DC Short Circuit Calculations

[Ingenieur]

For decades for a 1st cut number have simply used 300 NH per foot or 1 uH per meter. As good as anything for mast cases.

If critical applications, either mock it up and measure or use one of the electromagnetic FEA analysis programs

For aerospace use even the fancy/pricey full FEA programs relay on correct inputs, so nearly always do a mockup and measure, have never trusted models or calculations for critical/complex systems.

Just because you can calculate it does not mean your inputs are correct, as per the 1960 computer sayings, 'garbage in, garbage out".

the point is it is inconsequential for a 125 vdc ckt of conductors in a conduit
there is a reason the nec gives tables
but a typical nec conductor x/r is ~ 0.05 inductive, very minute ckt response influence

Example #10 Cu
Xl 0.05 or L 0.132 mH/1000'
R 1.2
X/R = 0.044
tc = 0.132 x 10e-3 / 1.2 = 0.11 x 10e-3

observe at 0.001 sec (0.06 cycle 60 Hz based)
i = max x e^- (0.001 x 0.00011) = max x 1.0000
current rises to max 100% in 0.06 cycles
does not matter unless dealing with very hi freq signals
and never worth measuring in this application
in fact pointless to even use the table values
does not matter either way
that is why it is usually ignored in power level calcs

 

[Ingenieur]

1 uH/m or 0.305 mH per 1000'
2.5 times the calculated or table values
still moot
using 1/0 Cu
R = 0.12 Ohm / 1000'
X/R = 0.95
tc = .000305/0.12 = 2.54 e-3
observe i at 0.001 sec / 0.06 cycle
i = max x 0.999997
or 99.99% in 0.001 sec or 0.06 cycles
moot

 

[Sahib]

Ingeniur: While a current limiting fuse operates at about 0.25 cycle, the DC short circuit current reaches its maximum value in about 0.06 cycle
So it seems circuit inductance does not matter in practical low DC voltage short circuit calculations, IMO also.