In certain cases two or more consecutive dips or interruptions may occur. Voltage variations are caused by the continuously varying loads connected to the network. These phenomena are random in nature and can be characterized in terms of the deviation from the rated voltage and duration. Voltage dips and short interruptions are not always abrupt, because of the reaction time of rotating machines and protection elements connected to the power supply network. For a short period, the rotating machines will operate as generators sending power into the network.
Some equipment is more sensitive to gradual variations in voltage than to abrupt change.
Most data processing equipment has built-in power fail detectors in order to protect and save the data in the internal memory so that after the mains voltage has been restored, the equipment will start up in the correct way. Some power fail detectors will not react sufficiently fast on a gradual decrease of the mains voltage. Therefore, the DC voltage to the intended circuits will decrease to a level below the minimum operating voltage before the power fail detector is activated and data will be lost or distorted. When the mains voltage is restored, the data processing equipment will not be able to restart correctly before it has been re-programmed. Consequently different types of tests are specified in this standard to simulate the effects of abrupt change voltage, and optionally, for the reasons explained above, a type test is specified also for gradual voltage change.