One thing that bothers me is the fact that there are no master radio switches on the 727 overhead panel. You need to have these switches so you can really turn off the captain's radios in the event of an electrical fire. If the avionics switches are left on the battery will transfer power to the captain's radio bus. The battery system in the 727 has the battery bus, the battery transfer bus and the hot battery bus. The hot battery bus is powered whenever the battery is installed in the plane. It powers stuff like the PA and the fire bottle buttons. When external power is connected the battery charger is powered and charges the battery. Normal A.C. power powers the battery as well through the T.R.'s (transformer rectifiers) that make 28 volt D.C. and thus keep the battery charged. If you were to kill all power to the 727 and left the essential radio bus turned on the battery would die. So to truly be cold and dark you need to be able to turn off the essential radio master switch as well.

Each engine driven generator is able to produce 54 KW for 10 mins and 36 KW continuous. The APU generator is the same as the engine driven generators, except it is rated at 54 KW continuous because it is better cooled. DON'T turn the frequency knobs to max - keep them around the middle. The AC system needs 400 cycle power +/- a few cycles. Here is how the generators are handled during engine start...
All fields and Bus Ties are closed.
1. - APU is supplying electrical power to the aircraft.
2. - Turn off packs to reduce electrical load (pack fans) and air load to start engines. Duct pressure should read around 40 PSI.
3. - Start engine one. As engine comes up to idle check AC meters to engine one and look for 115 volts and 400 cycles. Adjust CSD to obtain 400 cycles.
4. - Move the Essential power switch to the number one engine generator.
5. - Close Main Generator Relay (middle switch). This action will trip the APU generator off and connect the engine one generator to power the plane. The same would happen if you were using ground power. Closing the MGR on generator will trip other power sources.
6. - Start next engine. Move meter selector to that engine and observe volts and freqs.

Here is where it gets complicated. Until now only one power source was connected to the aircraft. The bus ties were used to connect the load buss in series. Just like a power strip with several outlets and items plugged in to them - it is still one power source providing all the power. When you start the second engine generator and connect it to the aircraft you are using more than one power source and something special happens. The bus ties now become part of the Sync-Bus and a whole different system of control comes into play.

With just one power source generator protection consisted of overload protection and high or low voltage protection with wide latitude in the limits. Look at the meters gauge for voltage and frequency. The limits are the full scale of either gauge. This all changes when you connect the second generator. Now the bus ties change protection and become a Sync-Bus. A very complicated electrical system comes into action and load sharing of the generators starts. Now the two generators work together and attempt to equalize their loads. The limits on voltage and frequency variation are lessened because of this paralleling of generators. Overload protection changes and other faults related to the sync-bus come into the system.

In normal operation all the bus ties should be closed (lights out). This will allow ground power or APU power to flow through the bus ties to their load buss.

Had enough? It is a somewhat complicated system and you would need to study the system for some time to be able to understand what is happening to all the electrons.

OK, now you asked for it! Here is a look at the 727 electrical system. Give it a good read and then if you have any questions, ask away.

http://www.smartcockpit.com/data/pdfs/plane/boeing/b727/systems/B727-Electrical.pdf


Lou