Parking Lot Lights Ground Fault

[MD84]

There is also the resistance of the ungrounded conductor. That is larger however.

Re: Gene B:

In any case, the greatest danger comes from the possibility of a compromised EGC, not the window of vulnerability between a ground fault occurring and the breaker tripping.

I see what you are saying and there is some truth to it. I do feel however that the duration of the fault contributes quite a bit to the overall hazard of the fault. Not only does the duration increase the probability of someone getting shocked it also increases the possibility of a lethal shock. The severity of a shock not only depends on the voltage exposure but also the duration current is flowing.

I do have some actual data to contribute.


480v Y
50A Seimens BQD 2 Pole CB (brown and orange phase)

130A measured at CB Load Side brown phase during fault
30A measured in first JB EGC during fault

150' 1 AWG Copper to first JB
300' 1 AWG Copper to first Pole
180' 8 AWG Copper to faulted Pole
3' 10 AWG Copper to fault location inside Pole

8 AWG Copper EGC returning the same route as above (630')

That is 0.172 ohms for the ungrounded conductor and 0.396 ohms for the EGC. A total of 0.568 ohms for the circuit. This does not include any resistance of splices or the impedance of the fault.

This is a relatively low resistance circuit and if the full circuit voltage was present there would likely be current in or approaching the instantaneous trip range.

 

[kwired]

There is also the resistance of the ungrounded conductor. That is larger however.

Re: Gene B:

In any case, the greatest danger comes from the possibility of a compromised EGC, not the window of vulnerability between a ground fault occurring and the breaker tripping.

I see what you are saying and there is some truth to it. I do feel however that the duration of the fault contributes quite a bit to the overall hazard of the fault. Not only does the duration increase the probability of someone getting shocked it also increases the possibility of a lethal shock. The severity of a shock not only depends on the voltage exposure but also the duration current is flowing.

I do have some actual data to contribute.


480v Y
50A Seimens BQD 2 Pole CB (brown and orange phase)

130A measured at CB Load Side brown phase during fault
30A measured in first JB EGC during fault

150' 1 AWG Copper to first JB
300' 1 AWG Copper to first Pole
180' 8 AWG Copper to faulted Pole
3' 10 AWG Copper to fault location inside Pole

8 AWG Copper EGC returning the same route as above (630')

That is 0.172 ohms for the ungrounded conductor and 0.396 ohms for the EGC. A total of 0.568 ohms for the circuit. This does not include any resistance of splices or the impedance of the fault.

This is a relatively low resistance circuit and if the full circuit voltage was present there would likely be current in or approaching the instantaneous trip range.

I didn't run any calculations, but seems to me the 8 AWG EGC should likely be larger if you ran 1 AWG ungrounded conductors here to comply with 250.122(B).

Did you have metal raceway? If you have 130 amps on the ungrounded conductor and only 30 amps on the portion of EGC you measured, obviously there was a lower resistance path carrying the remaining 100 amps somewhere.

 

[MD84]

With the parking lot example about the only place a person could come into contact with the fault return path is by touching the light pole. If he is touching the one with the fault, or poles that are connected to the same EGC down stream of the fault, he would be subjected to the full voltage drop of the fault return path.

I am still trying to understand how he would be subjected to the full voltage drop of the fault return path. I am with you as far as touching the faulted light pole. Where I have trouble is the other point. A voltage exists between two points. The man is touching the light pole and experiencing a voltage between what other point?

I have an idea but it does not make sense to me as far as being subjected to the full voltage drop of the fault return path.

 

[MD84]

I didn't run any calculations, but seems to me the 8 AWG EGC should likely be larger if you ran 1 AWG ungrounded conductors here to comply with 250.122(B).

Did you have metal raceway? If you have 130 amps on the ungrounded conductor and only 30 amps on the portion of EGC you measured, obviously there was a lower resistance path carrying the remaining 100 amps somewhere.

I agree with you here. I did not install this system. It starts out as metal raceway but I could not determine the construction once it leaves the first junction box. I do not think it is metal underground but I am not positive.

I am thinking that the network of light poles bolted into concrete are making a nice array of ground electrodes. The panel is bonded to building steel and here to likely has a large contact area with earth. The earth can act as a very good conductor.

 

[Gene B]

I do feel however that the duration of the fault contributes quite a bit to the overall hazard of the fault. Not only does the duration increase the probability of someone getting shocked it also increases the possibility of a lethal shock. The severity of a shock not only depends on the voltage exposure but also the duration current is flowing.

