"电压瞬时跌落" can be translated into English as "voltage instantaneous sag" or "voltage transient dip".
"Voltage instantaneous sag" emphasizes the sudden and temporary decrease in voltage.
"Voltage transient dip" also conveys the idea of a brief, temporary drop in voltage, with "transient" indicating the short-lived nature of the event.
Both expressions are commonly used in electrical engineering contexts to describe this phenomenon.