Auxillary Grounding Electrode Required?

[Dumbo2929]

I have a NEC question. We are installing a new electrical service and panel (outdoors) for a lighting project. The service is single-120/240V and is grounding at the service entrance (outdoor cabinet, panelboard, utility meter) via 2 ground rods. A grounding conductor is run with each branch feeder circuit to each light pole and the metal pole is bonded to the grounding conductor.

The local inspector says a auxilliary grounding electrode is required at each pole location. Is this correct? Section 250.54 in the 2008 NEC tells me if i install an auxilliary electrode to connect it to my system ground conductor, but not that is required. Does anyone know where this is required?

 

[rbulsara]

If the local inspector says it is required, it is required. Also it has been the common practice for light poles. You would want the pole firmly grounded even when the circuit or its ground wire is disconnected. Codes are just minimum safety standards and not end it all requirements. You pick your battles.

Rafiq Bulsara

http://www.srengineersct.com

 

[dpc]

This has been commonly done for a long time. I'm not aware of an NEC requirement, but I haven't looked for awhile. In the past, utilities would rely only on the ground rod and not run a grounding conductor, which is a potential safety hazard.

I agree with Rafiq, it's probably not worth a battle with the inspector. It could be a local requirements and/or custom.



David Castor

http://www.cvoes.com

 

[Dumbo2929]

I know it's commonly done and we specify it on larger projects. This is a small project and I was looking for a requirement in the NEC, because i don't believe there is one.

 

[rbulsara]

Are you implying that you do the small design badly? Why is it different for a small project? Light pole is a light pole. It also acts as lightning rod, not just another indoor piece of equipment. That is a compelling reason to independently ground a light pole. Not having a specific reference is not an argument. NEC is not the scope of your work.



Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

There is no NEC requirement for a ground rod at the poles, but NFPA 780 Standard for the Installation of Lightning Protection Systems ¶4.2, Masts, Spires, Flagpoles says

These slender structures shall require one strike termination device, down conductor, and ground terminal. Electrically continuous metal structures shall require only bonding to ground terminals.

Maybe the local inspector is requiring the ground for lightning protection.

 

[rcw retired EE]

A few years ago, we tested some light pole foundations for use as a grounding connection, like a Ufer ground. Our test was simple - compare the measured ground resistance of the concrete encased rebar with a driven ground rod using the FOP method.

We measured two structures and in both cases the foundation had lower resistance than the rod, as expected. I've seen other articles and blog comments that say the same thing.

I believe that that if the anchor bolts are connected the the rebar in the conrete foundation, that the resistance to earth is better than a single ground rod. I would think that the impedance to lightning strike current is also less.

One concern is that the lightning current will blow the foundation apart by overheating the rebar and vaporizing mositure in the conrete. (I saw remnants of a windmill foundation where this occured.) I don't know of any actual lightning stike on a street light pedastal where the foundation failed. Maybe it has happened.

If the inspector wants a ground rod, put one in. It probalby won't help much but it will make him feel better,

 

[alehman]

Not strictly required, but I wouldn't put my seal on a drawing without ground rods at each pole.

If you have a ground fault and the ground wire is the same size as the phase wire, the light pole will be at 1/2 the phase voltage until the circuit breaker operates. With long circuits, that could be enough time to injure someone leaning against the pole. Better to add a ground rod to make the pole and surrounding earth closer to the same voltage.



Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[resqcapt19]

If you have a ground fault and the ground wire is the same size as the phase wire, the light pole will be at 1/2 the phase voltage until the circuit breaker operates. With long circuits, that could be enough time to injure someone leaning against the pole. Better to add a ground rod to make the pole and surrounding earth closer to the same voltage.

But that only helps if you are standing on or very close to the rod. If you are on the opposite side of the pole (assuming 3' away from rod) you would still be subjected to about 85% of voltage on the pole. In you example about 42% of the voltage...to me that is really the same hazard as without the rod. I guess, in some cases you will be closer to the rod and there may be some benefit. The best solution is feeder and grounding conductors sized so that a fault opens the OCPD very quickly.

We really have this same issue anytime there is a ground fault to any electrical equipment. That equipment will be energized with a voltage that is equal to the voltage drop on the EGC, and like you said this voltage will be there until the fault is cleared.

 

[alehman]

resqcapt19,

The best solution is feeder and grounding conductors sized so that a fault opens the OCPD very quickly.

I agree with that.

I think you have disputed me previously on this topic. If you want to recommend against grounding electrodes at the poles and are willing to put your stamp on it, that's fine with me, but I won't.

I think a concrete pole base itself may be the best electrode for this purpose, but it has be constructed with the necessary reinforcing steel for a proper Ufer system. I'm not sure that I would trust it would be installed correctly since it cannot be inspected after the fact. Therefore I'll stick with ground rods.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[resqcapt19]

Alan,
I won't recommend against it, but I see no real use for it. It is my opinion that the perceived safety benefit of installing the grounding electrode for light pole bases is far greater than the real one.

As a contractor, it is rare for me to find a set of stamped drawings that call for the installation of a ground rod or other grounding electrode for a light pole base. I have no issue with installing one when the drawings call for it as I get paid by the hour and that increases the labor hours.

25 years ago, it was common to find a grounding electrode requirement on the drawings, but those drawings often left out the required EGC.

 

[rbulsara]

Ground rod has nothing to do with equipment grounding conductor or fault clearing. It is intended as grounding for diverting currents due to lightning. Pole base ufer grounding may work if it is intentionally designed and installed for the purpose and verified. Having a separate ground rod is also a very cheap insurance against liabilities.



Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

I agree with Rafiq. If the ground rod were there for ground fault clearing in the event of loss of the EGC, then it would not do a very good job. Assuming generously that you could get 25 ohm ground resistance, a direct short with 120V would only result in 120/25 = 4.8A, not enough to trip the breaker.

You could reasonably argue that lightning protection is not required, but if the inspector or contract requires the ground rod, then you would lose the reasonable argument most of the time.

 

[alehman]

I did not mean to imply that the ground rod would change the fault clearing time. My point was to reduce touch potential.

It's not required by the NEC, and certainly cannot substitute for an equipment grounding conductor.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.