Calc L-N Fault

chris kennedy

Senior Member
Miami Fla.
60 yr old tool twisting electrician
I have a 300kVa with a 480Y/277 secondary, 5.35% z.

To calculate line to neutral fault current do I use 1/3 FLA and Z?


Julius Right

Senior Member
Cooper Bussmann does not indicate how to calculate 3 phase transformer phase-to-neutral short-circuit current, indeed.
IEEE 80/2013 pp.76 [for instance] symmetrical one phase short-circuit
Ineutral=3*Io [in all three phase the same symmetrical component zero sequence current ]
The reactances X1,X2 and Xo are the total reactance of the supply system and the transformer. Xo is different and depends on transformer connection and the magnetic core construction [3 limbs or 5 limbs].
If we consider the short-circuit power of the supply system as infinite [Xsystem=0] then if the transformer connection is Delta/Yn then Xo≈X1 X2=X1[far from generators].
X1≈Z1=VL_L^2/S*Z%/100=0.041088 ohm
where VL_L=0.48 kV S=0.3 MVA
Isc1=277/0.041088=6741.6 A
Ifla=S[kVA]*1000/1.73/VL_L=360.8 A
That means Isco[symmetric component per phase] is indeed Ifla/Z/3=2248.25 A
But you have 3 Io[one per phase] then IN=3*2248.25=6744.75
Also IEC 60909-0[formula 29]:
c=1 Un=380 V
Z=0.041088 ohm
Ik=1*480/1.732/.041088=6744.9 A