Circuit Protection and Switching Devices

Every aircraft electrical circuit needs two kinds of control. Switching devices start and stop the current: some are worked by hand, others by an electromagnet, or by temperature, pressure, rotational speed, elapsed time or the position of a moving part. Circuit protection devices, the fuse, the circuit breaker and the current limiter, stop the current by themselves when it becomes dangerous.

The distinction matters to pilots because the protection devices are the ones they handle in flight. A tripped circuit breaker is one of the commonest electrical indications on any flight deck, and what the crew does with it, reset it or leave it out, is a safety decision. The underlying circuit theory is in electrical fundamentals and DC circuits; the way protection is arranged across the buses is in electrical power distribution.

What circuit protection is for

Circuit protection exists to protect the wiring and equipment from excessive current. Excess current has two main causes:

• A short circuit is an unintended low-resistance path that bypasses the load, often a conductor touching another conductor or the airframe. The current rises to a level limited only by the source and the wiring, and wiring can melt and catch fire.
• An overload is a more gradual excess, such as a motor labouring against a jammed mechanism.

An open circuit is the opposite fault: a break in the path, such as a broken wire, a blown fuse or a failed switch. No current flows and the load stops working, but nothing overheats.

Every protection device is connected in series with the load, so that all the load current passes through it, and it is rated to open the circuit before the wiring overheats. This is why a fuse is never replaced by one of a higher rating: the rating is matched to the wire, not to the equipment at the end of it.

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Fuses and HRC fuses

A fuse is a wire or strip of low-melting-point metal in series with the load. Normal current warms it slightly; excessive current heats it faster than it can cool, the element melts, or "blows", and the circuit is broken. Fuses are rated in amperes, the current they can carry continuously without blowing. The time to blow falls as the overcurrent rises: the further above its rating, the faster a fuse goes.

The high rupture capacity (HRC) fuse is used on high-current circuits. Its element sits in a ceramic body filled with powdered quartz sand or marble, which quenches the arc when the element melts. Compared with a glass cartridge fuse it operates more accurately, without flame, does not deteriorate with age and is less affected by ambient temperature.

A blown fuse may be replaced once. If the replacement blows too, the circuit has a fault that must be found; fitting a higher-rated fuse, or bridging the holder with wire, removes the protection and invites a wiring fire.

For operations under EASA Air Operations, CAT.IDE.A.110 requires spare fuses of the ratings needed for complete circuit protection, for replacing the fuses that may be replaced in flight. Its AMC sets the number: 10 per cent of the number of fuses of each rating, or three of each rating, whichever is greater.

A dummy fuse is a non-conducting plastic plug of standard fuse size, usually red and often carrying a red streamer, fitted in place of a real fuse to isolate a circuit during maintenance. The equivalent for a circuit breaker is a warning flag or collar clipped to the pulled breaker, showing that the circuit is deliberately dead.

Current limiters

A current limiter, also called an airfuse, is a heavy-duty thermal device similar to a fuse but with a higher melting point and a slower response. It lets a considerable overload flow for a short time without rupturing, which suits circuits that see brief heavy currents in normal operation, such as the inrush current of a motor. It still opens under a sustained fault. Current limiters are found in heavy-duty power distribution, for example at the output of a transformer rectifier unit (TRU).

Circuit breakers

A circuit breaker (CB) combines the protection of a fuse with the action of a switch. It trips on overcurrent and can then be reset rather than replaced, and it can be pulled or switched off to isolate a circuit in normal operation. The tripping element is:

• thermal, a bimetallic strip that bends as the current heats it and releases a latch, giving a delay that grows shorter as the overload grows;
• magnetic, a coil whose field trips the mechanism quickly on a heavy fault; or
• both, which is common.

Aircraft circuit breakers are usually push-pull buttons that pop out when they trip, sometimes with coloured collars that show which bus feeds them. They are trip-free: the mechanism opens the contacts on an overload or fault whatever the position of the button, so a faulty circuit cannot be held on by hand. CS 25.1357 and 14 CFR 25.1357 require this of every resettable protective device on a large aeroplane.

Resetting in flight

A circuit breaker protects the wiring, not the equipment, and a trip means current has exceeded what the wiring can safely carry. The accepted practice is:

• reset once at most, after a short cooling period, and only if the service is needed for the safe continuation of the flight;
• if it trips again, leave it out: the fault is still present;
• never hold a breaker in;
• never reset a breaker that tripped together with smoke, fumes or a burning smell.

