Exposed live conductor in contact with earth.

[jap]

Dug up and repaired an underground 120/240v aluminum feeder to a building that had burnt up.

Made me wonder what hazard the exposed live conductors would have when the insulation burnt off and were in contact with the earth.

This feeder passed through a muddy barn lot where #2 urd was ran to feed an outbuilding.
The tractor tires sinking in the lot nicked the cable and once exposed went downhill from there.

The feeder was energized but was burnt almost completely in half.

To my knowledge it did not affect the cattle stepping on and around the muddy place where the conductors had burnt in two.

If voltage and amperage does try to get back to it's source in such an instance, how much of a danger is it to persons or livestock that may be around it, and , how far would that voltage radiate from the damaged conductor?

There were no grounded parts close to the break in the cable.


JAP>

 

[ptonsparky]

We do a lot of those repairs and eventually the conductors create a high resistance fault to earth. I've seen faults in sand that form glass and dirt can dry up to about like concrete. Depends on the luck of the draw as to what it affects. Dairy situations could have a major impact on production. I think typically the step potential drops off fairly quickly but again that would depend on your situations.

 

[steve66]

Never thought about it before, but it seems like the closer a grounding electrode is to the break, the more likely there is to be a step potential that might be dangerous.

Assume its a 120V line that is exposed to the dirt, and assume 120' to the nearest ground rod, with evenly moist soil all the way back. The 120 volts is dropped across 120 feet, giving a 1 volt/foot step potential.

Now assume someone is barefoot and standing between the break and the ground rod, and assume their feet are about 2' apart. They would only have about 2 volts between their feet. (1 volt/foot * 2 feet = 2 volts.)

Now assume the ground rod is only 12' away from the break. Now the voltage is dropped across 12', giving 10 volts per foot. Now the same person standing between the break and the ground rod would have 20 volts between their feet.

Seems counter-intuitive. Or am I missing something?

Of course, if our same person manages to touch anything grounded, it quickly becomes much more dangerous, especially if they are close to the break.

 

[jap]

Never thought about it before, but it seems like the closer a grounding electrode is to the break, the more likely there is to be a step potential that might be dangerous.

Assume its a 120V line that is exposed to the dirt, and assume 120' to the nearest ground rod, with evenly moist soil all the way back. The 120 volts is dropped across 120 feet, giving a 1 volt/foot step potential.

Now assume someone is barefoot and standing between the break and the ground rod, and assume their feet are about 2' apart. They would only have about 2 volts between their feet. (1 volt/foot * 2 feet = 2 volts.)

Now assume the ground rod is only 12' away from the break. Now the voltage is dropped across 12', giving 10 volts per foot. Now the same person standing between the break and the ground rod would have 20 volts between their feet.

Seems counter-intuitive. Or am I missing something?

Of course, if our same person manages to touch anything grounded, it quickly becomes much more dangerous, especially if they are close to the break.

That seems to be key.
Nothing grounded in the lot.

Would one even notice anything standing over a break in the mud? or, do you just become part of the current and not really notice anything?

JAP>

 

[ptonsparky]

That seems to be key.
Nothing grounded in the lot.

Would one even notice anything standing over a break in the mud? or, do you just become part of the current and not really notice anything?

JAP>

Depends, are you, from one foot via legs to the other foot, of significantly less resistance than what the path of the puddle is? The current is trying to find a path back to source and the other end of the conductor is just inches away. It's an open switch.

You do not want to become part of the current. Remember 5 ma hurts.

 

[jap]

I'm just thinking there are probably tons of underground conductors out there with damaged or missing insulation in direct contact with the dirt.

Just not sure why that doesn't create more of an issue than what we hear about.

JAP>

 

[electrofelon]

Never thought about it before, but it seems like the closer a grounding electrode is to the break, the more likely there is to be a step potential that might be dangerous.

Assume its a 120V line that is exposed to the dirt, and assume 120' to the nearest ground rod, with evenly moist soil all the way back. The 120 volts is dropped across 120 feet, giving a 1 volt/foot step potential.

