What if the line Isc (prospective short-circuit current) is greater than the values stated in the Altivar catalogue tables?


20 June 2022

To get the answer we have to first understand how diodes work on the input bridge of the drive.

A diode will conduct when the anode is greater than the cathode  Current flow is then permitted and the diode is forward biased. When the forward voltage becomes negative the Current flow is prohibited; the diode is reversed biased.  In actuality, a very small amount of current can and does go through a reverse-biased diode for a brief period of time (micro seconds) before returning to zero at which stage the diode is off. This current is leakage current or otherwise known in the VSD world as commutating current.

This current is then limited only by the system impedance. Installing a VSD on an electrical system with a higher Isc than the VSD rating can cause overheating of the diodes, this can result in premature failure due to the higher peak and RMS current.
VSD's are designed for a specific expected “maximum prospective short-circuit current .” The power component selection and the power section layout of the VSD design determines at what level of Isc the product will be rated.
Schneider Electric publishes this rating under the column heading “maximum prospective line Isc” in the VSD catalogues.

The resulting commutating current can only be reduced with inductance (Line Choke) in series with the diode bridge/rectifying bridge. Line chokes can be selected based on drive model in the associated drive catalogue.