Agreed, but there's no correlation between touching the pole and a ground fault. They are independent events. Unlike a handheld power tool, for example, where the tool is likely to be touched when the fault occurs.

 

[MD84]

Agreed, but there's no correlation between touching the pole and a ground fault. They are independent events. Unlike a handheld power tool, for example, where the tool is likely to be touched when the fault occurs.

Yes the probability of touching a light pole during a fault is much much lower than touching a power tool during a fault. There really isn't any good reason to touch a light pole anyway. It is not like a bus stop or something designed to be used in a way where direct contact is necessary. I suppose someone somewhere has been shocked by touching a light pole.

I appreciate all the feedback. I understand I may be looking too far into this. I am hungry to learn about my craft and this is a great way to do it. I think there may be some issues lurking in that installation however. Since the client did express interest in understanding the hazards I feel as though this is a good one to really dissect. Maybe myself and others will learn something along the way.

Perhaps tomorrow I will find some time to check if the size of the EGC is proper. If I go back to that location I will see if I can determine the raceway construction. I would also like to do some circuit analysis to see If I can approximate the resistance of the faulted line to the pole. It will take a few assumptions since I am unsure of the actual resistance of the return path of the fault current.

 

[kwired]

I am still trying to understand how he would be subjected to the full voltage drop of the fault return path. I am with you as far as touching the faulted light pole. Where I have trouble is the other point. A voltage exists between two points. The man is touching the light pole and experiencing a voltage between what other point?

I have an idea but it does not make sense to me as far as being subjected to the full voltage drop of the fault return path.

That other point is ground, but the earth around the pole likely has voltage gradient zones around it, so the further away from the pole you are measuring to, the higher the measurement will be. Now throw in the insulating properties of footwear the potential victim is wearing and you change what voltage they may be exposed to as well.

Those voltage gradient zones will be different if there is an attached EGC taking some current away then if the pole is not bonded to the electric system at all and earth has to carry all the fault return current, so voltage drop of both the ungrounded supply conductor and any grounding return conductor have in impact on what will be measured, and the more current that is flowing the more that drop will be. If the grounding and ungrounded conductor are same size and type then you should get fairly equal drop on each conductor (in the absence of other return paths) and voltage to ground at the fault point will be half the applied voltage to ground less any drop because of source impedance which would typically only be a significant factor on a small capacity source like a control transformer.

So in your case where you measured current of 130 amps at the source end of the branch circuit and only 30 amps on a return path at the pole, that means there is another return path taking the rest of the current, where that current is flowing will have an impact on what voltage to earth is at different points along the circuit, and the circuit in such a case includes the EGC and all other bonded objects when they are carrying current, under normal conditions they are not considered a part of the circuit because they don't carry current under normal conditions.

 

[MD84]

Thank you kwired. This is exactly my understanding. All things considered I do not believe a very large voltage would have been experienced by someone who happened to be touching the pole.

 

[CONDUIT]

Can you put a in line fuse at each pole where the power comes in at the base. Depending on where the fault was inside the pole.

 

[augie47]

From you data in Post #22, am I correct that your EGC is #8 for the full length of the installation ?

 

[kwired]

Can you put a in line fuse at each pole where the power comes in at the base. Depending on where the fault was inside the pole.

You can, it will not change how much voltage will be present on the pole during the fault, depending on trip curve characteristics of the fuse it could lessen the duration though.

Should you happen to select a fuse with longer trip time then the branch circuit device the shock hazards at the pole will remain unchanged and the branch circuit device will trip before the (poorly chosen) supplemental device can trip.

No matter what there is some time period after the fault occurs and up to the time of overcurrent device operation where there is a voltage rise on the light pole, other conditions will determine how much that rise is.

 

[MD84]

From you data in Post #22, am I correct that your EGC is #8 for the full length of the installation ?

Yes. Correct. #8.

I think it could have been upsized perhaps to #3 while it runs with the upsized #1.

 

[don_resqcapt19]

I am still trying to understand how he would be subjected to the full voltage drop of the fault return path. I am with you as far as touching the faulted light pole. Where I have trouble is the other point. A voltage exists between two points. The man is touching the light pole and experiencing a voltage between what other point?

I have an idea but it does not make sense to me as far as being subjected to the full voltage drop of the fault return path.

The earth stays at zero voltage as long as the pole base is not acting as a grounding electrode and the person touching the light pole is subject to a shock that is equal to the voltage drop on the fault return path. That voltage drop raises the voltage of the pole as measured to earth around the pole.

