bonding metal street light poles

[don_resqcapt19]

Re: bonding metal street light poles

Jim,

I admit that there are situations where if the neutral opens up in the first run or so on both sides of the pole, you will have a hot pole, but in general having two things go wrong at one time is unlikely

The problem happens when the supply side neutral opens. The load side neutral does not provide an effective fault clearing path.
Don

 

[jtester]

Re: bonding metal street light poles

by Don

How does the fact that the utility neutral is bonded at multiple points make it better than an EGC?

The impedance of the utility neutral will be less than the egc because the neutral is connected in parallel with the earth at multiple points. Not only that, but it is connected in parallel with all other utility neutrals through the earth connection as well. This brings the earth and other neutral wires into the fault impedance equation as parallel conductors. My old Westinghouse Distribution Manual lists earth impedance as 2.888 ohms/mile. It isn't the earth that is a significant impedance, it is the connection betwen neutral and earth.

Any one connection might be relatively high, but hundreds of them together drop the earth impedance part of the circuit significantly.

Jim T

 

[TOMWELDS]

Re: bonding metal street light poles

Good point Don. That's what really exist. Even more, i would worry about the hot/neutral getting reversed somewere in the circuit...BAD situation there...no OC device, bonded neutral and you have a hot pole. An EGC and OC device would be a must, but in reality..doesnt happen (here, anyhow). That's in regards to 'utility co.' street lighting.

 

[jtester]

Re: bonding metal street light poles

quote by Don

The problem happens when the supply side neutral opens. The load side neutral does not provide an effective fault clearing path.

Again, I look at a utility neutral differently because of the multiple ground connections and the inherent multiple parallel paths.

If the neutral opens up near the pole on the line side, there will be near line potential on the pole when the light is on, unless the load side neutral has comparatively low impedance relative to the source.

If this is a totally underground system where the load side neutral feeds a couple more lights downstream, then you are correct. However if the system feeds an entire distribution system downstream with lots of shared neutrals, the loss of the line side neutral will be much less significant due to parallel paths.

Again, multiple grounds and shared neutrals throughout will lessen the impedance of the neutral system.

Jim T

 

[haskindm]

Re: bonding metal street light poles

Here is an illustration that I use when students tell me that elctricity wants to return to "earth" ground. "Let's go out to your car and attach one condutor from a 12-volt light bulb to the positve terminal of your battery. Then let's attach the other conductor to a series of ground rods. How many ground rods will we need to drive until the restance gets low enough to light the bulb?" The students recognize that the bulb wil never light until we make a connection to the negatve terminal of the battery. Why is 120-volt AC any different? We must provide a low-impedance path back to source NOT TO GROUND. If you wish to install a supplemental grounding electrode at the pole "for lightning protection" go ahead, but this does not excuse you from providing a "ground fault return path". Just because "we've always done it" does not make it right or even safe!

 

[don_resqcapt19]

Re: bonding metal street light poles

Jim,

Again, multiple grounds and shared neutrals throughout will lessen the impedance of the neutral system.

I agree that it does lessen the impedance, but I don't believe it lesses it enough to make it safe, that is clear the fault. If the fault is not cleared, then there is hazardous voltage between the pole and the earth around the pole.
Don

 

[TOMWELDS]

Re: bonding metal street light poles

In regards to all steel poles, whether a ground rod is installed or not, the same scenario exists. The 'J' bolts are acting the same as a gnd rod. So unless the pole is completly isolated from the earth, that problem will always be there.

 

[don_resqcapt19]

Re: bonding metal street light poles

Tom,

So unless the pole is completly isolated from the earth, that problem will always be there.

The shock hazard is the same, even if the pole has no connection to earth.
Don

 

[jtester]

Re: bonding metal street light poles

Don

I agree that many utility secondary faults will not clear. That is because there is no overcurrent protection on the secondary at all. That was my stand early in this topic.

My recent posts began when pierre said that an egc would be better than a utility neutral (no offense pierre). I still contend that a neutral will be far better at conducting fault current. The real problem is that there is no OCPD to operate.

The utility that says the high side fuse will blow is not being very truthful. The vast number of different system high voltages and different transformer sizes combine to assure us that there is little or no coordination between high side fuses and low side protection.

Jim T

 

[don_resqcapt19]

Re: bonding metal street light poles

Jim,

My recent posts began when pierre said that an egc would be better than a utility neutral (no offense pierre). I still contend that a neutral will be far better at conducting fault current. The real problem is that there is no OCPD to operate.

Now I understand what you are saying, I think. If we bond the grounded conductor to the pole, it provides a better fault clearing path than a NEC sized EGC. I agree.
That being said, the current installation methods being used by utilities and municipality to power light poles and traffic lights is not safe. The need overcurrent protection on the circuit and an effective fault clearing path.
Don

 

[karl riley]

Re: bonding metal street light poles

As I read this excellent thread I do not see where Tom has understood that a ground rod at a light pole will not reduce the deadly shock hazard between pole and earth, such that if a person touches the pole while contacting the earth they may die.

