AC Faults

[brian john]

In an AC distribution system where you have multiple ground paths, (possible) copper EGC, metallic conduit, rebar, structural steel metal studs, metallic water piping and the like, I know the current will take all available paths but does the majority of the current follow the path of the feeder conduit and/or copper/Alum EGC.

 

[raider1]

I believe that the majority of the fault current will flow on the metallic raceway as opposed to the wire type EGC. The raceway will have a much greater surface area then wire type EGC.

Chris

 

[jusme123]

I believe that the majority of the fault current will flow on the metallic raceway as opposed to the wire type EGC. The raceway will have a much greater surface area then wire type EGC.

Chris

...which do you think will have the lower resistance?

 

[brian john]

I have been involved in many discussions conduit versus EGC of AL/CU, my question now, is does the fault follow the path of the AC feeders or if there as a more direct path with less resistance would the fault take that path. Does the fault want to stay near around the feeder?

 

[GoldDigger]

I have been involved in many discussions conduit versus EGC of AL/CU, my question now, is does the fault follow the path of the AC feeders or if there as a more direct path with less resistance would the fault take that path. Does the fault want to stay near around the feeder?

The issue for this case is the impedance, in particular the inductance, of the wires. Even an isolated straight wire will have some inductance. But if instead you have two wires in close proximity carrying equal but opposite current, then the magnetic field at a distance from the wires will be close to zero and as a result the inductance of the complete circuit will much lower than for either wire individually.

A fault current carrying conductor which follows a separate path from the rest of the wires will have a higher inductance and will carry less of the fault current for that reason.
Whether the effect of the higher inductance or the lower resistance will be greater depends on the details of each situation.

 

[marti smith]

And this, via Soares: This indicates that the inductive reactance of a current path closely paralleling the phase conductors offers lower impedance to the groundfault current than any other current path regardless of a lower resistance of the other current paths. Inductive reactance will be the predominate factor indetermining current division n parallel ground return paths in heavy/larger circuits. This has been proven by actual tests where it is shown that there is a tendency for ground return currents to take a path physically close to the outgoing current power conductor. (And the really, really cool part Usually the conduit or metallic raceway that encloses the conductors provides an excellent fault return path. The presence of magnetic material in the power conductor enclosure (conduit/raceway) introduces additional inductive effects tending to confine the return ground currents within the magnetic enclosure.

So, it is as GoldDigger's last sentence. Depends on the situation. The weakest link rule prevails.

 

[don_resqcapt19]

There is a section in the IEEE green book that shows the increase in the impedance of the fault return path when it is not run with the circuit conductors. As I recall, the impedance almost doubles with a 3' space between the circuit conductors and the EGC as compared with the EGC run in very close proximity to the circuit conductors.

 

[brian john]

Thanks,

I have both books and will pull them out when I get home.

 

[Haji]

If the EGC is run through a metal conduit, bonding or not of the conduit at both ends determines fault current distribution as well as the magnitude between the two.

 

[mivey]

As I recall, the impedance almost doubles with a 3' space between the circuit conductors and the EGC as compared with the EGC run in very close proximity to the circuit conductors.

A rough calculation of a single-phase overhead line with #8 CU:
0.2" separation: 0.7403 +j0.2658 ohms/kft
36.2" separation: 0.7019 +j0.2877 ohms/kft

An inductance calc for bare #3 Cu single-phase line:
1 ft spacing: 0.225 ohms/kft
132 ft spacing: 0.450 ohms/kft

 

[Phil Corso]

Mivey...

It is often easer to measure the ground-return-path impedance, than it is to calculate it!

Regards, Phil Corso

 

[mivey]

Mivey...

It is often easer to measure the ground-return-path impedance, than it is to calculate it!

Regards, Phil Corso

No doubt.

 

[Phil Corso]

Mivey...

In fact, without!

Phil

 

[don_resqcapt19]

A Table in the IEEE green book shows an increase in the voltage drop on an EGC that is spaced away from the circuit conductor.
The test set up was 200 feet of 500 kcmil as the circuit conductor and 200' 4/0 as the EGC. With a 5,500 amp ground fault, the voltage drop on the 4/0 EGC was 86 volts with a 2" seperation, 114 volts with a 8" seperation and 143 volts with a 30" seperation.