Fault current path

[Isaiah]

Why does fault current on a service always run back to the source? For example a solidly grounded Delta-Wye transformer? is it because the XFMR center tap is always at very low resistance?


Sent from my iPhone using Tapatalk

 

[Shaneyj]

Why does fault current on a service always run back to the source? For example a solidly grounded Delta-Wye transformer? is it because the XFMR center tap is always at very low resistance?


Sent from my iPhone using Tapatalk

The source is where the circuit is completed.
We intentionally create a low as possible impedance to the grounded side of the source. (Ungrounded systems notwithstanding)
That allows the bulk of the fault to flow as directly back to the source as possible.
This allows a fault, when it happens, to shoot to orders of magnitude larger than intended allowing the OCPD to clear.


Sent from my iPhone using Tapatalk

 

[Isaiah]

The source is where the circuit is completed.
We intentionally create a low as possible impedance to the grounded side of the source. (Ungrounded systems notwithstanding)
That allows the bulk of the fault to flow as directly back to the source as possible.
This allows a fault, when it happens, to shoot to orders of magnitude larger than intended allowing the OCPD to clear.


Sent from my iPhone using Tapatalk

Excellent response!
If the “bulk of the fault” will flow back to the source. Does this mean a small amount of current will also go to earth?


Sent from my iPhone using Tapatalk

 

[Shaneyj]

Excellent response!
If the “bulk of the fault” will flow back to the source. Does this mean a small amount of current will also go to earth?


Sent from my iPhone using Tapatalk

Recall that current flows in direct inverse proportion to impedance. So whatever paths are available (and because we earth ground) some of that will flow to earth. But in an optimally functioning system, it’s small in comparison to what flows back on the “grounded conductor”.
Properly installed, etc: with respect to the ungrounded conductor, the grounded conductor (in a circuit as intended) will have the lowest impedance of all paths.


Sent from my iPhone using Tapatalk

 

[Shaneyj]

Recall that current flows in direct inverse proportion to impedance. So whatever paths are available (and because we earth ground) some of that will flow to earth. But in an optimally functioning system, it’s small in comparison to what flows back on the “grounded conductor”.
Properly installed, etc: with respect to the ungrounded conductor, the grounded conductor (in a circuit as intended) will have the lowest impedance of all paths.


Sent from my iPhone using Tapatalk

I guess I should clarify, flow to the source using earth as a path.
If you draw it up you can trace the path.
Earth Ground the service. Earth Ground the transformer (like most utilities do) and you’ll see


Sent from my iPhone using Tapatalk

 

[Isaiah]

Recall that current flows in direct inverse proportion to impedance. So whatever paths are available (and because we earth ground) some of that will flow to earth. But in an optimally functioning system, it’s small in comparison to what flows back on the “grounded conductor”.
Properly installed, etc: with respect to the ungrounded conductor, the grounded conductor (in a circuit as intended) will have the lowest impedance of all paths.


Sent from my iPhone using Tapatalk

Do you have any idea how much resistance is typically at the transformer XO? If the GEC ties to an earth electrode at 25 ohms—the max allowed by NEC—what would be usually seen at the transformer ?


Sent from my iPhone using Tapatalk

 

[Shaneyj]

Do you have any idea how much resistance is typically at the transformer XO? If the GEC ties to an earth electrode at 25 ohms—the max allowed by NEC—what would be usually seen at the transformer ?


Sent from my iPhone using Tapatalk

We have to speak in terms of relativity.
Resistance between what points?
If you’re asking X0 to X1/2/3 then it’ll be transformer windings.
If it’s X0 to service grounded point then it would be the value of the cable.
If you wanted to know X0 to ungrounded conductors, you’d have to lift the connections at X1/2/3 then measure.
And I think that 25 ohm value is only applicable when using just 1 rod. If you can’t prove 25 or less you use 2 rods and to my knowledge there’s no further confirmation required.
Also, those testers are not cheap so I think most just drive 2 rods and be done. There was a thread recently talking about that.


Sent from my iPhone using Tapatalk

 

[Isaiah]

We have to speak in terms of relativity.
Resistance between what points?
If you’re asking X0 to X1/2/3 then it’ll be transformer windings.
If it’s X0 to service grounded point then it would be the value of the cable.
If you wanted to know X0 to ungrounded conductors, you’d have to lift the connections at X1/2/3 then measure.
And I think that 25 ohm value is only applicable when using just 1 rod. If you can’t prove 25 or less you use 2 rods and to my knowledge there’s no further confirmation required.
Also, those testers are not cheap so I think most just drive 2 rods and be done. There was a thread recently talking about that.


Sent from my iPhone using Tapatalk

Thanks for great response!


Sent from my iPhone using Tapatalk

 

[jim dungar]

Excellent response!
If the “bulk of the fault” will flow back to the source. Does this mean a small amount of current will also go to earth?


Sent from my iPhone using Tapatalk

Not really, but probably.

Current takes all possible paths, think parallel resistors. Earth/dirt is not a good conductor at less than 1000V, so current is more likely to flow on metallic paths.

 

[don_resqcapt19]

Excellent response!
If the “bulk of the fault” will flow back to the source. Does this mean a small amount of current will also go to earth?


Sent from my iPhone using Tapatalk

It won't go to earth, but some of the current may flow through the earth back to the source.