Ground fault- Why doesn't anyone get shocked?

mbrooke

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Ok, say you have a 7,200 volt single phase line running down your street. A a tree falls on the primary and it comes down on the neutral conductor.


According to electrical theory 3,600 volts will drop across the phase conductor and 3,600 volts across the neutral going back to the substation.


My question is, why doesn't 3,600 volts appear on the service neutral and thus everything in the home thats grounded?
 

GoldDigger

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One reason is the multiple earth electrode grounds along the primary neutral. At the current and voltage present they are effective alternate fault current paths.

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mbrooke

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One reason is the multiple earth electrode grounds along the primary neutral. At the current and voltage present they are effective alternate fault current paths.

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But its still not zero ohms back to the substation- even if they halve the neutral resistance, there would still be around 1,800 volts? Remember that there are distribution systems up to 34.5kv.
 
Last edited:

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My question is, why doesn't 3,600 volts appear on the service neutral and thus everything in the home thats grounded?
Let's say that does happen.

Your system neutral, your water piping, your grounding electrodes, the earth around your house, everything around you, all energized to 3600v relative to the grounded neutral back at the sub-station.

You'd never know it because all of those things are at the same potential. You'd be like a bird on a primary wire or a lineman in a metallic suit tied to the line he's working on; no potential difference.

That's what equipotential bonding is all about. Preventing voltage gradients means preventing current flow. The absolute voltage is not as important as relative voltages between conductive surfaces.
 

GoldDigger

Moderator
Staff member
One serious risk would be that if you had an isolated earth electrode to a machine or PV array, then the exposed metal could be at a high potential to neutral unless a solid EGC connection to the main GES is also provided.
Not all earth in the vicinity of the building will be at the same potential, and there can be serious touch/step potential near the electrodes.

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romex jockey

Senior Member
Location
Vermont
One serious risk would be that if you had an isolated earth electrode to a machine or PV array, then the exposed metal could be at a high potential to neutral unless a solid EGC connection to the main GES is also provided.
Not all earth in the vicinity of the building will be at the same potential, and there can be serious touch/step potential near the electrodes.

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That's a study in of itself GoldOne.....~RJ~
 

mbrooke

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Let's say that does happen.

Your system neutral, your water piping, your grounding electrodes, the earth around your house, everything around you, all energized to 3600v relative to the grounded neutral back at the sub-station.

You'd never know it because all of those things are at the same potential. You'd be like a bird on a primary wire or a lineman in a metallic suit tied to the line he's working on; no potential difference.

That's what equipotential bonding is all about. Preventing voltage gradients means preventing current flow. The absolute voltage is not as important as relative voltages between conductive surfaces.


Makes sense- but what about a construction site or being outside with a grounded object? Technically remote earth is at my feet?
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Makes sense- but what about a construction site or being outside with a grounded object? Technically remote earth is at my feet?
I suppose the 3600v would create a voltage gradient between your "local" earth and the sub-station. Luckily, we don't have a great step length.

How far between your hypothetical fault and your hypothetical source? Divide that by 3600.

Obviously, you could see a voltage between earth and a point on the neutral that is not well earthed locally. You could do that at home in miniature.
 

mbrooke

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I suppose the 3600v would create a voltage gradient between your "local" earth and the sub-station. Luckily, we don't have a great step length.

How far between your hypothetical fault and your hypothetical source? Divide that by 3600.

Obviously, you could see a voltage between earth and a point on the neutral that is not well earthed locally. You could do that at home in miniature.
Never though about it like that...

Hmmmm...
 

GoldDigger

Moderator
Staff member
Only when the distance to the substation is comparable to the size of the sphere of influence of the GES at both ends.
Almost all of the voltage drop at the surface occurs within ten times the radius of the sphere of influence of the electrode system.
 

mivey

Senior Member
Let's say that does happen.

Your system neutral, your water piping, your grounding electrodes, the earth around your house, everything around you, all energized to 3600v relative to the grounded neutral back at the sub-station.

You'd never know it because all of those things are at the same potential. You'd be like a bird on a primary wire or a lineman in a metallic suit tied to the line he's working on; no potential difference.

That's what equipotential bonding is all about. Preventing voltage gradients means preventing current flow. The absolute voltage is not as important as relative voltages between conductive surfaces.
Ditto.
 
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