We’ll also cover how the DC side works under normal and emergency conditions — Page 222, Lesson 241

I want to walk you through what happens in a split-bus AC electrical system when one generator fails, and then when both fail. We’ll also cover how the DC side works under normal and emergency conditions. Let’s start with the failure of a single alternator. If one alternator fails, the two main AC bus bars—No. 1 and No. 2—are automatically connected by the Bus Tie Breaker. Once that breaker closes, the two bus bars effectively become one bus bar. That means the remaining good alternator now supplies power to both sides. Power supplies to all the bus bars are thereby maintained—nothing essential is lost yet. Now, in flight, you can then start the APU—the Auxiliary Power Unit. Its generator can be used to restore full power by connecting to either AC bus 1 or bus 2. So the APU becomes a backup source to bring the system back to normal. Let’s look at how the loads are split under normal conditions. Each alternator separately supplies its own AC non-essential services and its associated TRU—that’s a Transformer Rectifier Unit, which converts AC to DC. But the essential AC loads are supplied from only the No. 1 main bus bar, and they get there via a changeover relay. So essential AC comes exclusively from bus 1, not from bus 2. A key point: the main AC bus bars are normally isolated from one another. That means the alternators are not paralleled—they don’t run in sync feeding a common bus. They stay separate until a failure forces the bus tie breaker to close. Now, what if both alternators fail? In that case, the AC non-essential services—which are normally supplied from the main AC bus bars—are isolated. They get disconnected to save power. The changeover relay between the No. 1 main bus bar and the essential AC bus bar will automatically switch over. This connects the essential AC bus bar to an Emergency Static Inverter. That inverter takes DC from the batteries and converts it back to AC. If the batteries are in a fully charged state, this inverter can supply the essential AC bus bars for 30 minutes. So you have a limited but critical backup. Now let’s move to the DC side, which is shown in Figure 13.2. Under normal conditions, the DC supply comes from two independent TRUs and the batteries. The No. 1 TRU supplies essential DC loads, and the No. 2 TRU supplies non-essential DC loads. In normal operation, the two bus bars that supply essential and non-essential DC loads are connected together by the Isolation Relay. The batteries are connected directly to the Battery Bus Bar, and through the Battery Relay they will feed the essential DC bus bar. So the batteries are always ready to back up the essential DC side. If one alternator fails, both TRUs are still supplied—because the bus tie breaker has closed, connecting the remaining good alternator to both sides. So both TRUs keep working, and all DC consumers are still supplied. But if both alternators fail, the DC Isolation Relay will open and separate the essential and non-essential bus bars. Non-essential DC loads are now no longer powered. But the AC and DC essential loads will be fed from the battery bus bar. The AC loads, as we said, come from the static inverter, which draws from the battery bus bar. Finally, a note about ground operations. External power or supplies from the APU can be used to feed all electrical services in the aircraft on the ground. But in flight, the APU generator may only be capable of supplying one bus bar. So you can’t necessarily restore full dual-bus power with the APU in the air—it depends on the aircraft design.