Aircraft electrical systems operate within precisely defined voltage windows. A modern light aircraft with a 28-volt DC system, for example, depends on bus voltage staying within a few volts of that nominal value under all conditions. When voltage climbs too high — even briefly — sensitive avionics, solid-state electronics, and the battery itself can suffer permanent damage. When voltage drops too low, flight-critical instruments and radios may lose power entirely. Overvoltage and undervoltage protection devices are the automatic sentinels that detect these excursions and take corrective action before damage or mission failure occurs. Every AMT working on aircraft electrical systems must understand how these devices operate, why they are required, and how to troubleshoot them effectively.
The Nature of Voltage Excursions
In a typical aircraft DC electrical system, the engine-driven alternator (or generator) supplies current to the bus and simultaneously charges the battery. A voltage regulator holds the alternator output at a set value — commonly around 14 volts for a 14-volt system or 28 volts for a 28-volt system. Problems arise from two directions. An overvoltage condition occurs when the regulator fails in a way that allows alternator output to climb above the upper design limit, often above 31–32 volts in a 28-volt system. An undervoltage condition occurs when the alternator output falls below the minimum threshold needed to sustain bus voltage, forcing the battery to carry the entire electrical load and depleting it toward exhaustion.
Both conditions can develop rapidly. A shorted field transistor inside a solid-state regulator can drive output voltage into the mid-30-volt range within seconds. Conversely, an open field circuit, a broken alternator drive belt, or a faulty regulator that simply stops commanding output will instantly drop bus voltage toward battery terminal voltage, which itself declines as the battery discharges. Protection devices must therefore act quickly — often within a fraction of a second for overvoltage events — to prevent cumulative harm.
Overvoltage Protection Devices
The Overvoltage Relay (Crowbar Circuit)
The most common overvoltage protection approach in general aviation aircraft uses an overvoltage relay, sometimes called a crowbar relay or overvoltage protector. This electromechanical or solid-state device continuously monitors bus voltage. When voltage exceeds the trip point — figures vary by aircraft and manufacturer, but many common 28-volt GA systems trip in the general vicinity of 30 to 32 volts — the relay trips and removes the alternator field current. Without field excitation, the alternator ceases to produce output and bus voltage falls back toward battery voltage. On many designs, the relay also opens the alternator output contactor, completely disconnecting the alternator from the bus.
The term crowbar comes from the analogy of literally dropping a metal bar across the circuit to short it out. In actual aircraft applications, the device does not create a short; rather, it interrupts the field circuit rapidly. On some legacy designs, a zener diode and silicon-controlled rectifier (SCR) combination provided a true crowbar function by triggering a low-impedance path that shorted or interrupted the field circuit directly (sometimes opening a fuse in the field line), but modern designs favor relay-based isolation because it avoids the high current spike of a true crowbar.
Solid-State Overvoltage Modules
Many contemporary aircraft use integrated solid-state voltage regulators with built-in overvoltage protection. The regulator monitors its own output and contains internal logic that shuts down the field drive if output exceeds the trip point. These units may automatically reset after a brief delay (auto-reset designs) or may require a manual reset — cycling the alternator master switch — before the alternator can be re-engaged. The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25) discusses this sequence: the pilot sees the alternator (ALT) warning light illuminate, verifies the ammeter shows a discharge condition, and follows the POH procedure, which typically involves cycling the alternator off then on to reset the protection device and re-examine whether the fault has cleared.
Crowbar OVP and the Alternator Circuit Breaker
On aircraft where the overvoltage module is designed to trip the alternator circuit breaker rather than open a relay, the circuit breaker serves as the isolation mechanism. The AMT must understand that repeatedly resetting a tripped alternator circuit breaker without identifying the root cause is dangerous — if the overvoltage condition persists, the repeated arc energy can damage the breaker or wiring. The proper maintenance response is to verify regulator output with a calibrated voltmeter before re-energizing the system.
Undervoltage Protection Devices
Undervoltage protection is equally important. In many aircraft, an undervoltage relay or low-voltage warning system monitors bus voltage and alerts the crew — or in some designs, automatically disconnects non-essential buses — when voltage drops below a defined threshold. This threshold is aircraft-specific and set close to expected battery-only bus voltage, often in the general range of the mid-20s in a 28-volt system, per the applicable maintenance manual.
The most basic form is a simple low-voltage warning light activated by a voltage-sensing relay. When bus voltage falls below the relay's calibrated dropout voltage, the relay de-energizes, completing a circuit that illuminates the ALT or VOLTS warning annunciator in the cockpit. This alerts the pilot that the alternator has failed and the battery is supplying the bus alone, providing time to shed electrical load and divert to the nearest suitable airport.
