IEC rated breakers and switchboards

[EddyWirbelstrom]

A test report for 13.8kV, 50 Hz switchboard is based on Standards IEC 60298: 1990 and IEC 62 271-100: 2001.
The test report lists following switchgear ratings :-
1.) "Rated short-circuit breaking current" of 31.5kA.
( This is the rms symmetrical current at breaker opening )
2.) "d.c. component of breaking current" of 41%.
( ratio of d.c. current to peak value of symmentrical breaking current)
3.) d.c. time constant of rated short-circuit breaking current = 45 msec
4.) "Rated short-circuit making current" of 80kA.
( peak or crest value of short-circuit current including the d.c. offset ie. i peak = sqrt2*I"k ( 1 + e^-pi*R/X ) ) From above1.) and 3.), ipeak = sqrt2*31.5(1+e^(-pi /14.137)) = 80.2kA.
5.) "Rated peak withstand current" = 100kA
Question
Why are 4.) and 5.) different ?

 

[Kiribanda]

mnewman,

Yes, those two ratings cannot be different. They should be identical as per IEC 60271-100.

Also I have a comment on your calculation in item 4.

As per IEC 60271-100, the Rated Short Circuit Making Current should be equal to 2.5 times Rated Short Circuit Breaking Current (r.m.s) for 45 ms/ 50 Hz breakers. Then it should be 31.5 kA times 2.5 which will be equal to 78.75 kA or 79 kA. Therefore I donot understand why you should calculate and get get 80.2 kA, eventhough there is no big difference.

Best thing is you should contact the manufacturer and ask how the Items 4 & 5 are different, if they have tested as per IEC 60271-100 and specified as per IEC 60298.

Hope this helps.

Kiri