bonding metal street light poles

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by TOMWELDS:
It shows how adding a ground rod creates a scenario were someone can get shocked.

No, Tom, you misunderstood the picture.

The hot wire touching the pole creates the scenario for someone to get shocked. The picture is to demonstrate that a ground rod does not solve the problem.

Without any grounding electrode, the voltage is a clean 120 volts, measured from 1', 3', and 6' away from the pole.

With rod = shock hazard
w/o rod = shock hazard

Nothing changes adding a rod. Well, the potentials change a little bit, but it's still a shock hazard either way.

iwire · Dec 17, 2005

Re: bonding metal street light poles

Well said Mr. Stolz.

TOMWELDS · Dec 17, 2005

Re: bonding metal street light poles

I agree Mr Stolz. What the picture shows and seems to be the dilemma, is that, adding the ground rod 'increases' the potential hazard. The hazard is 'already there'. By the mounting base. Adding the ground rod does not 'increase' or 'decrease' the hazard. That's all ive been trying to point out.

jbwhite · Dec 17, 2005

Re: bonding metal street light poles

along these lines, i have a question related to this tipic.

Would adding a ground rod decreese the risk from a lightning strike? I ask because alot of designers for parking lot lights require an egc run with the conductors, and a ground rod at each pole.

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by TOMWELDS:
What the picture shows and seems to be the dilemma, is that, adding the ground rod 'increases' the potential hazard. By the mounting base.

No, it doesn't. Forget the other grounding electrodes for a second, imagine there is solely a ground rod. The base is just another type of ground rod, so if it helps, consider the two to be one rod. Better yet, just forget about the base.
-----------

The ground rod decreases the voltage measured from various points to the pole.

If you test from the ground rod to the pole, your meter will read zero volts. They're connected together.

The only safe way to touch this pole is to literally be standing on the rod itself. Once you step off the rod, keeping your hand on the pole, there will be a voltage between your feet and hands.

If you test from the pole (or rod) to 1 foot away from the rod, your meter would read 82V.
If you test from the rod to 3 feet from the rod, your meter would read 90V.
If you test from the rod to 6 feet from the rod, your meter would read 103V.
If you test from the rod to 50 feet from the rod, your meter would read 120V.

The ground rod is an ineffective attempt to raise the voltage of the ground around the pole to 120V. The only point at which this works is if you're physically standing on the rod itself.

The rod doesn't increase the hazard. It just doesn't do much to decrease the hazard. An 82V shock from one foot away from the rod will still kill someone.

Edit to add some words and stuff.

[ December 17, 2005, 01:15 PM: Message edited by: georgestolz ]

roger · Dec 17, 2005

Re: bonding metal street light poles

TOM, you're not reading all the text with the picture. The picture does not show or indicate that adding a rod will increase any danger, it shows that a rod does practically nothing to make it safer which some people believe it does.

Here is the picture again,

Notice the wording in the yellow block, there is nothing in the picture claiming a rod makes the situation more dangerous, or that the voltage gradients will not being present around the pole without it.

I was the one who (thinking hypothetically) said a rod can increase the danger around a pole.

If a pole were not earthed through bolts or base construction methods, adding a rod with out an EGC would in fact create the hazard.

Roger

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by jbwhite:
Would adding a ground rod decreese the risk from a lightning strike? I ask because alot of designers for parking lot lights require an egc run with the conductors, and a ground rod at each pole.

I'm starting to warm up to the idea that grounding electrodes prevent lightning strikes, to some degree. I believe the idea is, that if the entire area is at the same potential, then the lightning will be a bit more prone to strike at an area that isn't so even.

But, I dunno.

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by roger:
If a pole were not earthed through bolts or base construction methods, adding a rod with out an EGC would in fact create the hazard.

If nothing were grounded at all, sure.

But as long as the tranformer is grounded, then that pure 120V at the pole is going to want to get back it's source. The voltage at all points would be 120V, as indicated in my doodle above.

iwire · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by georgestolz:
I'm starting to warm up to the idea that grounding electrodes prevent lightning strikes, to some degree.

Based on what?

jbwhite · Dec 17, 2005

Re: bonding metal street light poles

if i am splitting this thread, let me know so I can start a new one.

I am wondering why a pole attached with J bolts to a concrete base would needs its own rod, in addition to a egc? is this just poor design, or is there some rationale?

roger · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by georgestolz:

Originally posted by roger:
If a pole were not earthed through bolts or base construction methods, adding a rod with out an EGC would in fact create the hazard.

