As I understand, you are talking about a loss of one incoming phase. So you test each phase voltage, so any capacitor connected across the phases will not change the voltage. The reason for a loss of phase detector is because with electric motors, if a phase goes missing, if the motor is heavily loaded then it would burn out due to excessive current in the connected phases or would trip its local current overload. If you had a factory with 100s of machines it would take for ever to reset them all! Best to turn the lot off until all three phases return.
Frank