Definition
An electrical control component used in aircraft generator systems that connects the generator to the main bus only when the generator's output voltage is higher than the bus voltage by a specified amount, and disconnects it if current begins flowing backward from the bus into the generator. It performs two functions: it senses the voltage difference between the generator and the bus before closing the circuit, and it opens the circuit if reverse current is detected, protecting the generator from being driven as a motor by battery power.
Plain English
A switch that automatically connects the generator to the airplane's electrical system only when the generator is producing more voltage than the battery, and automatically disconnects it if electricity starts flowing the wrong way -- back into the generator.
Context Anchor
Seen in older aircraft electrical systems that use direct-current generators, especially in charging-system descriptions and maintenance troubleshooting.
Derivation
The name describes exactly what it does. 'Differential-voltage' means it reacts to the difference between two voltages (generator vs. bus). 'Reverse-current' means it also reacts when current flows in the wrong direction. 'Relay' is an electrically operated switch -- one circuit controls the opening and closing of another.
Why Pilots Care
Prevents battery discharge and protects the generator from reverse-current damage when engine RPM is low.
Analogy
Think of it like a one-way gate that opens only when the generator is pushing harder than the battery side. If the push starts going the wrong way, the gate shuts.
Intuition Check
Do not read “reverse-current” as a different kind of electricity. It means normal electrical current flowing in the wrong direction for that circuit.
Example Sentence 1
When the generator output dropped below the bus voltage during the descent, the differential-voltage reverse-current relay opened and isolated the generator from the electrical system.
Example Sentence 2
A stuck differential-voltage reverse-current relay allowed the battery to discharge through the generator during taxi.