Medium Voltage Application Guide

13847.A
SWITCHGEAR
The calculated short circuit current of a transformer, I sc is often used to rate the downstream distribution switchgear
it is feeding. In reality, the expected short circuit current at the switchgear installation will be less than the calculated
short circuit current, due to any impedance in the feeder circuit (ie impedance of feeder cables, switchgear, busbars
etc). All switchgear has a short-time withstand current rating (I k ), which is typically type tested for 3 seconds (t k ).
I k > I sc
Transformer
Switchgear installation
1
I sc
2
Q1 Q2 Q3
I k
Example
To calculate the short-time withstand current rating of the downstream switchgear, I k , we must calculate the rated
secondary current and the short circuit current of the feeder transformer.
Transformer power S = 20 MVA
Secondary rated voltage U r = 11 kV
Impedance Z% = 8%
Assume infinite power system.
Rated secondary current, I r
r
The transformer rated secondary current is 1050 A.
Short circuit current, I sc
r
The transformer short circuit current is 13125 A.
Switchgear rating, I k /t k
I k I sc
I k 13125 A
An appropriate switchgear rating is 16 kA / 3 seconds.
Page 144 Medium Voltage Application Guide 710-12280-00A