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# Short circuit rating etap

## Short circuit rating etap

(OP)
Hi every one,
My distribution network is having 1 incomer source of 220 kv and 3 outgoing transformers of 220/33kV, 65 mva with Z= 14% with 7.5%. Source short circuit value is 50kA.
I have done short circut calculation manually and with etap but my results are not matching.
With etap SC current on 33kV bus is coming 28.5kA.
While with manual calculation is its 24.4 plz guide me if i am doing any thing wrong-
Zt=14%, with negative tolerance zt=12.95%
With 3 transformer in parallel= zt- 12.95/3=4.32%
Now contribution from source side-
Zs= 65000*100/1.732*220*50000
= 0.3412
Total impedance= zt+zs= 4.32+0.3412= 4.66
Isc= 65000*100/1.732*33*4.66
= 24.4kA
Strange thing i am getting if i am substracting source impedance from total transformer impedance than i am getting same result as etap-
Total impedance= zt-zs= 4.32-0.3412= 3.98
Isc= 65000*100/1.732*33*3.98
= 28.5kA
Please anyone help me im this regard if i am doing any fundamental mistake

### RE: Short circuit rating etap

I think ETAP follows IEC way to calculate maximum short-circuit current.
According to IEC 60909-0 Short-circuit currents in three-phase a.c. systems -Part 0: Calculation of currents Ik=c*Un/sqrt(3)/Zk [29]
Definition: 1.3.15 voltage factor c
ratio between the equivalent voltage source and the nominal system voltage Un, divided by √3.
NOTE The introduction of a voltage factor c is necessary for various reasons. These are:
_ changing of transformer taps,
_voltage variations depending on time and place,
_neglecting loads and capacitances by calculations according to 2.3.1,
_the subtransient behaviour of generators and motors.
Table 1 for maximum short-circuit current c=1.1
Then if you start the calculation neglecting system impedance [short-circuit power infinite]
and applying factor 1.1 you’ll get 29 kA.By the way if you take only -6% error for transformer
impedance you’ll get 28.5 indeed.

### RE: Short circuit rating etap

Normally ETAP results are fairly accurate. There are various types of faults such as 3-phase bolted, Line to Line, Double Line to Ground etc. Simplest calculation is 3-phasr fault calculation. Check the results corresponding to this in the ETAP. If it is matching, it is verified.

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