Perhaps it is not linear per unit distance? I think as you move away from the dirt-conductor contact area, you gain more paths geometrically as you move away. Probably whether the utility uses a MGN makes a difference to. If its just a delta distribution, it can only return to the rods at the house, outbuilding, and probably utility has one at the base of the pole. If its a MGN, it can get back to a zillion different places= lower resistance=more current= more voltage gradient? Sort of thinking out loud. Interesting question.

 

[Buck Parrish]

I had this happen to a garage circuit. But it was on the neutral. Their was a pool near by.
It was an in ground swimming pool and the pool panel fed off of the garage panel.
If all of the metal parts to the pool are bonded. Wouldn't that neutral go to one of them?

 

[kwired]

We do a lot of those repairs and eventually the conductors create a high resistance fault to earth. I've seen faults in sand that form glass and dirt can dry up to about like concrete. Depends on the luck of the draw as to what it affects. Dairy situations could have a major impact on production. I think typically the step potential drops off fairly quickly but again that would depend on your situations.

I do a lot of those repair also. No issue is ever discovered until the conductor is in an "open" condition. Aluminium conductors don't take as long to burn open as copper conductors. I can see current heating up the soil and reducing resistance at the point of contact, which will slow down degrading of the conductor, as well as reduce any surrounding voltage gradients in the soil. Put this in a place with high moisture or even high water table and it maybe isn't going to dry anything out.

Never thought about it before, but it seems like the closer a grounding electrode is to the break, the more likely there is to be a step potential that might be dangerous.

Assume its a 120V line that is exposed to the dirt, and assume 120' to the nearest ground rod, with evenly moist soil all the way back. The 120 volts is dropped across 120 feet, giving a 1 volt/foot step potential.

Now assume someone is barefoot and standing between the break and the ground rod, and assume their feet are about 2' apart. They would only have about 2 volts between their feet. (1 volt/foot * 2 feet = 2 volts.)

Now assume the ground rod is only 12' away from the break. Now the voltage is dropped across 12', giving 10 volts per foot. Now the same person standing between the break and the ground rod would have 20 volts between their feet.

Seems counter-intuitive. Or am I missing something?

Of course, if our same person manages to touch anything grounded, it quickly becomes much more dangerous, especially if they are close to the break.

EF hit on what I was going to reply with:

Perhaps it is not linear per unit distance? I think as you move away from the dirt-conductor contact area, you gain more paths geometrically as you move away. Probably whether the utility uses a MGN makes a difference to. If its just a delta distribution, it can only return to the rods at the house, outbuilding, and probably utility has one at the base of the pole. If its a MGN, it can get back to a zillion different places= lower resistance=more current= more voltage gradient? Sort of thinking out loud. Interesting question.

It is not linear. "voltage zones" are higher voltage but smaller zone near the fault. Also to get a shock of 120 volts you have to contact the "very close" to the fault point and a point away from there, though it won't be but a few feet away. If conductor is buried 24" deep, the surface directly above may only have less then 5 volts to points that are only 3-5 feet horizontally away. Human with shoes on won't feel it at all, barefooted possibly if you spread your feet far enough apart. Larger livestock possibly will feel it as they may have longer distance between front and rear feet - again is not at a voltage level that will likely be lethal to them, they are discomforted and they probably just move away. I do run into this with a fault in cattle water tanks (they have electric heaters in them to keep from freezing). Tank can be at full 120 volts, but cattle feel shock before they even get to the tank, and they just stay away from the tank. If one would happen to touch the tank they typically do get electrocuted, but that usually only happens if they are already there when the fault occurs, the rest of the time they just won't go near the tank. Pigs however are more curious creatures and are going to check it out. If there is a faulted tank around them - you will find dead pigs at the tank almost every time.

I had this happen to a garage circuit. But it was on the neutral. Their was a pool near by.
It was an in ground swimming pool and the pool panel fed off of the garage panel.
If all of the metal parts to the pool are bonded. Wouldn't that neutral go to one of them?

Current will likely flow to the bonded pool equipment, but properly bonded pool equipment is still safe for users of the pool, if properly bonded everything rises the same amount in voltage potential and there is no voltage between any points at the pool - the users are similar to a bird sitting on a high voltage line - though they are at same potential as the line, as long as they can't touch anything that is at a different potential they never will get shocked.