Forcing current through a damaged wire risks a fire in an inaccessible part of the airframe. Where a type's procedures are more restrictive, they take precedence. See in-flight fire, smoke and fumes.

Reverse current protection

A DC generator needs protection against current flowing the wrong way. If its voltage falls below the battery's, the battery would drive current back into the generator, discharging itself. A reverse current circuit breaker, or generator cut-out, disconnects the generator from the bus when current reverses, and a cut-out also connects it once its voltage rises above the battery's. An alternator does not need one, because its rectifier diodes block reverse current. The protection of AC generators is covered in generator control, protection and paralleling.

Relays, contactors and solenoids

A relay is an electromagnetically operated switch. A small control current energises a coil; the coil attracts an armature, which closes or opens the main contacts. A low-power signal can therefore switch a high-power load remotely, with the control circuit isolated from the load circuit. That saves running heavy cables to the flight deck.

A contactor, or electrical contactor, is a heavy-duty relay built to switch high currents, with larger contacts and sometimes arc-suppression chambers. Typical contactors are:

• the battery contactor, near the battery, closed by the battery master switch (see aircraft batteries);
• the starter contactor;
• generator line contactors and bus tie contactors, which connect generators and buses.

A solenoid is, strictly, a coil of wire that produces a magnetic field when current flows through it; the right-hand grasp rule gives its north pole. With a movable soft-iron core, or plunger, it converts current into a straight-line pull. The word is also used for devices built on that principle: a heavy-duty contactor is often called a battery solenoid or starter solenoid, and a solenoid valve opens or closes a fluid line or moves a hydraulic selector valve electrically. The electromagnetic principles are in electromagnetism and induction.

Relay contacts are described as normally open or normally closed, their state when the coil is de-energised. A normally closed relay that opens on command can, for example, separate the essential from the non-essential DC bus when both generators have failed, so that the battery supplies only the essential loads.

Sensing switches

Many switches are operated not by a person but by the condition they monitor.

Time switches

A time switch, or time relay, makes or breaks a circuit a preset interval after it is started. Uses include sequencing, such as the steps of landing gear operation, silencing a warning after a set time, and keeping an APU air intake open for a period after shutdown so the unit continues to cool.

Microswitches and pressure switches

A microswitch is a small switch operated by a very small mechanical movement. It detects the position of moving parts, such as a door open or closed, landing gear up or down, flap or throttle position, and its output lights an indicator or drives a relay in a sequence.

A pressure switch operates when a fluid or air pressure rises above or falls below a preset value. Low oil pressure warnings, hydraulic pressure indications and fuel pressure switches work this way. Like relays, pressure switches are normally open or normally closed, according to the job.

Thermal and centrifugal switches

A bimetallic switch, or thermal switch, uses a strip of two bonded metals with different rates of thermal expansion. As temperature rises, one expands more than the other, the strip bends, and at a calibrated temperature it makes or breaks the contacts. It is used in overheat and fire detection, oil temperature warnings and the thermal protection of NiCd batteries, where it disconnects the charging source when the battery overheats. The thermal element of a circuit breaker works on the same principle.

A centrifugal switch senses rotational speed. Flyweights move outwards against a spring as rpm rises, and at a set speed they operate the contacts. The classic use is the starter cut-out: once the engine reaches self-sustaining speed, the switch disconnects the starter so that it is not overdriven.

Proximity detectors and Hall effect sensors

A proximity detector senses the presence of a material without physical contact. Common types are inductive, which respond to metal close to a coil; capacitive, which respond to anything that changes an electric field; and magnetic, using a reed switch or a Hall effect sensor to detect a magnet. With no moving contacts to wear, arc or become contaminated, and a sealed case, proximity detectors are more reliable and last longer than microswitches, and on modern aircraft they have largely replaced them for landing gear, door and flap position sensing.

A Hall effect sensor relies on a property of a conductor, usually a semiconductor chip, that carries a current across a magnetic field: the field pushes the moving charges sideways, so a small voltage appears across the chip at right angles to both the current and the field. When a magnet on a moving part comes close, the Hall voltage appears; when it moves away, the voltage disappears. The sensor thus switches without any mechanical contact. Proximity sensors feed the aircraft's monitoring computers much as other transducers do; see sensors, transducers and system indicators.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.