Ground fault- Why doesn't anyone get shocked?

mbrooke

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It varies. Time to clear also matters. Go through the substation design steps.
It can get to 1.2 seconds coordinating reclosers in series vs a 15T that will blow in a few cycles. So the voltage must be below 300 volts I'd assume.


 

mivey

Senior Member
It can get to 1.2 seconds coordinating reclosers in series vs a 15T that will blow in a few cycles. So the voltage must be below 300 volts I'd assume.


The design deals with how many amps you can withstand for how long without electrocution.

Transferred voltage is what you are asking about I believe.
 

mbrooke

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The design deals with how many amps you can withstand for how long without electrocution.

Transferred voltage is what you are asking about I believe.


Neutral to remote earth voltage.


Guy using a metal drill or an electric barbecue in his backyard, feet are bare on grass.


The graph applies to all cases where voltage is high enough to push enough current across the body between two points like the hand and legs.
 

mivey

Senior Member
Neutral to remote earth voltage.


Guy using a metal drill or an electric barbecue in his backyard, feet are bare on grass.


The graph applies to all cases where voltage is high enough to push enough current across the body between two points like the hand and legs.
I'll make a model.
 

kwired

Electron manager
Location
NE Nebraska
But what if the home has plastic water pipes and just a single ground rod? Your outside using a corded drill with a metal frame and barefoot.
I would think risk increases, and would if lightning struck the MGN instead of having ungrounded conductor fall on it as well. I don't know how much that risk increases though. Isn't part of reason we have such grounding to lower potential for these kinds of events? MGN conductor, and the fact it is extended to EGC's is probably low enough impedance that voltage divide in the fault case is not 50-50 at the fault point, the MGN is probably lower impedance than the ungrounded conductor during this event, that would place higher voltage across the segment from supply to fault point. There would still be a rise on the grounded conductor above earth, but actual impedance of different portions of the circuit will determine how much, and it probably will vary from one location to another even though you still have a ungrounded to grounded conductor fault to start this thing.
 

mbrooke

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I would think risk increases, and would if lightning struck the MGN instead of having ungrounded conductor fall on it as well. I don't know how much that risk increases though. Isn't part of reason we have such grounding to lower potential for these kinds of events? MGN conductor, and the fact it is extended to EGC's is probably low enough impedance that voltage divide in the fault case is not 50-50 at the fault point, the MGN is probably lower impedance than the ungrounded conductor during this event, that would place higher voltage across the segment from supply to fault point. There would still be a rise on the grounded conductor above earth, but actual impedance of different portions of the circuit will determine how much, and it probably will vary from one location to another even though you still have a ungrounded to grounded conductor fault to start this thing.
Under typical conditions I would agree. But picture a scenario where the NESC allows the MGN to be smaller then the phase conductors and there are a limited number of parallel paths. 4 ground rods per mile and at every transformer is IMO substantially more impedance then copper water mains, gas mains and TELCO shields. The scenario I have in mind is predominately farms out in the boonies- no gas, well water and only a single thin phone line strung underneath the single phase line going for miles.
 

romex jockey

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. The scenario I have in mind is predominately farms out in the boonies- no gas, well water and only a single thin phone line strung underneath the single phase line going for miles.
I know of one farm between line extensions , i mean literally the cows are all standing on 'earth', or in mud that could be any potential

And yes, i've grounded everything, you name it , if you showed up in steel toed boots you'd be made into it all, etc, ad naseum....:p

But i digress.....how can 'equo' exist in any plane with changing parameters?
:?
~RJ~
 

kwired

Electron manager
Location
NE Nebraska
Under typical conditions I would agree. But picture a scenario where the NESC allows the MGN to be smaller then the phase conductors and there are a limited number of parallel paths. 4 ground rods per mile and at every transformer is IMO substantially more impedance then copper water mains, gas mains and TELCO shields. The scenario I have in mind is predominately farms out in the boonies- no gas, well water and only a single thin phone line strung underneath the single phase line going for miles.
I work with a lot of what you described. Mostly rural areas. Lines do fall onto other lines on occasion, nobody been shocked yet, don't know if it is just luck or if voltage just isn't as high as you are suggesting it can possibly be - meaning return path must be lower impedance than you are suggesting.

