DC Electrics - Aircraft Electrical Power Systems — Page 132, Lesson 134

I want to walk you through the DC electrical distribution system in an aircraft — specifically, how power gets from the source to the equipment that needs it, and what happens when things go wrong. Let's start with the big picture. The function of a distribution system is primarily a simple one: move electrical power from the source to the consumers. But it gets complicated because we have to meet additional requirements that concern a power source, or a power consumer system, operating either separately or collectively, under abnormal conditions. In other words, the system has to work not just when everything is normal, but also when a generator fails, a wire shorts, or a piece of equipment malfunctions. These requirements and abnormal conditions fall into three main areas. Let me go through each one. First: 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. So if your alternator fails in flight, the battery can still supply power, but only up to its capacity. If you try to draw more current than the battery can deliver, something has to give — and that's where prioritisation comes in, which we'll get to shortly. Second: faults on the distribution system itself — things like fault currents, or grounding or earthing at a bus bar — should have the minimum effect on system functioning and should constitute the minimum possible fire risk. A short circuit at a bus bar could dump huge current into the structure, creating heat and potentially fire. The system must be designed so that such a fault doesn't take down the entire electrical system, and doesn't start a fire. Third: power-consuming equipment faults must not endanger the supply of power to other equipment. If a single light bulb shorts out, it shouldn't cause the radios to lose power. Each piece of equipment needs to be isolated so that its failure doesn't cascade. Now, how are these requirements 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. Paralleling generators means connecting them so they share the load — if one fails, the other can still supply power. Circuit protection devices, like fuses or circuit breakers, are sized to open before a fault current can damage wiring or start a fire. And if a generator fails, it gets disconnected from the bus so it doesn't drag down the rest of the system. The operating principle behind these methods also involves arranging bus bars and distribution circuits so that they may be fed from different power sources. A bus bar is simply a common connection point — think of it as a metal strip or bar that distributes power to multiple circuits. By arranging bus bars cleverly, you can have one bus fed by the alternator, another by the battery, and critical equipment can be switched between them. In adopting this arrangement, it's usual to categorise all consumer services — that is, all the electrical equipment on the aircraft — into their order of importance. And in general, they fall into three groups: vital, essential, and non-essential. Vital equipment is the stuff you absolutely need to keep flying and land safely — things like flight instruments, maybe the primary radios. Essential equipment is important but not immediately critical — perhaps the cabin lighting or secondary avionics. Non-essential equipment is the comfort items — the cigar lighter, the cabin reading light, that sort of thing. When power is limited, the non-essential loads get shed first, then essential, and only as a last resort would you shed vital loads. Now, let me tie this to the diagram you can see on screen. This is Figure 8.9, showing a typical light aircraft single alternator DC system. You'll see the bus bar running across the middle — that's the main distribution point. Coming off it are various loads: the starter solenoid, starter motor, starter switch, lamp test, a 5-amp circuit, another 5-amp circuit, a 15-amp circuit, the alternator warning light, cabin light, cigar lighter, external power receptacle, external power solenoid, master solenoid, voltage regulator, overvoltage protector, a generator labelled 'G', a radio interference capacitor, the alternator, battery, master interlock, battery and alternator switch, alternator field, alternator source-power relay energising circuit, and separate battery and alternator switches. Notice the ammeter on the bus bar — that measures current flowing into or out of the battery. And the master solenoid is the big relay that connects the battery to the bus when you turn the master switch on. The alternator feeds the bus through its own relay, and the voltage regulator controls the alternator's output to keep the bus voltage steady. The overvoltage protector is there to disconnect the alternator if its output voltage goes too high — that could damage equipment. So the whole system is designed around that bus bar, with protection devices and source selection to ensure that vital loads stay powered even when something fails. That's the core idea of a DC electrical distribution system in an aircraft.