Voltage sags occur more often because they are a secondary result of other disturbances and they are broadcast over a larger area. When a fault (short between any two phases of the transmission lines) occurs, a power outage in the general vicinity of the fault is produced. The entire grid tries to feed the fault and the voltage level of a much larger area of the grid is pulled down. A voltage sag can last as long as the fault is in place. The fault is cleared either by self-destructing (burning itself out), or by the grid isolating the fault using its relay network. A fault might cause a power outage in an area within 1 or 2 miles of the fault and it may also cause a voltage sag within 20 or 30 miles of the fault.
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