Definition
An electrical protective device, used in aircraft direct-current generator systems, that automatically disconnects the generator from the battery and electrical bus whenever the battery voltage exceeds the generator voltage. This prevents current from flowing backward — from the battery into the generator — which would otherwise discharge the battery and could damage the generator by motoring it.
Plain English
A switch that watches which way the electricity is flowing between the generator and the battery. As long as the generator is producing more voltage than the battery, the switch stays closed and the generator charges the battery. The moment the battery starts trying to push current back into the generator, this device opens the circuit and disconnects them.
Context Anchor
Seen in discussions of older aircraft generator systems, charging circuits, and electrical system troubleshooting.
Derivation
The name describes the device exactly: a relay (an electrically operated switch) that responds to reverse current — current flowing the wrong way through the circuit.
Why Pilots Care
It keeps the battery from draining through the generator and prevents overheating or damage that could lead to electrical failure in flight.
Analogy
Think of it like a one-way gate in an electrical line. Power is allowed to go from the generator to the battery, but the gate opens to stop power from going backward from the battery into the generator.
Intuition Check
Current does not mean wind or trend here; it means electrical flow. Reverse-current does not mean the generator is charging in reverse; it means battery power is trying to flow backward into the generator.
Example Sentence 1
When the engine RPM dropped below idle during taxi, the reverse-current relay opened and took the generator off the bus.
Example Sentence 2
During the electrical system check, the pilot confirmed the reverse-current relay would disconnect if generator output dropped too low.