You might want to find out instantly whether or not the mains voltage is running low before starting an important task because the load switch ON might cause an already low voltage to dip below the critical level.
The mains voltage monitor circuit explained here is powered from the mains and sent across R1 and P1. The reference voltages are provided by R2, C1, C2, D1 and D2, and tallied across A1 and A2 in a set proportion of the mains voltage. D7 lights up when it is under 210 V and D8 lights up when it is over 250 V.
When both the LEDs are not illuminating, then D4 lights up due to T1 conducting to show that the voltage is within the safe range.
A multimeter and variac may be used to fix the acceptable voltage range with P1. If eyework is allowed, it can be preset approximately at the center of the full range of movement.
Since the mains AC voltage monitor circuit is directly associated with the mains AC, therefore it should be kept in a well secured fiber case.