Voltage fluctuation
In the real world power line voltage occurs frequently especially in industrial area. Every electrical equipments and devices do have a working voltage limit/tolerance. Some equipment are build to tolerate ±10% of nominal voltage while others ±5% or less depending on sensitivity.

Effects
The correct operation of electrical and electronic equipment depends on the voltage accuracy and stability. In the event of long time over voltage, it will lead to damage of the equipment; while long time under voltage will cause malfunction and computation errors of the electrical & electronic equipment