Duchess Electrical System

[cookmg]

I have always understood the bus tie fuse to be the only means for one alternator to supply power to the opposite bus. Now I'm curious, if the bus tie was not there (or was blown) couldn't #1 alternator power bus #2 by routing power through the battery bus and down into the second bus?

If so, then why the bus tie fuse?

Thanks

 

[SMOE]

Here's my take:

The wiring diagram I have shows a pair of current limiters between the Batt Bus and the Left and Right Busses. As I understand it, as long as the Left and Right Busses are self powered (via the Alternators), current only flows from the main busses. The current limiters prevent current flow from the battery bus and thus prevent powering one of the busses should the bus tie fuse go.

just a theory. Anyone else?
_________________________________________________________________
"The value of a twin-engine aircraft is that it gets you out of the trouble you wouldn't be having in the first place unless you were flying a twin."
- Murphy's Law of Multi-Engine Aircraft-

 

[TheBigH]

as far as I know current limiters are just high amperage fuses, normally diodes are used to control the direction of electrical flow


the POH states that the two alternator systems are separate except for the bus tie fuse, the mutual tie to the battery bus, and the paralleling circuit between the regulators

Also, the bus tie fuse is only present in the newer ME-103 serial #s and after

 

[JetBlast2000]

The previous posts have some good info.

There are 3 connections between the LH/RH sides; Bus Tie (BT), Battery Bus Bar (BBB), and Paralleling Circuit (Only regulates Alt. output between the two sides). The Bus Tie will blow first. At this point its time to reduce your load. You can then get power via the Battery Buss Bar to the other side. The BT Fuse is the most direct link (30-amp). If a problem does arise this is the first line of defense to prevent loosing one side of your system. A mechanic can only legally fix the fuse. The BBB has a “slow blow” 50-amp fuse. You can still power the opposite side buses from a failed alt.

As far as ME-103 it is a cut-off for several things, however I’m not sure that’s the case. I have flow a BE-76 S/N prior to ME-103. I can’t remember the details. And, I’ve never taught 2 versions of the electrical system (other than 24/12 volt.). But, I could be wrong. So ask your mechanic.

Hope this helps.

JB2k

 

[cookmg]

thanks everyone.

Jetblast: That's some great info. One of my students was asked what would be the voltage on the #2 bus should the #2 alternator fail and the bus tie fuse had not been working (i.e. had previously blown though not detected)?

The examiner said 0 as the bus tie fuse is the only way to power bus 2 from alternator 1. It seems clear that it can also do so via the battery bus, but there are limitations.

Am I right in understanding that if an overvoltage occurs in bus two, all the CBs on bus 2 will pop, the bus tie fuse will blow, and the overvoltage relay will take the #2 alternator off line? Now bus 2 can run off alternator 1 through the battery bus, but the load must be low enough not to blow the slow blow fuse? (is that the current limiter?)

Anyone able to give some specific scenarios that may better help me understand this? how about the isolation cbs? When would you use those? why are they necessary in addition to the field switches?

Thanks so much.

mike

 

[JetBlast2000]

First, I’m not an EE. So I’ll try to explain it the best I can. Also, TheBigH was correct about ME-103.



What will cause an over voltage? In most cases it’s a short to ground or a V spike from some electrical component. I would not expect ALL CB’s to pop at once. Though it might be possible. You also must remember that electrons flow from (-) to (+). So that’s one reason why your Bus Iso breaker is 50 amp and inline after the BBB. If the spike comes upstream of that point, that CB will pop first. If the Bus Iso breaker malfunctions, you have the Current Limiter (slow blow fuse) as a last line of defense. I have also had CBs pop without tripping an alt. off line. However, if the Bus itself sees a spike, then you most likely will trip an alt and might blow the BT fuse if you draw enough current. You can still power an opposite bus with an alt. as long as you have either the BT or BBB link intact. The Bus Iso breakers are for protection. As are all breakers. Next time you fly pull both Bus Iso breakers. Then turn off an alt. You can still power the opposite bus.

If you have a complete electrical failure, you will want to isolate the battery from the system. This is because it is most likely the cause. That’s another reason for the Bus Iso Breakers. You still have the BT for load sharing.

The Field Switch serves a different purpose. For that you have to review your alt. operation. Without a field, you have no alt. operation. The strength of that field (controlled by V. Regulator) determines alt. load output.

Do you have a electrical system schematic? If not, go to maintenance and look at the maintenance manual.

Hope this helps.

JB2k

I’ve got a good one for your students… What mechanism causes the prop to feather when the prop lever is moved past the detent and what does it do? (Just for fun).