View Full Version : Electrical system

11th Nov 2004, 22:48
Can anyone provide me with an EASY to understand answer to how a typical Electrical system works on a modern Jet?

I'm having a hard time understanding all of this "bus" stuff. Battery bus, transfer bus etc...Are these all individual bus bars?
Where does the generators come into this?


Ka8 Flyer
11th Nov 2004, 23:14
Hi there!

Think of a bus as a plug socket.

And your aircraft is a big room with many sockets.
Now the difference between your living room and your typical aircraft is that each socket in the plane has a different power source (not all, but many)
Some are only powered by the battery, others only by generators or by transformers.
Also, the sockets aren't directly connected to each other. If one of your sockets in your living room has a short circuit, most appliances in that room will fail as the circuit breaker trips. Not good.
In an aircraft, a complete bus can fail (an AC bus for example) without affecting the other systems.
Of course some busses are connected but to keep it simple I won't go into that. (If you need more info go ahead and ask)

I hope that helps?



12th Nov 2004, 00:08
That is a good analogy.

These sockets (buses) are then distributing power to various equipment? is that it?

Are the buses all the doing the same job essentially? I mean they have different names . "transfer" bus etc.

Thanks for your help.

12th Nov 2004, 05:09
Maybe if it's put in operational terms. The whole point is not to lose too many systems after one or more failures. Most aircraft have two main generators, one on each engine with another on the APU for use on the ground or in the air (if needed). Each one puts out enough electricity to keep your street powered and basically provides it to its side of the aircraft - No. 1 on the Left and No. 2 on the Right. Depending on where the fault may be the crew can use large remote switches known as Contactors to couple (or transfer) Buses.

This power is usually 3 Phase Alternating Current (AC) at 115V & 400 Hz and is brought down to much lower voltages (28 V DC) for radios, avionics & lighting by units called Transformer Rectifier Units (TRUs). Each one of these, and in some cases in pairs, is used to power a DC Bus. Again these have contactors & switches to allow the power to be moved around a fault and keep systems alive. Finally, exactly like your car, you have one, or more likely, two batteries to keep a minimum number of radios and other services alive if all else fails.

Hope this helps.

Ka8 Flyer
12th Nov 2004, 10:33
Hi Tom,

"Are the buses all the doing the same job essentially?"

Regarding it globally, then I'd say yes. In more detail, each bus will power different systems. More important systems (like flight instruments) can be supplied by different power sources and/or busses.
Also, not that important stuff, like cabin lighting, the ovens etc are all connected to a single bus, (or two busses) and can be easily shed (disconnected) if you lose an engine or a generator.
Important stuff, or sstems that always need electrical power to function (ship's clock) are connected to busses that are always powered (as long as the battery has enough power). These include the fire extinguishing system, as an example. This would then be your hot battery bus (Boeing term, not sure if it applies to Airbus).