Aircraft Electrical Systems practice questions

Answer: C) Trips the generator off line when output exceeds a preset limit

Overvoltage protection is a fault-protection device. It takes the generator off line when output voltage goes above a safe limit, protecting the bus and connected equipment.

• AIncorrect. That is the voltage regulator's job. The overvoltage device is a protective backup that acts when regulation has failed.
• BIncorrect. That is the reverse-current relay (cutout), which opens when generator output drops below battery voltage.
• CCorrect. If voltage rises above a safe limit, for example after a regulator failure, the overvoltage protection de-energizes the generator and disconnects it from the bus. This protects connected equipment from damage.
• DIncorrect. Load sharing between paralleled generators is handled by paralleling or equalizing circuits that adjust each generator's field, not by overvoltage protection.

Answer: A) To confirm the power sources can supply the total connected load, including the new equipment

An electrical load analysis checks that the aircraft's power sources can supply all of the connected loads in probable combinations, so it must be redone when equipment is added. Wire sizing, breaker selection, and weight and balance are separate tasks.

• ACorrect. A load analysis adds up the electrical loads in probable combinations and for probable durations. This shows whether the generators/alternators and other power sources can still supply every circuit after the new equipment is added.
• BIncorrect. Wire size is chosen from current, run length, and allowable voltage drop for each circuit. That is a separate task from checking the capacity of the whole system.
• CIncorrect. Sizing a circuit protective device protects a single wire run. It does not tell you whether the power sources can carry the total load.
• DIncorrect. That is a weight-and-balance calculation, not an electrical load analysis.

Answer: A) It opens the circuit even though the handle is being held in the closed position

A trip-free circuit breaker opens the circuit during an overload or fault even if its handle is held closed. 14 CFR 25.1357(c) requires this of each resettable circuit protective device.

• ACorrect. A trip-free breaker opens the circuit whenever an overload or fault exists, whatever position the operating control is in. 14 CFR 25.1357(c) requires this of each resettable circuit protective device, so holding the handle in cannot override the protection.
• BIncorrect. This describes a breaker that is not trip-free: holding the control in would defeat the protection and let an overloaded circuit overheat. Trip-free design exists to prevent this.
• CIncorrect. This describes an automatic-reset device. 14 CFR 25.1357(g) allows those only as integral equipment protectors (such as thermal cut-outs), and only when separate protection covers the cable. A trip-free breaker stays open until someone resets it.
• DIncorrect. A circuit breaker responds to overcurrent (an overload or fault), not to low bus voltage. Undervoltage is not what trips it.

Answer: A) The essential bus

The essential bus groups the loads needed for safe flight and can be powered from backup sources after a generator failure. Nonessential loads are shed to protect the remaining capacity.

• ACorrect. The essential bus carries the loads needed for safe flight and is arranged so that it can be powered from alternate sources, such as the battery or a remaining generator, after a power-source failure.
• BIncorrect. The nonessential bus carries loads that can be shed, and it is usually among the first to be disconnected when generating capacity is lost.
• CIncorrect. A tie bus connects or parallels other buses and power sources. Its job is not to hold the critical flight loads.
• DIncorrect. A ground service bus powers servicing loads, such as cabin and cargo lighting, from external or battery power on the ground. It is not intended for critical in-flight equipment.

Answer: D) To identify the wire and the circuit or function it is part of

Wire identification markings identify each wire and the circuit or function it belongs to, so the wiring can be traced against diagrams during maintenance. They do not show clamp locations, breaker ratings or test dates.

• AIncorrect. Clamp spacing and locations come from the installation drawings and standard practices. Wire identification codes do not mark support points.
• BIncorrect. Circuit protection ratings come from the wiring diagrams and electrical load data. They are not what the wire identification code is for.
• CIncorrect. Inspection and test results go in the maintenance records. Wire markings have to stay the same and stay legible for the wire's whole service life.
• DCorrect. Wire identification codes identify each wire and the circuit or function it serves, so technicians can trace the wiring against the wiring diagrams. 14 CFR 25.1711(a) requires wiring components to be identified by a consistent method that shows the component and its function, and 25.1711(c) requires the markings to stay legible for the component's service life.

Answer: C) Four

• AIncorrect. That exceeds the general limit.
• BIncorrect. Up to four are generally allowed.
• CCorrect. More than four terminals on one stud is not recommended.
• DIncorrect. The guidance sets a maximum of four.

Answer: B) The current to be carried and the allowable voltage drop

Aircraft wire is sized so it can carry the circuit current without overheating and so the voltage drop over the run stays within allowable limits.

• AIncorrect. The terminal lug is chosen to fit the wire after the wire size is set. Nominal voltage alone does not set conductor size. What matters is the voltage drop allowed for that system.
• BCorrect. The wire must carry the circuit's current without overheating. It must also keep the voltage drop over the length of the run within allowable limits. AC 43.13-1B Chapter 11 sizes wire using these two factors.
• CIncorrect. Each circuit's wire is sized for that circuit's own load, not for the total generator output. Splices are kept to a minimum, but they do not set the wire gauge.
• DIncorrect. Insulation color is for identification, and clamp spacing is about supporting the bundle. Neither one decides the conductor size.

Answer: C) The reverse current relay

A reverse current relay (cutout) disconnects the generator from the battery when generator voltage drops below battery voltage, so the battery can't discharge back through the generator. Voltage regulators, current limiters and overvoltage relays protect against other conditions.

• AIncorrect. The voltage regulator adjusts generator field current to hold output voltage at a set value. It does not open the line between the generator and the battery when current reverses.
• BIncorrect. The current limiter keeps the generator from putting out more than its rated current. It protects against overload, not against reverse current from the battery.
• CCorrect. The reverse current relay (cutout) opens the generator-to-bus circuit when generator voltage falls below battery voltage and current starts to flow back into the generator. This disconnects the generator from the battery, as 14 CFR 27.1351(c) requires.
• DIncorrect. The overvoltage relay (overvoltage control) acts when generator voltage goes too high, which is the opposite of the low-RPM condition in the question.

Answer: A) The wiring in that circuit from overheating caused by overcurrent

A circuit breaker is rated to protect the wiring in its circuit. It opens on overcurrent before the conductor overheats. Protection for the equipment itself is a separate function.

• ACorrect. The breaker rating is matched to the wire so the circuit opens before the conductor overheats and its insulation is damaged or catches fire. 14 CFR 25.1357(g) describes circuit protection as protecting the cable to the equipment.
• BIncorrect. Protection inside a unit, such as a thermal cut-out, protects the equipment itself. 25.1357(g) allows such protectors only if there is also circuit protection for the cable, which is the circuit breaker's job.
• CIncorrect. A branch breaker is sized for the current of its own circuit. Overload protection for the generator is handled separately in the power generation and control system.
• DIncorrect. A circuit breaker responds to excess current in its circuit. It does not sense battery state of charge or prevent a battery from being run down.

Answer: D) At intervals of not more than about 15 inches

• AIncorrect. Wire identification is required.
• BIncorrect. Markings are spaced along the wire.
• CIncorrect. That is too far apart.
• DCorrect. Markings also appear near each end.