Definition
A test instrument used to determine the sequence in which the three phases of a three-phase AC electrical system reach their peak voltage. Connecting the indicator's leads to the three phase conductors causes a small motor or visual indicator inside the instrument to rotate in a direction that shows whether the phase sequence is correct (typically A-B-C) or reversed.
Plain English
A small testing tool that checks whether the three wires of a three-phase electrical system are connected in the right order. If the wires are swapped, equipment connected to that system can run backwards or be damaged, so this tool confirms the order is correct before things are powered up.
Context Anchor
Seen during aircraft electrical maintenance, especially when checking three-phase power, motors, generators, or external power connections.
Derivation
Phase refers to the timing of each of the three alternating voltages in a three-phase system. Rotation refers to the order in which those voltages reach their peak. The instrument indicates the rotation of the phases — hence the name.
Why Pilots Care
Correct phase sequence prevents motors from running backward, which can damage aircraft systems or cause unsafe operation.
Grounding Statement
The instrument is checking the order of the electrical waves, not the amount of power.
Intuition Check
Do not read “rotation” here as aircraft rotation during takeoff or a propeller turning. In this term, rotation means the order of the three electrical phases.
Example Sentence 1
After reconnecting the external power receptacle, the technician used a phase-rotation indicator to confirm the phases were wired in the correct order.
Example Sentence 2
An incorrect reading on the phase-rotation indicator showed the leads had to be swapped to avoid running the motor backward.