Definition
An electrical relay designed to open its contacts when the current flowing through its sensing coil drops below a preset minimum value. It is used as a protective device to disconnect a circuit or signal a fault when current falls below the level required for normal operation.
Plain English
A switch that automatically trips when the electrical current flowing through it gets too low. It watches the current, and if the flow drops below a set point, it opens and breaks the circuit.
Context Anchor
Seen in aircraft electrical system maintenance, especially in generator, alternator, battery, and protective control circuits.
Derivation
Under' means below, 'current' is the flow of electricity, and 'relay' is an electrically operated switch. Combined: a switch that responds when current drops below a set level.
Why Pilots Care
Prevents the aircraft battery from discharging back through a failed generator, preserving power for radios, instruments, and other critical systems.
Intuition Check
Do not confuse undercurrent with low voltage. Undercurrent means the current, or amount of electric flow, is below the required value.
Example Sentence 1
When the generator output dropped, the undercurrent relay opened and disconnected it from the bus.
Example Sentence 2
The pilot's checklist included verifying that the undercurrent relay was functioning before a night flight.