Definition
An electrical protective relay that automatically disconnects a generator or alternator from the aircraft electrical bus when its output voltage drops below a preset minimum value. It prevents a weak or failed generator from drawing current backward from the battery or from other generators in a paralleled system.
Plain English
A switch that automatically cuts a generator off the electrical system if its voltage drops too low, so it cannot drag the rest of the system down with it.
Context Anchor
Seen in aircraft electrical system descriptions, charging system troubleshooting, and low-voltage warning circuits.
Derivation
From 'under-' (below) + 'voltage' (electrical pressure) + 'relay' (an electrically operated switch). The name describes exactly what triggers it: voltage that has dropped under the acceptable level.
Why Pilots Care
It prevents damage to avionics and other systems by responding automatically to low-voltage conditions before they cause failures or total power loss.
Intuition Check
An undervoltage relay does not repair low voltage or boost the voltage back up. It only senses that voltage is too low and then switches, disconnects, or signals another circuit.
Example Sentence 1
When the left generator's output fell below the threshold, the undervoltage relay tripped and isolated it from the bus.
Example Sentence 2
During the preflight check the mechanic verified that the undervoltage relay would activate if generator output fell too low.