Parking Lot Lights Ground Fault

[MD84]

I was called out to troubleshoot a tripping breaker for parking lot lighting. The the 50A breaker when reset and the load switched on would hold for about 20 seconds. I measured 130A during this time. The metal poles are powering 1000w lamps at 480v. One of the lines for a single lamp had faulted to the inside of the metal pole. It may have been a high impedance fault and/or the egc may have also had some poor connections. The pole was about 500' from the source.

I am assuming the fault current took parallel paths back to the source on the egc and through earth.

After repairs were made and while debriefing the maintenance staff it was asked if the pole was energized during the fault. My response was that the system operated properly to clear the fault. There was likely some voltage between the pole and ground but that it would be very small and definitely not the full system voltage.

Do you think any hazardous voltage could have been present from the pole to ground during the fault?

 

[iwire]

Do you think any hazardous voltage could have been present from the pole to ground during the fault?

Yes, not full circuit voltage but likely more than enough to harm someone if they are touching the pole and the earth.

 

[cowboyjwc]

And that is why it's so important to bond light poles.

 

[iwire]

And that is why it's so important to bond light poles.

Wiring 480 or 277 light poles incorrectly or making a bad EGC splice on the circuit to the poles is my biggest fear in my job. I always think of some poor innocent person touching a live pole on a rainy or slushy day.

 

[don_resqcapt19]

And that is why it's so important to bond light poles.

Even with it properly bonded to the EGC, if there is a fault to the pole, the pole will be energized with a voltage that is equal to the voltage drop on the fault return path. This will be a hazardous voltage.

 

[Gene B]

Since you know the current, you can make a rough calculation of the touch voltage. Look up the resistance of your EGC (assuming it's a wire) and multiply by the current.

Ideally, you would want to size the EGC so that at the instantaneous trip threshold of the breaker, the voltage across the EGC is under 30V or whatever you deem safe. Or you could install ground fault protection, which reduces the instantaneous trip threshold by a factor of 10,000.

 

[peter d]

Wiring 480 or 277 light poles incorrectly or making a bad EGC splice on the circuit to the poles is my biggest fear in my job. I always think of some poor innocent person touching a live pole on a rainy or slushy day.

Indeed, more often than not the EGC's in pole bases are spliced very poorly, usually as many #10's, 8's or 6's that can fit under one red or large blue wire nut. :roll:

 

[mivey]

Wiring 480 or 277 light poles incorrectly or making a bad EGC splice on the circuit to the poles is my biggest fear in my job. I always think of some poor innocent person touching a live pole on a rainy or slushy day.

You ever add GF protection and/or add in-line fuses at pole base?

 

[iwire]

You ever add GF protection and/or add in-line fuses at pole base?

I have never designed site lighting, I am following the prints or extending what is already in place or just repairing them.

I can't think of a time GF protection was specified on any print for site lighting. Fuses at the base are pretty common as a way to keep one shorted ballast from taking out the entire circuit.

A typical issue is the one Peter brought up, the EGC splice is done crappy due to the combination of wire sizes and number of them.

 

[meternerd]

Even with it properly bonded to the EGC, if there is a fault to the pole, the pole will be energized with a voltage that is equal to the voltage drop on the fault return path. This will be a hazardous voltage.

Kirchoff's Voltage Law.....all parallel circuits will see the same voltage. The difference is how much current you'll draw compared to the EGC. Too much and you're toast! That's where the idea of EPG (Equipotentiial Grounding) came from. Just sayin'

 

[don_resqcapt19]

Kirchoff's Voltage Law.....all parallel circuits will see the same voltage. The difference is how much current you'll draw compared to the EGC. Too much and you're toast! That's where the idea of EPG (Equipotentiial Grounding) came from. Just sayin'

The typical electrical equipment does not have EPG, and in many cases there will be a hazardous voltage on the equipment until the OCPD clears the fault. The idea is that the EGC will flow enough current to quickly clear the fault so that the hazardous voltage only exists for a very short time.

 

[MD84]

Thank you for the input. I will let the client know about he potential hazards and possible mitigation.

Does anyone here have experience with GF breakers on circuits like this where there can be quite long circuit runs. Would the charging current possibly be enough to trip the GF?

