The unit is designed to detect a broken neutral condition from the supply authority incoming supply.
Once a broken neutral condition is detected it creates a situation for the Residual Current Device (RCD or Earth Leakage unit) to operate and isolate the single-phase load from the incoming supply.
Without this device, a broken neutral condition will not be detected and create an unsafe safe environment with a high risk of electrocution in this condition. The device will also ensure that appliances connected to the single-phase load are protected under broken neutral overvoltage conditions.