During ground instruction, a technician explains the purpose of the overvoltage sensing relay in a light aircraft's electrical system. What does this relay do when it detects excessive alternator output voltage?
- AIt reduces engine RPM automatically to lower alternator output voltage
- BIt opens the field circuit to the alternator, disconnecting it and illuminating an alternator warning light
- CIt automatically increases battery charging rate to absorb the excess voltage safely
- DIt shuts down the entire electrical bus, including the battery, to prevent damage
Show answer & explanationAnswer & explanation
Correct answer: B. It opens the field circuit to the alternator, disconnecting it and illuminating an alternator warning light
The overvoltage relay senses when alternator output exceeds a safe threshold (typically around 16-17 volts in a 14-volt system) and opens the field circuit, effectively taking the alternator offline to protect the electrical system and avionics from damage. This triggers the alternator warning/failure annunciator, and the pilot must then rely on the battery and follow appropriate checklist procedures.
Why the other options are wrong
- A. The relay has no control over engine RPM; it acts purely on the electrical field circuit.
- C. The relay disconnects the alternator rather than increasing charging, which would worsen an overvoltage condition.
- D. The relay does not disconnect the battery or entire bus; the battery remains available after alternator disconnect.
Overvoltage Protection Relay
A safety device in the aircraft electrical system that disconnects the alternator field circuit if output voltage exceeds a safe limit, protecting avionics and the battery from damage.
- Typically trips around 16-17V in a 14V system (30-32V in 28V systems)
- Opens alternator field circuit to stop output
- Triggers ALT warning light; battery remains as sole power source
Memory trick: Overvoltage relay 'pulls the plug' on the alternator field to save the avionics