Click to expand...

If nothing were grounded at all, sure.

Isn't that what I said?

Roger

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by iwire:
Based on what?

Based on a conversation with a kindly old man who meant well but apparently didn't know his patoot about the matter. I should have taken his anecdote with a grain of salt, as his code knowledge wasn't very hot either.

He claimed that his home was struck by lightning several times, causing damage. The insurance company finally came down on him and said they were going to honor no more claims unless he did something about the problem. So, he installed a series of 10' copper rods around the structure and "hasn't had a problem since."

Important questions that I had were, how frequent were the strikes before the install, and how long has the system been in place, to get an idea of the effectiveness of the system. When I began to ask, he kinda interrupted with "It solved the problem."

On the occasion I had to speak with him, he wanted to require a ground rod on a PV-interactive generator. I argued that it wasn't an SDS so it was unnecessary, and he simply smiled and said that he wanted to see a ground rod, as the generator might get hit by lightning.

(I believe if it were an SDS, it would be required to be bonded to the real electrodes of the system, not to a lonely electrode kicking it on it's own. But that's just me.)

I argued, but it wasn't my battle (not my installation), so I moved along.

I just did a quick internet search to find something along the same lines, but there appeared a row of experts claiming just the opposite.

Live and learn, I guess.

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by roger:
Isn't that what I said?

I thought it was a given that the utility transformer would be grounded, sorry.

George Stolz · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by jbwhite:
I am wondering why a pole attached with J bolts to a concrete base would needs its own rod, in addition to a egc? is this just poor design, or is there some rationale? [/QB]

It doesn't need any of it. We as people 6' tall look at tall metal objects and visualize lightning hitting it. So, we're inclined to install grounding electrodes to give the lightning a path to the ground.

There's no NEC requirements for any electrode at a light pole without a panel hanging on it.

iwire · Dec 17, 2005

Re: bonding metal street light poles

Originally posted by georgestolz:
There's no NEC requirements for any electrode at a light pole without a panel hanging on it.

Or it has more than one branch circuit supplying it.

karl riley · Dec 17, 2005

Re: bonding metal street light poles

Tom, are you clear that neither a rod nor J-bolts will bring the pole voltage down to earth voltage? (In a ground fault situation) Average ground rod resistance in 4 midwestern states was found to be about 116 ohms. This means the pole would stay charged and remain a hazard indefinitely. Many people have died from touching poles in this situation.

Are you sure you are not missing something? Something you always thought was true but happens to be false? We all have this experience. It helps to step back and take a second look at our beliefs.

Karl

pierre · Dec 18, 2005

Re: bonding metal street light poles

George
"I'm starting to warm up to the idea that grounding electrodes prevent lightning strikes, to some degree."

I hope your temperature started out at -270 degrees

As far as why do some engineers not like to use the concrete pier with the j-bolts as the electrode? They think the lightning will crack the concrete. There a plenty of studies done that refute that idea.

BTW: in practice, you cannot stop the concrete pier from being used as an electrode, you can only ignore it and add more

don_resqcapt19 · Dec 18, 2005

Re: bonding metal street light poles

Pierre,

There a plenty of studies done that refute that idea.

Most of the studies were under specific conditions. It is my understanding that a concrete pole base is likely to be cracked by the current from a lighting strike if the rebar and bolts are not double tied or welded. The heat produced by the current as a result of the resistance of standard rebar connections causes a "steam explosion" that can damage the conduit. This occurs both where the rebar cage is used as a grounding electrode and where its not used as a grounding electrode. The addition of a ground rod does not change this.
Don

pierre · Dec 18, 2005

Re: bonding metal street light poles

If the bolts are in the concrete base, and they are in contact with the pole, is the base not by default an electrode, regardless of how many other electrodes may be present?

Don
Like you, I have read several studies. I have read the conflicting studies. From those studies, my experience, and my own understanding of the theory in our industry, I have come to a conclusion (that can under strong reason, be changed) that the concrete electrodes will under some conditions crack and under the average condition not crack.

George Stolz · Dec 18, 2005

Re: bonding metal street light poles

Originally posted by pierre:
If the bolts are in the concrete base, and they are in contact with the pole, is the base not by default an electrode, regardless of how many other electrodes may be present?

Functionally, or officially?

Officially, it's not an electrode, IMO.

Functionally, I believe current will travel through the bolts, into the concrete, and into the earth.

bonding metal street light poles

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