Grounding at light pole base
Grounding at light pole base
(OP)
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?






RE: Grounding at light pole base
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].
RE: Grounding at light pole base
RE: Grounding at light pole base
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.
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
Bill
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"Why not the best?"
Jimmy Carter
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
RE: Grounding at light pole base
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).
http://www.osha.gov/OshStd_gif/10RF_1.gif
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.
RE: Grounding at light pole base
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?
RE: Grounding at light pole base
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
RE: Grounding at light pole base
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.
RE: Grounding at light pole base
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?
RE: Grounding at light pole base
RE: Grounding at light pole base
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.