Every wire in an aircraft's electrical system carries a risk: if too much current flows through it, the wire overheats, insulation melts, and a fire can start in a place you may never reach in flight. Circuit breakers and fuses are the two primary devices used to prevent that outcome. Both work by interrupting current flow when it exceeds a safe level, but they do so in different ways, and the rules governing how they are selected, installed, and replaced differ significantly. For an Aviation Maintenance Technician working on airframe electrical systems, understanding these differences is not just exam knowledge — it is a fundamental safety responsibility.
This article walks through the operating principles of each device, the criteria used to select the correct rating, the regulatory framework that governs their use, and the practical guidance that separates a safe installation from a dangerous one.
How Circuit Breakers Work
A circuit breaker is a resettable overcurrent protection device. Inside the most common aircraft type — the thermal circuit breaker — a bimetallic strip carries the circuit current. When current exceeds the rated value, the strip heats up, bends, and mechanically trips a latch that opens the circuit. The breaker button pops out on push-to-reset types, providing an immediate visual indication of the trip; the exact pop-out distance varies by manufacturer and is not a standardized FAA-published figure. Once the fault is cleared and the strip cools, the breaker can be reset by pushing the button back in.
A second type, the thermal-magnetic circuit breaker, adds a magnetic element that responds to sudden, high-magnitude surges — the kind a motor starting load might produce — without nuisance tripping during normal inrush. This makes them well suited for motor-driven equipment such as electric trim, landing gear actuators, and flap motors.
A third variety, the manual reset (toggle-type) breaker, must be deliberately toggled off and back on by the technician or pilot; it does not pop out automatically. These are less common in newer designs but still found in legacy aircraft. Regardless of type, circuit breakers are designed so that the cockpit control and the protective element are the same device — one unit that both protects the wire and serves as the switch or indicator.
How Fuses Work
A fuse is a one-time overcurrent protection device. A calibrated fusible element — a short length of metal alloy with a precise melting point and cross-section — is enclosed in a glass or ceramic body. When current exceeds the rated value, the element melts and permanently opens the circuit. Unlike a circuit breaker, a fuse cannot be reset; it must be replaced with a new fuse of the correct type and rating.
Aircraft commonly use cartridge fuses (glass cartridge types such as AGC and AGX, which are fast-acting designs — AGC being a standard glass fuse and AGX a fast-acting variant; slow-blow or time-delay aircraft fuses are typically identified by separate designations rather than by the AGC/AGX distinction) and, in some applications, current limiters — heavy-duty fuses designed to protect large-gauge feeders and bus ties rather than individual branch circuits. Fuses generally respond faster than thermal circuit breakers to short-circuit conditions, which can be an advantage in sensitive avionics circuits where even a brief overcurrent could damage equipment.
Selection Criteria: Matching Protection to the Wire
The most important principle in selecting either device is this: the overcurrent protection must protect the wire, not the equipment. The FAA's guidance, grounded in advisory material on aircraft electrical system design, is explicit — the breaker or fuse rating must not exceed the allowable current-carrying capacity (ampacity) of the smallest wire it protects in that circuit segment.
Wire ampacity is determined by the wire gauge (AWG — American Wire Gauge, where a smaller number means a larger wire), the wire's insulation type, the installation environment (conduit vs. open air, bundled vs. single), and the operating temperature of that area. Larger bundles of wire derate individual wire ampacity because bundled wires cannot dissipate heat as effectively as single conductors. The specific amount of derating for a given wire depends on the wire gauge, bundle size, and ambient temperature, and must be determined from the applicable AC 43.13-1B wire derating curves rather than a single fixed figure.
Once the allowable ampacity of the wire is established, a breaker or fuse is selected whose continuous rating does not exceed that value. Never upsize a breaker or fuse to stop nuisance tripping. Nuisance tripping is a symptom of a fault or an undersized wire — solving it by installing a higher-rated protective device simply shifts the failure mode from a tripped breaker to a burning wire.
For equipment protection — for example, protecting a sensitive avionics box from overcurrent — additional fuses may be installed at or near the equipment itself, in series with the branch circuit breaker. This provides two layers of protection: the feeder breaker protects the wire, and the equipment fuse protects the box.
Replacement Criteria: The Critical Rule
For fuses, the replacement rule is straightforward: replace with a fuse of identical type, construction, and current rating. Substituting a fuse of a different physical size, a different time-delay characteristic (fast-blow for slow-blow), or a higher rating is prohibited. A physically larger fuse that happens to fit the holder may allow dangerous current levels before opening. Aircraft maintenance manuals and wiring diagrams specify exact fuse part numbers — follow them precisely.
For circuit breakers, the situation is more nuanced. In flight, guidance such as AC 91-42C and aircraft-specific flight manuals (AFMs) generally caution against resetting a tripped circuit breaker unless doing so is necessary for the continued safe flight of the aircraft, and any reset should follow the procedure specified in that aircraft's AFM rather than a universal wait time. However, if the breaker trips again after reset, it should be left open — attempting repeated resets risks starting or worsening an electrical fire. On the ground, a breaker that has tripped should be investigated before reset; the underlying fault must be identified and corrected.
When physically replacing a failed circuit breaker, the replacement must match the original in current rating, voltage rating, trip characteristics, and physical mounting configuration. Circuit breakers are calibrated devices — a 5-ampere breaker from one manufacturer may not trip at exactly the same point as one from another, and differences in calibration can matter. Always use the part number specified in the aircraft's maintenance manual or illustrated parts catalog.
Regulatory and Advisory Framework
Title 14 CFR Part 23 (for small airplanes) and Part 25 (for transport category) set airworthiness standards that require overcurrent protection for each circuit. Specifically, regulations require that each circuit have protection against overcurrent and that protective devices be accessible and resettable (or replaceable) during flight where the circuit is needed for continued safe flight and landing. The FAA's Aircraft Electrical Systems guidance, along with AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair), Chapter 11, provides the practical wire sizing and overcurrent protection guidance that technicians use daily. AC 43.13-1B Chapter 11's wire ampacity figures (Figures 11-1 through 11-6) and associated protection guidance are the standard reference for determining allowable wire current loads and corresponding protection ratings.
Key Numbers and Rules
- Protection protects the wire first: Breaker or fuse rating must not exceed the wire's ampacity for the installation conditions.
- Never upsize to stop nuisance tripping: Investigate the cause; do not substitute a higher-rated device.
- Fuse replacement: Identical type, rating, and time-delay characteristic — no substitutions.
- In-flight breaker reset: Follow the aircraft's AFM procedure; if it trips again, leave it open.
- Current limiters: Used on large feeder cables to protect bus structure and heavy wiring — not general branch circuit protection; specific ratings vary by installation.
- Bundling derating: Wire ampacity decreases as bundle size increases; always derate per the applicable table before selecting protection.
- Voltage rating: The replacement breaker or fuse must be rated for at least the system voltage (typically 14V or 28V DC in general aviation).