What is the term Symmetrical and Asymmetrical current while calculating the short circuit current rating of Circuit breaker for Generator Protection?

Symmetrical (RMS) Current

This term is widely used to identify the short circuit rating of breakers in low voltage systems. This is because the low voltage breakers take8 to10 cycles to break a circuit. In8 or more cycles (typically15), the fault current will decay to a symmetrical waveform which, ofcourse, would have no DC offset.

Low voltage panels too are rated by their symmetrical current rating.

Most modern circuit breakers implicitly list their ratings in symmetrical amps.

Asymmetrical (RMS) Current

During the first half of a cycle, the fault current is at its largest magnitude – occurring at a moment when the voltage wave is passing the reference axis. This asymmetry is brought on by the DC offset current (see Figure1 below.) At the half cycle mark, the peak RMS value of the asymmetrical current is about ~1.6 times the symmetrical current.