Does anyone here have any experience or input on a separately derived un-grounded system for parking lot lighting like this? It seems that this would be a good candidate for additional safety. One could run a Delta 480 secondary and leave it un-grounded. Use ground detectors and allow them to trip the circuit. This way the first ground fault would not be able to harm anyone. The fault could be repaired before a second ground fault occurs.

Kirchoff's Voltage Law.....all parallel circuits will see the same voltage. The difference is how much current you'll draw compared to the EGC. Too much and you're toast! That's where the idea of EPG (Equipotentiial Grounding) came from. Just sayin'

I would not define this scenario as a parallel circuit. The line that faulted to the inside of the pole was likely high impedance. A human touching the outside of the pole and ground would be in series with this impedance not directly connected to the line. Of course the pole is grounded and bonded. This starts to look like EPG excluding the resistance of the asphalt and soil.

 

[iwire]

Thank you for the input. I will let the client know about he potential hazards and possible mitigation.

In my opinion there is nothing to mitigate. Your clients situation is the same as every other site lighting. Every shopping center and business.

Does anyone here have experience with GF breakers on circuits like this where there can be quite long circuit runs. Would the charging current possibly be enough to trip the GF?

You would use adjustable GFP you could likely set the GFP high enough.

Does anyone here have any experience or input on a separately derived un-grounded system for parking lot lighting like this? It seems that this would be a good candidate for additional safety. One could run a Delta 480 secondary and leave it un-grounded. Use ground detectors and allow them to trip the circuit. This way the first ground fault would not be able to harm anyone. The fault could be repaired before a second ground fault occurs.

I believe that is prohibited for this type of application.

To me it seems you are trying to reinvent the wheel.

 

[MD84]

I understand that the lighting circuit is typical and meets code. The client asked so I feel that they should be given an accurate answer. My original response was accurate however I did not tell them that there is the potential for a hazardous voltage during a fault.

If they are concerned with it even though there circuit is the same as just about every other parking lot I will let them know some options. I am 90% sure they will not do anything. The adjustable GFP could be a possibility.

Just because it has been done a certain way in the past does not prohibit us from improving methods in the future. I think that we have a responsibility as those installing and servicing electrical equipment. These systems have the ability to kill people. Safety should be paramount. I do not think I am trying to re-invent the wheel. I am only trying to advise my client of possible hazards and options to mitigate them. If they are willing to increase the safety of a potentially hazardous installation then I am all for it.

 

[kwired]

There is a potentially hazardous voltage imposed on the EGC and objects bonded to it. The lower the impedance of the EGC and the supply conductors the faster overcurrent protection will respond to a fault. This is not limited to just parking lot light poles either.

Things to consider is how likley is there going to be someone touching the pole at the right time, and even then how well are they going to be grounded? If that chance were really high we would have enough statistical data and have different code requirements.

Also remember if the pole has a good bond to the EGC and there is a fault to the pole, electrically the pole is approximately at mid point of the voltage drop in the faulted circuit path, so touch voltage on that pole is reduced to around half of normal line to ground operating voltage also lessening the severity of what shock might be experienced, the pole/base probably has reasonable grounding electrode properties or even a ground rod - which doesn't automatically bleed off potential voltage, but will further mitigate some of the touch potential, so if you have a 120 volt circuit fault to the pole, around 60 volts is the mid point, then maybe you get the electrode to raise the potential where you are standing another 10-30 volts and the shock you receive maybe only has 30 to 50 volts behind it instead of full 120. Now at this point unless you are standing there barefoot or are otherwise contacting something at true ground potential, you probably have enough other insulation that you aren't even exposed to a significant enough voltage potential to feel.

Now put electrical items in or around a swimming pool and the risks do change and the reason those applications do require equipotential bonding methods.

 

[don_resqcapt19]

I understand that the lighting circuit is typical and meets code. The client asked so I feel that they should be given an accurate answer. My original response was accurate however I did not tell them that there is the potential for a hazardous voltage during a fault.

If they are concerned with it even though there circuit is the same as just about every other parking lot I will let them know some options. I am 90% sure they will not do anything. The adjustable GFP could be a possibility.