There may be some earth potential rise very close to the pole if the pole has a base that is acting as a grounding electrode, but that potential rise drops very quickly as you move away from the pole. At 1' from the grounding electrode the earth voltage would rise about 32% so the shock hazard would be ~68% of the fault return path voltage drop. At 3' the shock hazard is 75% of the fault return path

 

[MD84]

The earth stays at zero voltage as long as the pole base is not acting as a grounding electrode and the person touching the light pole is subject to a shock that is equal to the voltage drop on the fault return path. That voltage drop raises the voltage of the pole as measured to earth around the pole.

There may be some earth potential rise very close to the pole if the pole has a base that is acting as a grounding electrode, but that potential rise drops very quickly as you move away from the pole. At 1' from the grounding electrode the earth voltage would rise about 32% so the shock hazard would be ~68% of the fault return path voltage drop. At 3' the shock hazard is 75% of the fault return path

This was what I was thinking since the pole is bolted to the concrete and acting as a ground electrode.

Are you saying that the resistance of the earth relative to the person touching the pole would effect the shock hazard?

 

[jumper]

Graphic for what Don said:

View attachment 14609

 

[MD84]

I think there is a contradiction here. It seems as though the resistance of earth does play a role in the level of shock hazard. I am trying to confirm what I believe to be correct or learn an error I may have made.

In terms of the shock hazard, the resistance of the earth does not really change anything.

Graphic for what Don said:

View attachment 14609

The attached graphic shows a system that does not have a low impedance EGC. Please explain how this graphic would be modified when a ground electrode and an equipment grounding conductor are connected.

 

[augie47]

Yes. Correct. #8.

I think it could have been upsized perhaps to #3 while it runs with the upsized #1.

'

If I did the math correctly, a #3 would be the correct EGC. Correcting this might well improve the situation and remove the liability of a none Code compliant insrtall.

 

[don_resqcapt19]

...
The attached graphic shows a system that does not have a low impedance EGC. Please explain how this graphic would be modified when a ground electrode and an equipment grounding conductor are connected.

The only difference would be that the pole would not be at 120 volts to "remote" earth (earth outside the influence of a grounding electrode) but would be a voltage equal to the voltage drop on the fault return path.

 

[kwired]

I think there is a contradiction here. It seems as though the resistance of earth does play a role in the level of shock hazard. I am trying to confirm what I believe to be correct or learn an error I may have made.





The attached graphic shows a system that does not have a low impedance EGC. Please explain how this graphic would be modified when a ground electrode and an equipment grounding conductor are connected.

I was going to point that out - there is no EGC in that graphic. If you have an EGC the touch voltages are going to be lower, but you still will have different voltage levels in the earth as you get further from the pole. How much voltage to true ground you have at the pole itself will depend on how much voltage drop you have in the EGC, the presumed undersized #8 will have more resistance then a correctly sized EGC in your installation, and that extra resistance means less voltage has dropped at the pole point of the fault path, thus higher touch potential at the pole.

 

[don_resqcapt19]

I think there is a contradiction here. It seems as though the resistance of earth does play a role in the level of shock hazard. I am trying to confirm what I believe to be correct or learn an error I may have made. ....

The fault energizes the earth around the grounding electrode when there is voltage source connected to the grounding electrode. As you move away from the grounding electrode the amount of voltage that is on the "earth" surrounding the electrode, as measured to remote earth, drops.
The voltage values shown in the graphic are based on the assumption that you have almost 100% of the earth resistance between an grounding electrode and the earth at a point 25' away from the electrode. While this is not completely accurate, it is very close. Studies show the there is very little increase in the resistance as you move away from the rod more than 25'.
The values in the graphic are base on this information from the IEEE Green Book.

Table 9
Electrode Resistance at a Radius r ft from a 10 ft (3 m) Long by ⁵/₈ in (16 mm) Diameter Rod (Where Total Resistance
at r = 25 ft (7.6 m) =100%)

Distance from Electrode Surface (r) ft/M	Approximate Percentage of Total Resistance
0.1	0.03	25
0.2	0.06	38
0.3	0.09	46
0.5	0.15	52
1.0	0.3	68
5.0	1.5	86
10.0	.3.0	94
15.0	4.6	97
20.0	6.1	99
25.0	7.6	100
(100.0)*	30.5	(104)
(1000.0)*	305.0	(117)

*These figures show that for the most practical reasons the majority of the resistance to remote earth occurs within 25 ft of the electrode, i.e., at 1000 ft the resistance is only 17% higher than that of 25

The table did not come out exactly correct, the first column is the radius distance in feet from the ground rod and the second is in meters.