A downed power line is most likely to cause a ___.

A downed power line is most likely to cause a phase failure. This occurs when one or more phases of a three-phase system become unintentionally disconnected or interrupted. When a power line falls, it can disconnect the line from its source, leading to an imbalance in the electrical supply. This imbalance results in a phase failure, which can cause equipment to malfunction or systems to operate erratically due to the loss of a phase, impacting performance.

In contrast, a power surge refers to a temporary increase in voltage that exceeds normal levels but may not necessarily be linked to the physical disconnection of a power line. A voltage spike involves a rapid, brief increase in voltage, often caused by switching operations or lightning strikes, rather than a physical failure of the power lines. A ground fault occurs when a live wire makes unintended contact with a grounded surface, leading to a current flowing through the ground instead of the intended path, but does not directly relate to the scenario of a downed power line. Hence, the most fitting consequence of a downed power line is a phase failure, reflecting the disruption in the electrical system caused by the physical disconnection.