 

[kwired]

I have hand dug around such lines for around 30 years, even still energized lines (sometimes not even knowing there is an insulation problem, just happened to be digging for other reasons and trying to locate a line so we don't damage it) and never have gotten shocked that I can recall unless I actually touched the exposed conductor, that voltage drops off pretty fast as you get away from the fault. Over 600 volt systems however you don't want to be digging near such a fault while it is still energized.

 

[meternerd]

Perhaps it is not linear per unit distance? I think as you move away from the dirt-conductor contact area, you gain more paths geometrically as you move away. Probably whether the utility uses a MGN makes a difference to. If its just a delta distribution, it can only return to the rods at the house, outbuilding, and probably utility has one at the base of the pole. If its a MGN, it can get back to a zillion different places= lower resistance=more current= more voltage gradient? Sort of thinking out loud. Interesting question.

It's not linear....the voltage is the square root of the distance. Double the distance and you have 1/4 the voltage. Of course, that's in a soil with exactly the same resistance per foot over the total distance. Not likely. Higher resistance, more voltage drop. Same for radiation, sound, etc.

 

[wwhitney]

the voltage is the square root of the distance. Double the distance and you have 1/4 the voltage.

I think you mean "inverse square" of the distance, which is what your second sentence expresses.

Cheers, Wayne

 

[meternerd]

I think you mean "inverse square" of the distance, which is what your second sentence expresses.

Cheers, Wayne

Yup...that's the correct term....been a while since college. Glad I'm retired....I hated math!

 

[ggunn]

I think you mean "inverse square" of the distance, which is what your second sentence expresses.

Cheers, Wayne

The inverse square law refers to a spherical dispersion; this would be a hemispherical dispersion. Wouldn't the voltage drop off as the square of the distance divided by two? I'm not sure the inverse square law applies here, though.

 

[GoldDigger]

The inverse square law refers to a spherical dispersion; this would be a hemispherical dispersion. Wouldn't the voltage drop off as the square of the distance divided by two? I'm not sure the inverse square law applies here, though.

The difference between x², x²/2, and (x/2)² is just a constant (1:.5:.25) so the fall off with distance would be exactly the same.
What is different is that close to the rod it looks like current radiating from a line while farther away it looks like current radiating from a point.
As long as r is smaller than L, the length of the rod, the voltage gradient (current density) is proportional to 1/r while farther away it is is proportional to 1/r². As an approximation the step potential will be the step distance times the voltage gradient.

This means in turn that the absolute voltage, relative to the rod as zero, is going to be the integral over r of that function.

 

[kwired]

And unless soil is extreme high moisture saturation, chances are it gets dried out some near the fault - meaning even more voltage drops just in the first few inches away from the fault location.

 

[ggunn]

The difference between x², x²/2, and (x/2)² is just a constant (1:.5:.25) so the fall off with distance would be exactly the same.
What is different is that close to the rod it looks like current radiating from a line while farther away it looks like current radiating from a point.
As long as r is smaller than L, the length of the rod, the voltage gradient (current density) is proportional to 1/r while farther away it is is proportional to 1/r². As an approximation the step potential will be the step distance times the voltage gradient.

This means in turn that the absolute voltage, relative to the rod as zero, is going to be the integral over r of that function.

Interesting, but doesn't that assume that the voltage drops at the same rate in all directions? Wouldn't the voltage gradient be at least mostly pointed toward the grounding point of the source of the current to the fault?

 

[GoldDigger]

Interesting, but doesn't that assume that the voltage drops at the same rate in all directions? Wouldn't the voltage gradient be at least mostly pointed toward the grounding point of the source of the current to the fault?

That (a directional effect) would be seen only if the distance between the two ground electrodes in question was small compared to the size of the larger of the two electrodes.
If, for example, two 8' long rods were 500 feet apart and there were no other electrodes present within a mile, I doubt that you would see a measurable directionality within 20' of either electrode. But I have not actually done the math.

Sent from my XT1585 using Tapatalk

 

[jap]

So in summary, not much of a shock hazard at all even though the energized conductor is in direct contact with the earth ?


JAP>

 

[ptonsparky]

I wouldn’t grab the end of it and shut it off before digging.