Several years ago we did have a 38.5kV conductor drop onto a 4.16kV local distribution line, it caused damages to many things all over the small town. But was enough hit and miss that I think one can say if you weren't on the phase it dropped on you probably had no damages. I don't know how much risk of shock may have been with this event, but AFAIK it dropped on a phase conductor and not the MGN, which maybe changes things some.
 

mbrooke

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I work with a lot of what you described. Mostly rural areas. Lines do fall onto other lines on occasion, nobody been shocked yet, don't know if it is just luck or if voltage just isn't as high as you are suggesting it can possibly be - meaning return path must be lower impedance than you are suggesting.

Several years ago we did have a 38.5kV conductor drop onto a 4.16kV local distribution line, it caused damages to many things all over the small town. But was enough hit and miss that I think one can say if you weren't on the phase it dropped on you probably had no damages. I don't know how much risk of shock may have been with this event, but AFAIK it dropped on a phase conductor and not the MGN, which maybe changes things some.
:eek:


Did you blow out any lightning arrestors on the 4.16kv?


Around here a few years back there was a really bad ice storm. On one home the 13.2kv conductor fell on the 120/240 secondary, several homes needed a complete and I mean complete re-wire. Everything that could arc did.
 

kwired

Electron manager
Location
NE Nebraska
:eek:


Did you blow out any lightning arrestors on the 4.16kv?


Around here a few years back there was a really bad ice storm. On one home the 13.2kv conductor fell on the 120/240 secondary, several homes needed a complete and I mean complete re-wire. Everything that could arc did.
Don't know about the lightning arrestors, that would have been POCO issue. I got several service calls, some electronic control boards in appliances were pretty common failure items, maybe some control transformer failures in HVAC units, I replaced many GFCI receptacles that failed, couple houses just about every GFCI in the house failed. Would get people all on same block fed by same phase or even same transformer that had problems, across the street (probably on another phase) no issues at all.
 

mbrooke

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Don't know about the lightning arrestors, that would have been POCO issue. I got several service calls, some electronic control boards in appliances were pretty common failure items, maybe some control transformer failures in HVAC units, I replaced many GFCI receptacles that failed, couple houses just about every GFCI in the house failed. Would get people all on same block fed by same phase or even same transformer that had problems, across the street (probably on another phase) no issues at all.
Was there any visible damage on control boards or GFCIs?
 

kwired

Electron manager
Location
NE Nebraska
Was there any visible damage on control boards or GFCIs?
Too long ago to be able to answer, but seems like in most GFCI instances no, never opened any of them up either though. I didn't get too involved with appliance control boards - let the appliance repair guys do that, but with easier availability today with internet suppliers, I may entertain the idea a little more if it were to happen now.
 

mbrooke

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Too long ago to be able to answer, but seems like in most GFCI instances no, never opened any of them up either though. I didn't get too involved with appliance control boards - let the appliance repair guys do that, but with easier availability today with internet suppliers, I may entertain the idea a little more if it were to happen now.
Always take pics, never regret it latter on. But I'm the same, back then I rarely thought to look at anything in depth. The internet has truly sparked curiosity in humanity.
 

mivey

Senior Member
Using 34.5 kV, 3ph OH primary wye MGN, large wire (556 ph, 336 neu), I_fault at sub = 9.47 kA LLL, 11.5 kA LN.

N-E voltage for L-N primary fault at service transformer:

1 SPAN (11.1 kA): 216 V
1/4 MILE (9.7 kA): 916 V
1/2 MILE (8.4 kA): 1.52 kV
1 MILE (6.7 kA): 2.2 kV
2 MILES (4.9 kA): 2.4 kV
3 MILES (4.0 kA): 2.2 kV
4 MILES (3.3 kA): 1.8 kV
 
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