductor belongs to the EGC alone. Other paths are ok, but the EGC must be able to do the job all by itself.
Don

 

[bennie]

Re: Parking lot light pole ground rod

Don: Fault current to the neutral conductor bus, is only part way home. There, it splits up and takes every path available. The neutral conductor and the earth.

The combination of two parallel resistors total less than the smallest resistor. The earth will have some effect. This is Ohms law, not mine.

 

[brian john]

Re: Parking lot light pole ground rod

Don:

While I agree with your statement somewhat, last winter duiring two different snow storms we had lightning (doesn't mean the ground was frozen).

My kids were amazed they had never heard thunder in the winter, much less seen the flashes on a snow.

Brian John

This, of course, assumes that the ground is not frozen, but we have, for the most part, agreed that the grounding elecrode at a light pole is only for lighting protection and lightning strikes when the ground is frozen are somewhat rare.

 

[don_resqcapt19]

Re: Parking lot light pole ground rod

Brian,
I have seen cloud to cloud lightning in snow storms, but never cloud to ground. I'm sure that it does happen, but I think it is rare.
Don

 

[don_resqcapt19]

Re: Parking lot light pole ground rod

Bennie,
Yes current does travel on all available paths, but my point is that the code requires the EGC to be large enough to do the job even if there are no other paths. If these other paths are need to flow enough current to open the OCPD in a reasonable time, then the EGC is not code compliant.
Don

 

[bennie]

Re: Parking lot light pole ground rod

Don: There is a lot of variables in a service entrance. Maybe the loop impedance is such that the breaker will trip. "Maybe" is not good enough when there is one other path to assist the Neutral conductor. Remove this path, the total impedance goes up.

Ground fault breakers, depend on the earth path in a lot of situations.

Spare the rod, spoil the path

[ July 28, 2003, 11:39 AM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: Parking lot light pole ground rod

bennie,

Maybe the loop impedance is such that the breaker will trip.

Any installation where the loop impedance of the EGC is too high to open the OCPD is not code compliant.
Don

[ July 28, 2003, 07:21 PM: Message edited by: don_resqcapt19 ]