Bus Rating - IEC 60909

[NickParker]

Which parameter(I"k or Ith or Ik) from IEC 60909 needs to be considered for Busbar rating?
Most commonly bus bar rating is specified something like 25kA RMS for 1s.
Bus bar has to withstand both the Mechanical stress and thermal stress. Why bus bar rating is mentioned in RMS, though the peak short circuit current (ip) is responsible for Mechanical stress of the bus bar during the fault?

 

[ppedUK]

The answer to your question is given in BS EN (IEC) 60909-0:2016 Sect 7.1.1 Table 2.

 

[7anoter4]

In my opinion there are indeed 3 different rated currents: Ith [Rated normal current (lr)], another Rated short-time withstand current (Ik) and Rated peak withstand current (lp).
The IEC Standard IEC 60909-0 presents a calculation way of the short-circuit level at the point in which the equipment is located. IEC 62271-1 states the equipment rated
current. See for instance: Schneider Electric _Basic for MV Cubical Design.