Definition
A protective circuit used in three-phase electrical systems that detects when the phase sequence of the incoming AC power is reversed and disconnects the load before damage occurs. Reversed phase sequence causes three-phase motors to run backwards and can damage equipment designed to operate with a specific phase rotation.
Plain English
A safety circuit that watches the order in which the three power lines of an AC system arrive. If the order is wrong, it shuts the system off so motors don't spin backwards and equipment isn't damaged.
Context Anchor
Seen in aircraft electrical system maintenance, especially around three-phase AC power equipment, motors, and external power connections.
Derivation
Phase refers to the timing of each of the three alternating currents in a three-phase system. Reversal means two of those phases have been swapped, changing the sequence in which they peak. The protection circuit guards against that swap.
Why Pilots Care
Reversed phases can cause fuel pumps, hydraulic pumps, or actuators to operate in the wrong direction, leading to immediate equipment failure or unsafe flight conditions.
Grounding Statement
If two lines in a three-phase power connection are swapped, phase-reversal protection can stop the equipment before it runs the wrong direction.
Intuition Check
Do not read “phase” here as a stage of flight. In this term, “phase” means the timing order of AC electrical power.
Example Sentence 1
When the ground crew connected external power, the phase-reversal protection tripped because the cart was wired incorrectly.
Example Sentence 2
Technicians test phase-reversal protection during preflight checks to confirm all three-phase motors will rotate correctly.