Rated short-circuit making current

On breakers there is a difference between what the main contacts can interrupt, once, and what the contacts can carry without interrupting. And it would be the same for bus bar in that it has a carry capacity.
The carry capacity is typically a I^2T function and the ability to disapate heat and remain below the melting tempenture.
The interrupting rating is the ability of the main contacts to handle the arc interrupting on the current for one operation. If the breaker is expected to interrupt a fault more than once between contact replacments it needs to have a greater interrupting rating to allow for the contact wear factor.
Also you will find there should be an additional rating called the capacitor switching rating, which is a measure of the contact speed and opening distance and the ability to operate with near 90 degree angular difference between the current and voltage.

Probally to much information, but I sometimes can't stop in mid stream.

What cranky108 is describing is actually the withstand rating. The making rating is the current that the breaker can withstand as it is closing where the act of closing the breaker initiates the fault. This current will be as fully offset as it can be for that fault, much more so than even two or three cycles later, and the magnetic forces associated with the fault current may work to attempt to keep the contacts from closing.

"What cranky108 is describing is actually the withstand rating"

Yep, but it was a good description.

"Probally to much information, but I sometimes can't stop in mid stream."

Pfizer makes a pill for that

This current will be as fully offset as it can be for that fault

Click to expand...

davidbeach What do you mean by "offset"?

Thanks

Offset due to the asymmetry of the fault, as a function of the X/R of the current. The breaker will be rated in symmetrical amps but the rating accounts for an expected amount of asymmetry.

The IEC and ANSI define "rated short-circuit making current" differently for MV and HV breakers.

The IEC standard refers to the peak value of the off-set current waveform 0.5 cycle after fault inception.

IEC 62271-100 Clause 4.103
At 50Hz, and time-constant of 45mSec
"rated short-circuit making current" = 2.5 x rated breaking current.
Your quoted "rated short-circuit making current" / "rated short-circuit breaking" currents of 62.5kA / 25kA or a ratio of 2.5 suggest an IEC breaker.


The ANSI standard refers to the first cycle rms asymmetrical current. Pre 1965 terminology was "momentary rating", now "Close and Latch Capability".