An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment
failures, and control malfunctions.

The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.

Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption.

The duration of an interruption due to equipment malfunctions or loose connections can be irregular. Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system.

The voltage sag occurs between the time a fault initiates and the protective device operates. Figure 2.5 shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.

The difference between long (or sustained) interruption and interruption is that in the former the supply is restored manually, but during the latter the supply is restored automatically. Interruption is usually measured by its duration.

For example, according to the European standard EN-50160 [24]:
- A momentary interruption is between 30 cycles and 2 seconds;
- A temporary interruption is between 2 seconds and 2 minutes; and
- A sustained interruption is longer than 2 minutes.

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