The Battery

Systems Power Electrical Aircraft - Electrics DC 8

Figure 8.8 General arrangement - single-engine light aircraft

In most types of aircraft, the output from the generating sources is coupled to one or more low impedance conductors referred to as bus bars.

The bus bars are the collection and distribution centre for a generator or alternator power supply. They use solid copper bars which can be drilled to permit supply and distribution cables to be attached to them.

Bus bars are usually situated in junction boxes or distribution panels located at central points within the aircraft, and they provide a convenient means for connecting power supplies to the various consumer circuits; in other words, they perform a ‘carry-all’ function.

Bus bars vary in form dependent on the methods to be adopted in meeting the electrical power requirements of a particular aircraft type.

In its simplest form a bus bar can take the form of a strip of interlinked terminals, while in the more complex systems main bus bars are thick metal (usually copper) strips or rods to which input and output supply connections can be made.

The strips or rods are insulated from the main structure and are normally provided with some form of protective covering. Flat, flexible strips of braided copper wire are also used in some aircraft and serve as subsidiary bus bars.

Bus Bar Systems

Figure 8.9 A typical light aircraft single alternator DC system

The function of a distribution system is primarily a simple one, but it is complicated by having to meet additional requirements which concern a power source, or a power consumer system operating either separately or collectively, under abnormal conditions.

The requirements and abnormal conditions may be considered in relation to three main areas, which are summarized as follows:

•Power-consuming equipment must not be deprived of power in the event of power source failures unless the total power demand exceeds the available supply.

•Faults on the distribution system (e.g. fault currents, grounding or earthing at a bus bar) should have the minimum effect on system functioning and should constitute minimum possible fire risk.

•Power-consuming equipment faults must not endanger the supply of power to other equipment.

These requirements are met in a combined manner by paralleling generators where appropriate, by providing adequate circuit protection devices, and by arranging for failed generators to be isolated from the distribution system.

The operating principle of these methods is concerned with the additional one of arranging bus bars and distribution circuits so that they may be fed from different power sources.

In adopting this arrangement it is usual to categorise all consumer services into their order of importance and, in general, they fall into three groups:

•Vital

•Essential

•Non-essential

DC Electrics - Aircraft Electrical Power Systems 8

Systems Power Electrical Aircraft - Electrics DC 8

Vital services are those which would be required after an emergency wheels-up landing, e.g. emergency lighting and crash switch operation of fire extinguishers. These services are connected directly to the battery.

Essential services are those required to ensure safe flight in an in-flight emergency situation. They are connected to DC and AC bus bars, as appropriate, and in such a way that they can always be supplied from a generator or from batteries.

Non-essential services are those which can be isolated in an in-flight emergency for load shedding purposes (see below), and are connected to DC and AC bus bars, as appropriate, and are supplied from a generator.

Figure 8.10 illustrates, in a very simplified form, the principle of dividing categorized consumer services between individual bus bars; this is an example of a parallel bus bar system.

In this example, the power distribution system is one in which the power supplies are 28 volts DC, from engine-driven generators operating in parallel, 115 volts 400 Hz AC from inverters, and 24 volts DC from batteries.

Parallel Bus Bar System

Figure 8.10 shows that each generator has its own bus bar to which are connected the non-essential consumer services.

Both bus bars are in turn connected to a single bus bar which supplies power to the essential services. Thus, with both generators operating, all consumers requiring DC power are supplied.

The essential services bus bar is also connected to the battery bus bar so ensuring that the batteries are maintained in the charged condition.

The battery bus bar may be referred to as a ‘hot bus’ or ‘hot battery bus’ because it is always connected to the battery.

In the event that one generator should fail it is automatically isolated from its respective bus bar and all bus bar loads are then taken over by the operating generator. In the event of a generator failure the pilot will commence “LOAD SHEDDING” (page 131).

Should both generators fail, however, non-essential consumers can no longer be supplied, but the batteries will automatically supply power to the essential services and keep them operating for a predetermined period calculated on the basis of consumer load requirements and battery state of charge. (Normally a minimum of 30 minutes).

In the case of the system represented in Figure 8.10, the DC supply to power the inverters is taken from bus bars appropriate to the importance of the AC operated consumers.

Thus, essential AC consumers are operated by the No. 3 inverter and so it is supplied with DC from the essential services bus bar.

No. 1 and No. 2 inverters supply AC to non-essential services and so they are powered by DC from the No. 1 and No. 2 bus bars.