Dear student,
The consequences of high voltage are,it causes stress in the insulation of electrical equipments and lightly to cause damage if it occurred frequently.O Ver voltages caused by surges can result in Spark over and flash over between ground and phase in the weakest point of the network.
The consequences of low voltage on appliances that are not working are negligible.Suppose if an appliance is rated as 240V and supplied a low voltage then more current will be drawn to get the required power supply.This will cause insulated conductor to damage easily due to overheating.
Regards.