Electromagnetic induction is the effect in reverse: where current creates a magnetic field, a changing magnetic field creates (induces) a voltage in any conductor it sweeps across. Move a magnet past a wire, or grow and collapse a field beside it, and the wire's ends acquire a voltage with no contact at all. Induction is the principle of the generator and the transformer, both subjects of the generation lesson later in this module, but it also has an unwanted face: two electrical systems can affect each other through it. A cable carrying a heavily switched or alternating current wraps its neighbours in a changing field, and every neighbouring wire has small voltages induced in it: interference, superimposed on whatever signal that wire was carrying. This is why signal wiring is twisted, screened and routed away from power cables, and why a sensitive circuit can misbehave when a heavy consumer nearby is switched.
Fuses and circuit breakers
The last lesson ended with a warning: collapse a circuit's resistance and the current multiplies. Protection exists to disconnect a circuit before its fault can do damage, and it comes in two classic forms:
Fuse — a deliberately thin conductor in the circuit's path: current above its rating melts it, the melted wire opens the circuit, and the fuse is spent, replaced only on the ground.
Circuit breaker — the same job done with a mechanism instead of a sacrifice: excessive current trips a latch, a button pops out to show it, and the circuit is open until the breaker is reset by pushing the button back in.
A fuse's rating is the current it carries continuously without melting; sufficiently far above that rating it melts, and the further above, the faster.
Two circuit-breaker principles divide the work:
Thermal circuit breaker — senses the magnitude of the current: the current warms a bimetallic element (two bonded metals that expand at different rates, so it bends as it warms), and sustained excess warps it far enough to trip the latch. The warming takes time, which is a feature: brief spikes pass, sustained overcurrent trips.
Magnetic circuit breaker — senses the direction of the current: built around a coil, it trips when current flows the wrong way through the circuit it guards. Its classic station is generator protection, where power must flow from the generator to the system and never backwards into it: a failed generator would otherwise be driven like a motor from the battery.
A circuit breaker is also a maintenance handle for the aircraft's computers: pulling a system's breaker and resetting it removes and restores that system's power, forcing its computer through a cold restart. Some abnormal procedures direct exactly this, naming the breaker and the wait. The discipline matters as much as the trick: a breaker is cycled to reset a system when a written procedure says so, not as improvised troubleshooting, because the same action that reboots a computer also reapplies power to whatever fault tripped the breaker in the first place.
The short circuit
A short circuit is Ohm's law with the resistance removed. Suppose chafed insulation lets a 28-volt supply wire touch the structure directly, bypassing its consumer. The circuit's resistance falls from the consumer's designed ohms to the near-zero of bare wire, and current, voltage divided by resistance, multiplies enormously: a circuit built for 5 amperes may briefly conduct hundreds. Power dissipated is voltage times current, so it multiplies with the current, and all of it lands as heat in the wiring and at the fault, the only resistances left. Energy is power times time: every second the fault persists, hundreds of watts pour into a few grams of copper and insulation. That is the fire mechanism: conductors glow, insulation melts and ignites, and neighbouring wires join in. Any circuit subjected to overcurrent overheats the same way, short circuit or not; a wire carrying more current than its cross-section was sized for is a heater in the wrong place. This is what fuses and breakers exist to interrupt, and why their response is matched to the wiring they guard.
Not every overcurrent is a fault: overcurrent situations may be transient. An electric motor draws a heavy inrush current in the moment of starting, several times its running current, and a cold heater draws more than a warm one. Protection is built to ride these out, the thermal breaker's warming delay being exactly the tolerance required: a spike passes before the bimetal warps, a sustained fault does not.
Two abuses defeat the protection. Resetting a tripped breaker more than once means repeatedly reconnecting a live supply to what is, until proven otherwise, a fault: each reset delivers another dose of fault current and another pulse of heat into damaged wiring that has had no time to cool, and enough doses start the fire the breaker existed to prevent. One considered reset of an essential circuit may be justified; cycling a breaker that keeps tripping is feeding a fire. Replacing a blown fuse with a higher-rated one is the same error made permanent: the wiring was sized to the original rating, and behind a too-large fuse the wire becomes the sacrificial element, overheating before the fuse ever notices.
Semiconductors and heat
The last lesson introduced semiconductors as the material of the aircraft's electronics; heat is their particular enemy, in two ways:
Function — a semiconductor conducts more as it warms, so temperature shifts a device's operating point, and a hot transistor drifts, leaks current where it should block, and can run away entirely as its own heating raises its conduction further.
Longevity — sustained heat ages semiconductor junctions, and a component run hot lives a markedly shorter life than the same component run cool.
Both are why avionics bays and instrument panels are ventilated and cooled: the cooling air is not a comfort, it is what keeps the computers inside their operating points and their service lives.
The five logic gates
An aircraft's systems constantly make small decisions: sound the warning if this AND this, unless that. The FCOM draws these decisions as logic gates: symbols with inputs on the left, one output on the right, and a rule connecting them. Each input and output is either true (on, 1) or false (off, 0). Five gates, in their ANSI/MIL symbols (the American standard the FCOMs use), cover the syllabus:
AND — a D-shaped symbol: the output is true only if every input is true.
OR — a curved-backed, pointed symbol: the output is true if at least one input is true.
NOT — a triangle with a small circle (a bubble) on its output: one input, and the output is its opposite (an inverter).
NOR — the OR symbol with a bubble on the output: true only when no input is true (OR, then inverted).
NAND — the AND symbol with a bubble on the output: true unless all inputs are true (AND, then inverted).