IEC 61000-4-12 Ed.1, IEC 61000-4-18 Ed.1 and ANSI C37.90 describe an oscillating wave generator with defined open circuit waveforms parameters.
The oscillating wave, also called a “damped sine” is used to test the immunity of electronic relays and relay systems mounted in an electrical substation or similar harsh environment.
IEC product standards such as IEC 60255-22-1 include this wave shape too.