View Full Version : Split Bus AC Generator Question

3rd Jun 2008, 12:36
Hi all

The below question assumes the airframe in particular has a Split Bus System.

If a twin engine jet's Number 1 AC generator fails inflight, I am aware the Bus Tie Breaker will close and therefore supply all the DC loads (thanks to the Number 2 AC Generator), however in doing so is the Number 1 Main AC Bus Bar supplied with power from the Number 2 Generator?

I think the answer is NO but I would like to confirm this.

If the answer is indeed no, would it be a good idea to switch on the APU to power the Number 1 AC Bus essential loads or would you use the battery to do this? I reckon the battery would only be used if both AC Generators fail.

Thanks guys. Look forward to your professional opinion!

3rd Jun 2008, 13:36
In a split bus system on most a/craft, the AC Buses are Tied by a relay when 1 engine driven generator goes U/S (ie engine shutdown). DC buses are still supplied by their normal main AC bus thru their T/Rs (transformer rectifiers)
Automatic load shedding is also employed to ensure enough power for essential services, ie galley power reduced or switched off.

Capt Pit Bull
3rd Jun 2008, 16:41
Probably as many variations on the theme as there are aircraft types.

One method is to have your essential services on a transfer bus that is powered by the main bus on the side in question. Power the non-essentials direct from the main bus

Then with a 'suitable arrangement of relays' (tm) arrange the transfer buses to become powered by the opposite sides main bus if their own main bus fails.

E.G. Gen 1 fails. Main AC bus 1 falls over. AC 1 transfer bus now fed by Gen 2. (Gen 2 continues to feed Main bus 2 and transfer bus 2).

This way you've load shed the non essentials and load transferred the essentials.

Hope that's of some use.


3rd Jun 2008, 19:25
Thanks PB et al.

I am trying to avoid starting another thread. Can someone please give a definition for 'galley power'?

3rd Jun 2008, 19:27
Galley power = din-dins.

4th Jun 2008, 09:45

What's that? Elaborate please.

4th Jun 2008, 09:48
Din Dins = Food, Galley power = Ovens & Bev Makers

4th Jun 2008, 14:07
Thanks guys. Silly me!

4th Jun 2008, 15:22
A split bus is normally operated as a single bus. Power placed on any point on the bus powers the whole bus. If the bus is AC, then any power source placed on the bus needs to be regulated in voltage, frequency, and phase. When you say "generator failure," you're not specific as to the nature of the failure. It's possible to have a generator problem, with a generator that can still serve it's respective bus, but won't parallel with other generators on the main bus; in this case the bus can be split, or the generator isolated. It's failed in the sense that it doesn't play well with other generators, but is still performing a useful job. Where the split system breaker or bus tie is normally closed, it may be opened for a fault, or to isolate each generator from other generators, or from other faults.

So long as DC voltages match, you're golden. You can put them together. AC is another animal, and has more requirements.