DC Electrics - Aircraft Electrical Power Systems — Page 132, Lesson 136

I want to walk you through the classification of electrical services on an aircraft and how they are distributed through a parallel bus bar system. This is a fundamental part of understanding aircraft electrical power distribution. Let's start with the three categories of electrical services, because how they are connected determines what stays powered in an emergency. Vital services are those you would need after an emergency wheels-up landing. Think of things like emergency lighting and the crash switch that operates fire extinguishers. These are connected directly to the battery, with no generator or bus bar in between. That way, even if the entire electrical generation system is destroyed, those services still have power from the battery. Essential services are those required to ensure safe flight during an in-flight emergency. These are connected to DC and AC bus bars, as appropriate, and they are wired so that they can always be supplied either from a generator or from the batteries. So they have a backup path. Non-essential services are those that can be isolated in an in-flight emergency for what we call load shedding. They are also connected to DC and AC bus bars, but they are supplied only from a generator. If the generator fails, these loads get dropped first. Now, Figure 8.10 illustrates the principle of dividing these categorized consumers between individual bus bars. This is an example of a parallel bus bar system. In this example system, the power supplies are: - 28 volts DC from engine-driven generators operating in parallel - 115 volts, 400 Hz AC from inverters - 24 volts DC from batteries Let me walk you through how the parallel bus bar system works, based on Figure 8.10. Each generator has its own bus bar. Connected to that generator's bus bar are the non-essential consumer services. So Generator 1 powers Bus Bar 1 with non-essential loads, and Generator 2 powers Bus Bar 2 with non-essential loads. Both of those bus bars are then connected to a single bus bar which supplies power to the essential services. So with both generators operating, all consumers requiring DC power are supplied. The essential services bus bar is also connected to the battery bus bar. This ensures the batteries are maintained in a 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. Even with all generators off, that bus bar is live from the battery. Now, what happens when a generator fails? If one generator fails, it is automatically isolated from its respective bus bar. 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 — which we cover on page 131. If both generators fail, the 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. That period is calculated based on consumer load requirements and battery state of charge. Normally, this is a minimum of 30 minutes. Finally, let's look at how the AC inverters are powered in this system. The DC supply to power the inverters is taken from bus bars appropriate to the importance of the AC-operated consumers. - Essential AC consumers are operated by the No. 3 inverter. That inverter is supplied with DC from the essential services bus bar. - No. 1 and No. 2 inverters supply AC to non-essential services. So they are powered by DC from the No. 1 and No. 2 bus bars respectively. So the hierarchy is clear: vital services go straight to the battery, essential services have a dual feed from generator and battery, and non-essential services rely solely on the generator and get shed first in a failure.