More sophisticated systems include load shedding relays that automatically disconnect non-essential electrical buses (entertainment systems, certain lighting circuits, non-flight-critical avionics) when undervoltage is detected. This conserves battery energy for flight-critical systems — the primary flight display, transponder, communication radio, and lighting — extending the time available before total electrical failure. Transport category aircraft feature elaborate load shedding logic managed by bus power control units (BPCUs), but even some advanced general aviation aircraft incorporate automatic load shedding through their avionics bus architecture.
The Alternator Control Unit (ACU) and Integrated Protection
Modern aircraft increasingly consolidate both overvoltage and undervoltage sensing into a single Alternator Control Unit (ACU) or voltage regulator/protector module. This unit simultaneously regulates field current to maintain output voltage, monitors for overvoltage, and monitors for loss of alternator output. When the ACU detects an overvoltage, it removes field excitation. When it detects that the alternator has dropped off line (undervoltage relative to a reference), it triggers the warning system. (Frequency monitoring, by contrast, is a function found on AC generator systems with constant-speed drive units — such as those used on transport-category aircraft — to detect irregular generator speed, and is not a function of DC alternator ACUs.) Replacing an ACU requires careful part-number verification; an ACU calibrated for a 28-volt system must never be installed in a 14-volt aircraft.
Why These Devices Matter
The consequences of unprotected overvoltage are severe. Even a brief excursion to 35 volts can destroy the internal circuitry of glass-panel avionics costing tens of thousands of dollars, degrade battery electrolyte chemistry through overcharging, and potentially cause a thermal runaway event in a sealed lead-acid or lithium-based battery. Wiring insulation rated for normal system voltage may be stressed by elevated voltage, accelerating insulation breakdown that can lead to arcing faults and fire.
Undervoltage failures are insidious because they develop gradually. A failing alternator may provide reduced output for some time before dropping off line entirely, and the battery will silently deplete. By the time the pilot notices symptoms — dimming lights, erratic radio behavior — battery reserve may already be critically low. Automatic detection and annunciation buys the pilot the decision-making time that the Risk Management Handbook (FAA-H-8083-2) identifies as essential to avoiding cascading system failures.
Key Numbers and Rules
- Nominal system voltages: 14-volt systems (12-volt battery, ~14.2-volt regulated output) and 28-volt systems (24-volt battery, ~28-volt regulated output) are the two most common in general aviation.
- Typical overvoltage trip point: aircraft-specific, but many 28-volt system designs trip in roughly the 30–32 volt range; many 14-volt system designs trip in roughly the 16–16.5 volt range. Exact values are aircraft-specific and found in the maintenance manual.
- Typical undervoltage warning threshold: aircraft-specific, generally set near expected battery-only bus voltage (often in the mid-20s for a 28-volt system) during alternator-off battery operation; consult the applicable maintenance manual for the exact figure.
- Response time: solid-state overvoltage modules typically trip within milliseconds to protect sensitive electronics; electromechanical relays respond in tens of milliseconds.
- Regulatory basis: 14 CFR Part 23, Subpart F (Equipment) — including §§23.1351, 23.1353, and 23.1357 — addresses generating system function, storage battery design, and circuit protection requirements for normal-category aircraft; specific design standards are detailed in applicable technical standard orders and manufacturer data.
- Maintenance authority: overvoltage module calibration and replacement must follow the aircraft manufacturer's maintenance manual (MM) and the component manufacturer's overhaul manual (CMM); field adjustment of trip points is generally not permitted without engineering approval.
- Do not reset repeatedly: if an overvoltage protector trips more than once per the MM guidance, the underlying fault must be identified before returning the aircraft to service.
Common Test Traps
- Confusing the warning light logic: the low-voltage (ALT) warning light illuminates when bus voltage falls below the threshold — not when it exceeds it. Overvoltage may actually extinguish or peg the voltmeter rather than trigger the same annunciator, depending on the aircraft.
- Assuming the circuit breaker is just a breaker: on some aircraft the alternator circuit breaker doubles as the overvoltage protection isolation device. Resetting it without identifying the root cause can expose the system to repeated fault energy.
- Misidentifying auto-reset vs. manual-reset modules: some overvoltage modules automatically reconnect the alternator after a set delay; others require manual pilot action. An AMT must verify which type is installed when troubleshooting a recurring ALT light complaint.
- Neglecting part-number specificity: a voltage regulator/ACU is calibrated for a specific system voltage and alternator model. Installing the wrong unit will cause incorrect trip points and can result in either nuisance trips or, worse, failure to protect the system.
- Overlooking load shedding bus architecture: on aircraft with essential and non-essential bus splits, an undervoltage event should cause non-essential loads to drop off. A test question may ask the technician to diagnose why certain equipment lost power during an alternator failure — the answer is intentional load shedding, not a wiring fault.