Just because it has been done a certain way in the past does not prohibit us from improving methods in the future. I think that we have a responsibility as those installing and servicing electrical equipment. These systems have the ability to kill people. Safety should be paramount. I do not think I am trying to re-invent the wheel. I am only trying to advise my client of possible hazards and options to mitigate them. If they are willing to increase the safety of a potentially hazardous installation then I am all for it.

Then they need to talk about this same issue for every item that is fed from a grounded system. This is not an issue that is exclusive to parking lot lights....this issue applies to every circuit supplied from a grounded system both inside and outside of structures.

 

[MD84]

The issue I had was that the fault lasted for 20 seconds before tripping. This increases the probability of causing harm to someone.

I was worried that a hazardous voltage would be present during that time. It sounds like it is agreed that is the case.

The hazardous voltage would be the voltage drop of the fault current return path. I do not completely agree as the touch voltage would not be across the entire fault current return path but a small section of it. This would reduce the voltage drop across the man. There is also the resistance of the earth and other insulation variables.

The reason I think the parking lot lighting is special is because of the very long runs that can occur. It seems as though it is common to have a high impedance fault current return path in these installations. This I believe warrants consideration for steps that would decrease fault clearing times or eliminate touch voltage.

I will take a closer look at the installation and see if the egc was properly sized. Is it common to have these long clearing times on these installations?

If we are saying that all grounded system circuits exhibit this issue than is it very common for ground faults to last for multiples of seconds? I would hope a ground fault would clear in fractions of a second.

 

[don_resqcapt19]

The issue I had was that the fault lasted for 20 seconds before tripping. This increases the probability of causing harm to someone.

Yes the hazard is there for the length of the fault....if the grounding system is in good condition, and the OCPD is functioning correctly, the longer clearing time usually means less fault current and there fore less voltage drop on the fault return path and less voltage to cause a shock.

I was worried that a hazardous voltage would be present during that time. It sounds like it is agreed that is the case.

The hazardous voltage would be the voltage drop of the fault current return path. I do not completely agree as the touch voltage would not be across the entire fault current return path but a small section of it. This would reduce the voltage drop across the man. There is also the resistance of the earth and other insulation variables.

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. At upstream poles on the same EGC the voltage drop and shock hazard would be less. In terms of the shock hazard, the resistance of the earth does not really change anything.

The reason I think the parking lot lighting is special is because of the very long runs that can occur. It seems as though it is common to have a high impedance fault current return path in these installations. This I believe warrants consideration for steps that would decrease fault clearing times or eliminate touch voltage.

Yes the longer the runs, the more likely it is to have this hazard.

I will take a closer look at the installation and see if the egc was properly sized. Is it common to have these long clearing times on these installations?

Looking at the trip curve for one 50 amp breaker, the time to trip for a 130 amp fault would be between 15 and 60 seconds. Your trip time was well within that trip curve. Remember the size of both the ungrounded conductors and the fault return path act to lower the amount of current that the fault can draw and the lower the current the longer it takes for the fault to clear.

What was the size of the supply conductors and the EGC? From that you could calculate what the voltage drop would have been on the EGC.

If we are saying that all grounded system circuits exhibit this issue than is it very common for ground faults to last for multiples of seconds? I would hope a ground fault would clear in fractions of a second.

There is no question that is happens on all grounded systems.
To clear in fractions of a second you often need to be in the instantaneous trip region of the trip curve, often requiring the fault to pull at least 8 to 10 times the rating of the OCPD.

To be sure of clearing in fractions of a second the trip curve for the 50 amp breaker that I looked at would require a fault current of between 450 and 1000 amps.

 

[MD84]

Thank you Don. That is very good information. I will see if I can determine the conductor sizes. The ungrounded conductor starts larger and gets smaller as it gets out to the subject pole. The egc is the same througout. It looked like #4 bare stranded copper.

 

[Gene B]

Thank you Don. That is very good information. I will see if I can determine the conductor sizes. The ungrounded conductor starts larger and gets smaller as it gets out to the subject pole. The egc is the same througout. It looked like #4 bare stranded copper.

500 feet of #4 copper is nominally 0.124ohms (at 100degreesF). Multiply by your measured 130 amps, and you get 16V. So good news, dangerous voltage may not have been present during those 20 seconds. You were right.

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.