Figure 13.1 shows the general arrangement for a twin-jet aircraft — Page 222, Lesson 239

I want to walk you through the AC electrical distribution systems used on modern aircraft. We're starting a new chapter called "AC Electrics - Practical Aircraft Systems," and I'll begin with the overall power distribution layout and then move into the split bus system. Let's start with the basic layout. Figure 13.1 shows the general arrangement for a twin-jet aircraft. One generator is driven by and mounted on each engine — so that's two engine-driven generators. There's also a third generator mounted on the APU, the Auxiliary Power Unit, though it's not shown in that particular figure. The feeder cables from each generator are routed through the aircraft wings and fuselage, and they all meet at a central distribution compartment. That compartment is usually located beneath the flight deck or the cabin floor. Inside that distribution compartment, you'll find many of the components I've already described in earlier chapters: Generator Circuit Breakers, or GCBs; Bus Tie Breakers, or BTBs; Generator Control Units, or GCUs; voltage regulators; current transformers; the main bus bars; bus bar protection circuitry; the battery; and the battery charger. Some bus bars and bus bar extensions may also be found on the flight deck itself, behind the rear, side, and overhead circuit breaker panels. Now let's move into the first specific system: the Split Bus System. This system uses 115-volt, 200-volt, 400-hertz, three-phase constant frequency alternators as its primary power source. Let me break that down: 115 volts AC is the phase voltage, 200 volts is the line voltage, 400 hertz is the frequency, and three-phase means three alternating currents offset by 120 degrees. These alternators are not designed to run in parallel with each other. That's an important design choice — because they don't run in parallel, you don't need complex paralleling and load sharing circuits. That simplifies the system. For DC power, the split bus system provides a 28-volt DC supply using two Transformer Rectifier Units, or TRUs. Each TRU converts the 115-volt AC from one of the two separate AC bus bars into 28 volts DC. So you have two independent AC bus bars, each feeding its own TRU, giving you two separate DC supplies. There's also a battery in the system. That battery serves several purposes: it provides power to start the APU, it supplies limited emergency power to the essential bus bars if the main generators are lost, and it can supply air and electrics on the ground when the engine-driven generators are off line — meaning when they're not running. Now here's a key operational feature. If either alternator should fail, the main bus bars are automatically connected together by the Bus Tie Breaker — the BTB. When that happens, the two bus bars effectively become one single bus bar. Power supplies to all the bus bars are thereby maintained. So if you lose one generator, the BTB closes and the remaining good generator feeds both sides of the system. Let me show you the diagram that illustrates this.