CB Short time rating 1

[Prot123]

Hi,
I have a breaker with following specification
short time rating: 25kA for 1 sec.
I want to work out time for 30kA fault level. Can this sort of approximation be done?
Cheers

 

[aussiejohn2]

Prot123,
At higher currents the time to meet the *thermal* constraint can be calculated. But how will you know that the higher mechanical forces can be withstood?
AJ2

 

[wbd]

What is the AIC rating? That should be higher than the short time rating. What are you trying to do? Coordination with the inst function turned off? Is there an inst override?

 

[ScottyUK]

No, you can't interpolate upwards, using the adiabatic equation for example, because the limiting factors at high fault level are likely to be mechanical rather than thermal. AussieJohn2 has got it right.

 

[7anoter4]

However, the thermal short-circuit current is also limited.
According IEC 60865-1/1993 Short-circuit currents –Calculation of effects –Part 1efinitions and calculation methods 3.2.4 Calculation of the thermal short-circuit strength for different durations of short-circuit current. 3.2.4.1 Electrical equipment:
If the Tk=<Tkr then Ith=<Ithr where Tk is the actual duration Tkr is the rated duration
Ithr is the rated short-circuit withstand current.
If the Tk=>Tkr then Ith=<Ithr*sqrt(Tkr/Tk)
That means for time less than the short-circuit rated duration it will stay rated or less.
For longer duration you will reduce the current.

 

[bacon4life]

The end user can't interpolate. However, there is a chance the manufacturer may be able to provide a special rating for you. Sometime equipment undergoes a single test to get both ANSI and IEC ratings. Since the standards don't precisely align, the equipment may exceed both standards but in different ways. During actual testing, the CB could have been tested at above 25 kA, but not all the way up to the next standardized rating. We recently had a voltage regulator where the manufacturer provided a special rating about 10 percent higher than the IEEE standard value.

 

[Zogzog]

Are you trying to calculate clearing time @ 30kA?

 

[Prot123]

Hi All, Thanks for the reply. I'm trying to calculate clearing time at 28kA.

 

[ausphil]

Operating CBs above their rating.

It'll do it pretty quick, but only once.

Ausphil