Tom, can you verify that you understand this? If not I second the advice given that you read Mike Holt's writings on what ground rods are for and what they do and do not do.

The pole remains energized because the impedance of the rod-to-earth junction is too high to reduce the voltage to safe levels. Ohm's law shows this. This is not theory; this is fact. Death is fact, not theory.

Karl

 

[jtester]

Re: bonding metal street light poles

Don

The definition of effectively grounded is very important. In the NESC, it says

Intentionally connected to earth through a ground connection or connection of sufficiently low impedance as having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazard to connected equipment or to persons.

There is no reference to clearing a fault, just controlling potentials. We have come just past full circle. In my second post on this thread, I suggested that utilities seem to rely on limited step and touch potentials, not fault clearing, to comply with the NESC.

Obviously, the tragic deaths we have read about indicate that there always aren't adequately limited touch potentials, particularly with bare feet and wet grass. Utilities must seem to weigh added systemwide protection against the cost of occasional litigation and settlement as well.

That is the only explanation that I can see, and I worked for utilities for 20 years before going into private practice.


Jim T

 

[don_resqcapt19]

Re: bonding metal street light poles

Jim,
If you can't clear the fault, you can't prevent the build up of voltage. If you energize a pole at 120 volts to earth in the area of the pole, the only reduction in this voltage that can occur with out clearing the fault is the voltage drop on the ungrounded supply conductor. Assuming that this ungrounded conductor will almost always have a lower impedance than the fault return path, the minimum voltage would be more than 60 volts. That voltage will be there until the fault is cleared.
Don

 

[TOMWELDS]

Re: bonding metal street light poles

Not beating this to death, but what im trying to say is, the diagram shown may be correct. I could run the test myself and see the results. But, as ive already stated, the theory works if the pole is 'isolated' from the earth. When was the last time you saw a steel pole, mounted on plastic, with plastic 'J' bolts? Not in this world. The theory shown may be correct, but it doesnt serve me a whole lot of good in the electrical world. No offense to anyone here.

 

[don_resqcapt19]

Re: bonding metal street light poles

Tom,

But, as ive already stated, the theory works if the pole is 'isolated' from the earth.

What "theory" are you talking about. Isolating the pole does not change anything in the event of a fault to the pole.
Don

 

[George Stolz]

Re: bonding metal street light poles

Tom, welcome to the forum.

If the pole were insulated utterly from the earth, and the neutral were opened, then the only path for electrons seeking the neutral terminal of the utility transformer would be through people.

No ground rods, j-bolts, rebar in concrete - no grounding method - is going to save people from being shocked by these poles if they become energized.

Do you understand this?

 

[George Stolz]

Re: bonding metal street light poles

Tom here's some pictures to clarify the problem.
)</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">The neutral and EGC are not connected after that point.</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">Overcurrent protection is installed at the "service"</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">(Optional) ground rods are connected to the EGC and metal non-current-carrying parts only.</font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">The optional ground rods are for lightning protection,and to keep the ground rod industry robust. </font>[*]<font size="2" face="Verdana, Helvetica, sans-serif">The EGC is firmly connected to all non-current-carrying metal surfaces.</font>[/LIST]<font size="2" face="Verdana, Helvetica, sans-serif">
Under this setup, with an open neutral, the lights do not function, but they do not energize the poles they reside in either.

<a href="http://img.photobucket.com/albums/v334/georgestolz/Electrical/NECPolesGF.jpg" target="_blank">

During a ground fault, the EGC provides a low resistance path back to the source, causing the fuse to open the ungrounded conductor to the poles, safely disconnecting the hazard until such a time that trained personnel can come fix the problem.

The NEC provides a profoundly more sensible system than the one the utilities commonly use. Let's face it, they can make mistakes too, and the NEC's method has more redundancy, and is safer, than what the NESC allows.

Hope this helps,
George

 

[TOMWELDS]

Re: bonding metal street light poles

You guys are totally missing what im saying. An animated example was shown on this thread. It shows how adding a ground rod creates a scenario were someone can get shocked. It even shows the voltages at different distances. Now, if you take away the ground rod, the same problem exist because the pole base and 'J' bolts are doing the same thing as the ground rod...correct? Im not talking about neutrals..just that one diagram that was shown.

 

[mdshunk]

Re: bonding metal street light poles

Originally posted by bphgravity:

Originally posted by TOMWELDS:
...Why would you want to jepordize safety by not grounding it in some manner?..

There is no added benefit or safety by adding a ground rod to a light pole...

until the pole gets struck by lightning.

 

[iwire]

Re: bonding metal street light poles

Originally posted by mdshunk:

Originally posted by bphgravity:

Originally posted by TOMWELDS:
...Why would you want to jepordize safety by not grounding it in some manner?..

There is no added benefit or safety by adding a ground rod to a light pole...

until the pole gets struck by lightning.

mdshunk do you really think a ground rod is going to provide a 'better' path for lightning than the large concrete encased electrode (otherwise known as the pole base) that the pole is firmly bolted to?