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Grounding at light pole base

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anbm

anbm

Electrical
Dec 10, 2007
36
I saw several details for parking lot light pole base, where they run #10 ground wire and tie to rod inside the pole base. Why do we need this ground wire and rod inside pole base? For lightning protection? If the circuit feeds this parking lot light has equipment ground wire, do we still need this ground wire and rod inside pole base?
 
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As you already remarked, the ground rod is required to protect against lightning.
The lightning stroke contains high frequency and high voltage component and in order to discharge it to the ground a low reluctance path to ground has to be provided and this is the straight vertical direction with minimum deviations possible.
See NFPA 780 [or IEC 62305-1 or IEC 61024-2]
The equipment grounding wire is required in order protect against low voltage insulation failure which may energize metallic parts [usually are not live parts] and to facilitate the return of the failure current to the source and determine so a rapid trip of the breaker [in order to protect person against contact and equipment against fire hazard].
 
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Sorry,I meant "low inductance path" instead of "low reluctance path" ,of course.
 
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Grounding at the pole is also aimed at reducing the possibility of elevated contact voltage. At Con Ed in New York, they have had considerable problems with failures on low voltage splices leading to elevated voltages in their splice boxes and on the metal street light masts. They have a fairly extensive program now where they check the potential contact voltage at street lights and other metallic structures that may come in contact with the public.

While I would say that Con Ed's experience is not typical, in most cases, lightning would be the main reason for installing a ground rod at each street light.
 
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If you have metal poles and reinforced concrete foundations, you would do just as well to tie the anchor bolts to the rebar, making a ufer ground.
 
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The most important safety requirement is the equipment grounding conductor. The ground rod at the pole is not hurting anything, but it is not really doing much to protect anyone either.

 
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For metal poles, I've always been more comfortable with an earth connection at the pole to help with the touch potential until the breaker trips.
 
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The ground rod does very little to reduce the earth potential if there is a fault, unless you are standing very close to the electrode. If you are 3' away from the electrode you will have a potential of about 75% of the line to neutral votlage. This is still a hazard.
 
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The ground rod does very little to reduce the earth potential if there is a fault, unless you are standing very close to the electrode. If you are 3' away from the electrode you will have a potential of about 75% of the line to neutral votlage. This is still a hazard.
Click to expand...
If there is an equipment grounding conductor (EGC), the voltage from the pole to earth will be less than 50% of line to neutral voltage. If the source impedance is large (low fault current), the voltage will be considerably less than this. Unless the pole is insulated from the earth, the voltage will be lower still.
 
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I worked street lights many years ago. At that time the city had had some issues with people getting touch potential shocks from metal lamp standards. This was before the days of equipment grounding conductors. The problem was worse after the first rain after a long hot dry spell. The solution was to ground each neutral at every standard in addition to the local ground rods.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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If there is an equipment grounding conductor (EGC), the voltage from the pole to earth will be less than 50% of line to neutral voltage. If the source impedance is large (low fault current), the voltage will be considerably less than this. Unless the pole is insulated from the earth, the voltage will be lower still.
Click to expand...
Yes, the installation of an EGC will result in a much lower voltage on the pole, and this voltage will only exist until the OCPD clears the fault. My point is that the ground rod does little to decrease the touch potential with or without an EGC. If you are touching the pole and standing on the earth 3' away from the ground rod, you will be subjected to 75% of the voltage that is on the pole.
 
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This can be improved by bonding the base if possible (reinforcing steel for concrete bases).
 
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This can be improved by bonding the base if possible (reinforcing steel for concrete bases).
Click to expand...
Yes, that will put the person closer to the grounding electrode than you might be with a single rod, but as you touch the pole and move away from the concrete base, the voltage potential goes up. At 1' away you would have about 65% of the voltage on the pole itself.
 
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resqcapt19,
100% - 65% = 35% between the earth and pole. If we assume the loop circuit gives us 50% of L-G during a fault, we then have 17.5% as the touch potential. I think that's a lot better than 50% (which would be the case for no earth connection).


I see two possible solutions if you want to further reduce the touch potential.
1. Oversize the equipment ground (as compared to the phase conductor), to further reduce the potential rise.
2. Construct a ground ring or mat around the pole.
 
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Several posts refer to tripping an overcurrent device for a short in a metal light pole. Utilities that I have worked for don't typically have a protective device between their light poles and the distribution transformer serving them. A hot leg contacting the light pole would energize the pole at all times, an open neutral would result in the pole being energized only when the lamp was burning (with reduced light output).
We almost never knew about energized light poles except when someone complained about reduced light output, or about their dog running into the bushes after lifting his leg next to one of them. Once in a while, we would encounter a blown bayonet fuse for a secondary fault, but very rarely.
Are there any utilities that place an overcurrent (or ground fault) device in the circuit between a transformer and a street light? Or run separate ground and neutral conductors?
 
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We do consulting for small utilities and we design these circuits with fuse protection and separate neutral and equipment grounding conductors. But I would not say this is a standard practice. Many of these utilities use a lot of wood poles for street lighting and the problems not as severe in that case. When they use steel poles, they sometimes want to use the same wiring techniques that they have used for wood poles, and we have to do a little persuading. We won't sign off on a design without OC protection and a separate ground wire.

You won't have to search too hard to find case histories of people being killed by contacting an energized light pole. It happens somewhere in the US about once a year it seems like.

I think Mike Holt's website has a few stories on this.

Regards,

Dave
 
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I'm trying to figure out in the above posts why there will 75% of the voltage at a distance of 3ft. The way I think I understand is is because of the resistance of the earth and therefore the the voltage drop across this resistance.

As an example an 120V L-G fault would cause 4.8A on an 25ohm grounded system. It is this little bit of current flow in the earth that causes the voltage drop leading to a voltage of 75% at 3ft.
 
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If the resistance we are reading to ground is indeed about 100ohm corrolating to a current of 2.7A, is there a way to see if this current exists? Possibly with some kind of distortion or something on a scope?
 
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I think I found the answer to my question:

The voltage at any point on the earth surface a distance away from a ground rod connected to faulted equipment can be represented by.

V=I*P/2*pi*rx

Where:

V= Voltage on earth surface at distance rx
I = Magnitude of fault current
P = Soil Resistivity
rx = distance away from ground rod/ring.

Therfore what I theorized earlier with the voltage drop through the earth causing a difference in potential at a distance away from the faulted/grounded equipment is correct.
 
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