DC Electrics - Aircraft Electrical Power Systems — Page 132, Lesson 133

I want to walk you through the concept of bus bars, which are the backbone of any aircraft electrical power distribution system. Let's start with the big picture from Figure 8.8 — the general arrangement of a single-engine light aircraft. You can see the alternator, the battery, and then a thick vertical line labelled 'BUS BAR'. Everything feeds into it or draws from it. In most types of aircraft, the output from the generating sources — that's your alternator or generator — is coupled to one or more low impedance conductors referred to as bus bars. 'Low impedance' simply means they offer very little resistance to current flow, so they can carry large amounts of electrical power without significant voltage drop. The bus bars are the collection and distribution centre for a generator or alternator power supply. Think of them as the central hub: all the electrical power that's generated comes into the bus bar, and all the electrical power that's needed by the aircraft's systems goes out from the bus bar. They use solid copper bars which can be drilled to permit supply and distribution cables to be attached to them. So you literally drill holes in a solid copper bar, bolt your cables on, and that's your connection point. Bus bars are usually situated in junction boxes or distribution panels located at central points within the aircraft. They provide a convenient means for connecting power supplies to the various consumer circuits — 'consumer circuits' being any electrical load, like your lights, radios, instruments, and so on. In other words, they perform a 'carry-all' function. Everything that needs power comes to the bus bar, and the bus bar carries it all. Now, bus bars vary in form dependent on the methods to be adopted in meeting the electrical power requirements of a particular aircraft type. There's no one-size-fits-all design. In its simplest form, a bus bar can take the form of a strip of interlinked terminals — just a row of terminals connected together by a metal strip. That's about as basic as it gets. 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. These are substantial pieces of metal designed to handle the full electrical load of the aircraft. The strips or rods are insulated from the main structure — you don't want them shorting out against the aircraft's metal airframe — and they are normally provided with some form of protective covering to prevent accidental contact or damage. Flat, flexible strips of braided copper wire are also used in some aircraft and serve as subsidiary bus bars. These flexible strips can be routed into tighter spaces or across moving parts where a rigid copper bar wouldn't work, and they still perform the same collection-and-distribution function, just on a smaller scale. So to summarise: a bus bar is a low-impedance conductor — usually solid copper — that acts as the central collection and distribution point for all electrical power in the aircraft. It sits in a junction box or distribution panel, it's insulated from the airframe, and it comes in forms ranging from a simple strip of terminals to thick copper bars or even flexible braided strips for subsidiary circuits. Everything that generates power connects to it, and everything that consumes power connects to it. That's the bus bar.