help to detect a voltage n turn on or off alarm based on the output.

i want to make a circuit which can do the following work---
suppose there is an instrument which works on 400 V. if a wrong voltage is applied is may get damaged so i want to make a circuit which should check the input voltage and if the voltage is not 400V then it makes a alarm to ring or if the voltage is 400V then it is applied to the instrument.


help me with this please...😔