Fault Current and Impedance of Path

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crossman

Senior Member
Location
Southeast Texas
One of the EGC threads spurred my interest, and I am trying to gain a better understanding of the following:

We have a large AC 60hz source, 480Y/277. (In the diagram, I am showing only one phase and the neutral.)

We have 400 feet of 500 kcmil copper wire, in the first diagram, it is run for 200 feet in PVC, then it loops back and connects to the neutral terminal. This is a direct short from phase to neutral.

In the second diagram, same thing, but the neutral side of the wire is not in the conduit, and is routed completely away from the PVC. Any thoughts on "in the ballpark" current levels for the two arrangements? Would there be any differences in the current from diagram 1 to diagram 2?

groundfaultpvc.jpg


groundfaultoutpvc.jpg
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Well, to start with Ch. 9, Table 8, we get 0.0258 Ohms/1Kft, so 400' would be (0.4 x 0.0258) 0.01032 Ohms. Then, using Ohm's Law, we calculate (277/0.01032) 26,841 amps.

Now, as far as the difference in impedance between the paralleled circuit and the loop, that's something you'll need to wait to hear about from Rattus or one of the other theoreticists.

I'm guessing you didn't need my help on this. :)


To whom it may concern: Merry Christmas!
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I don't have my IEEE Green Book here, but I think there is a table in there that shows the change in impedance for short separations...nothing like the 100' shown here. As I recall you can almost double the impedance with a 5' or so separation.
Don
 

crossman

Senior Member
Location
Southeast Texas
Thanks for the replies, gentlemen. Larry, you are correct that I could do that calculation. I am interested in the theory of the two situations compared to each other.

Don, I did some google searches, but didn't find any studies to help me out. I did find a couple of IAEI articles, but they did not give any actual data or reasons why there would be a difference between the two situations.

The only difference between the two diagrams is that in the first one, the magnetic fields of both sides of the loop will interact, in the second diagram, they will not. I am interested in how much this affects the current flow.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
That big loop is essentially and air core (soil core??) inductor.

You can find quite a few _approximate_ inductance calculators online. For example http://www.cvel.clemson.edu/emc/new-induct/square.html

If you enter the parameters presented (100 foot square, wire 0.707" in diameter) it will calculate an inductance of 180uH. I'd trust this number as being in the ballpark, but not particularly accurate.

From the inductance you can calculate the inductive reactance at 60Hz, I get 0.07 ohms.

For a big loop like this, the inductance dominates the short circuit current calculation.

-Jon
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
From the inductance you can calculate the inductive reactance at 60Hz, I get 0.07 ohms.
When compared to Larry's resistance only calculation that is almost 7 times the impedance.
Don
 

crossman

Senior Member
Location
Southeast Texas
Just thinking off the top of my head:

In the first situation, the two magnetic fields will cancel each other, leaving very little reacive effects on the wires.

In the second situation, the magnetic field is cutting across the conductor which produced it, producing a back emf in the wire. This back emf opposes the source voltage, thereby reducing the total current flow.

Does that sound reasonable?
 
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