Grounding Electrodes as Equal Potential Devices

[mbrooke]

I was watching a video where Mike energized a ground rod, and had a huge voltage gradient which actually surprised me to be honest:

I was wondering. When a 19,920 volt single phase primary faults to the MGN, do the voltage gradients that form around NEC mandates rods and ground electrodes actually elevate the voltage around a property such that the potential between a grounded object like a tool or hot tub is reduced to the earth someone is standing on? Or does it make it worse? Or there is no effect?

I'm imagining 9,960 volts coming in through the service neutral. Feet on the ground, hand on an electric grill, ect.

 

[don_resqcapt19]

The grounding electrode does not keep things at an equal potential...they just create step potentials around the rod and to other conductive items that are not connected to the electrical grounding system. If you want equalpotential, you have to install a grid like they do in substation yards.

For your hand on something connected to 10kV and standing on the earth, unless you are standing on the grounding electrode itself, you are toast. So in general it has no effect on reducing any type of shock hazard.

 

[mbrooke]

The grounding electrode does not keep things at an equal potential...they just create step potentials around the rod and to other conductive items that are not connected to the electrical grounding system. If you want equalpotential, you have to install a grid like they do in substation yards.

For your hand on something connected to 10kV and standing on the earth, unless you are standing on the grounding electrode itself, you are toast. So in general it has no effect on reducing any type of shock hazard.

Even if you are in a potential gradient shell? I'd guess it would reduce this voltage.

 

[kwired]

Even if you are in a potential gradient shell? I'd guess it would reduce this voltage.

Most cases the gradient shells are not very big though and you can easily contact more than one shell at a time especially when the electrode is a simple ground rod- can easily be electrocuted just walking over the rod when it is energized by medium voltage levels.

 

[mbrooke]

Most cases the gradient shells are not very big though and you can easily contact more than one shell at a time especially when the electrode is a simple ground rod- can easily be electrocuted just walking over the rod when it is energized by medium voltage levels.

There is that. How does the code deal with MV faults coming into the building?

 

[kwired]

Shells can be relatively small even when voltage is 120 volts. Dry soil around a rod and you might drop all 120 volts in just a foot or so.

 

[mbrooke]

Shells can be relatively small even when voltage is 120 volts. Dry soil around a rod and you might drop all 120 volts in just a foot or so.

What about 10,000 volts? I'd imagine a large area.

 

[kwired]

What about 10,000 volts? I'd imagine a large area.

Possibly, I'm not going to walk through the area in question to find out though.

 

[mbrooke]

Possibly, I'm not going to walk through the area in question to find out though.

I get you lol.

But I've always wondered about MV-LV faults and their impact on the consumer.

 

[kwired]

I get you lol.

But I've always wondered about MV-LV faults and their impact on the consumer.

It happens. Usually only to nearby consumer from the point where the fault occurs.

Remember during ice storms we had about 15 years ago, primary line dropped on secondary lines and nearby house called fire department. Don't know the details, and has been long enough I forgot some of what I did know, but something started on fire as a result in that house, might be lucky nobody was in wrong place at wrong time and risked electrocution.
Same town different incident had transmission conductor drop onto a distribution conductor. Took entire town off line when transmission line was interrupted, but when things were going again there were damaged items all around town. Seemed to be in clusters though. for instance I had service calls for two or three houses on same block but nobody on adjacent blocks. My guess is all those on the one block were all supplied by same primary phase that the transmission line contacted and adjacent block maybe on a different phase. One house I remember had damaged electronic range control board, maybe computer or other electronics damaged, house behind them had nearly every GFCI in the house no longer working but not much else damaged.

 

[mbrooke]

It happens. Usually only to nearby consumer from the point where the fault occurs.

Remember during ice storms we had about 15 years ago, primary line dropped on secondary lines and nearby house called fire department. Don't know the details, and has been long enough I forgot some of what I did know, but something started on fire as a result in that house, might be lucky nobody was in wrong place at wrong time and risked electrocution.
Same town different incident had transmission conductor drop onto a distribution conductor. Took entire town off line when transmission line was interrupted, but when things were going again there were damaged items all around town. Seemed to be in clusters though. for instance I had service calls for two or three houses on same block but nobody on adjacent blocks. My guess is all those on the one block were all supplied by same primary phase that the transmission line contacted and adjacent block maybe on a different phase. One house I remember had damaged electronic range control board, maybe computer or other electronics damaged, house behind them had nearly every GFCI in the house no longer working but not much else damaged.

Sounds like stuff that happened around here. In the case of 7,200 getting onto the secondary hots that usually means a total re-wire. Tranmission into distrbution tends to raise the voltage by a few times so no side flashing but plenty of cooked electronics.

 

[kwired]

Sounds like stuff that happened around here. In the case of 7,200 getting onto the secondary hots that usually means a total re-wire. Tranmission into distrbution tends to raise the voltage by a few times so no side flashing but plenty of cooked electronics.

The primary onto the 120/240 I mentioned happened to be 2400 to ground, 7200 probably would have risk for even more damages.

Add: but 7200 possibly would have had faster clearing time, so total incident energy is a factor.

 

[mbrooke]

The primary onto the 120/240 I mentioned happened to be 2400 to ground, 7200 probably would have risk for even more damages.

Add: but 7200 possibly would have had faster clearing time, so total incident energy is a factor.

True, time also plays a role. High voltage also means more current and faster fuse blowing, of one that is typically rated less.

 

[hillbilly1]

Got to see the damage done when the high line hits a parking lot pole. An auto accident knocked down the high line (don’t know what the line voltage was) onto a steel light pole, flashed over every contactor in the cabinet, burning the wiring back to the wire way below it. Amazingly they didn’t loose any electronics in the store. Since it was during the day, the contactors were not closed, so that may have helped.

 

[kwired]

Got to see the damage done when the high line hits a parking lot pole. An auto accident knocked down the high line (don’t know what the line voltage was) onto a steel light pole, flashed over every contactor in the cabinet, burning the wiring back to the wire way below it. Amazingly they didn’t loose any electronics in the store. Since it was during the day, the contactors were not closed, so that may have helped.

Most current was likely on EGC's some probably found a way to lighting ungrounded conductors and if flashed over the contactors may have tripped feeder or branch circuit breakers but by the time the surge reached that point it was lessened enough to not be too damaging to electronics in the store, plus surge protectors both external and internal to the electronics had some chance to divert whatever surge made it that far as well.

Sort of no different than lightning striking distribution pole outside the store. Some surge may come into the store but a lot of it is dissipated out at the pole and at other distribution items between it and the store.