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Why does a master switch turn off devices?

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dan(at)syz.com
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Posted: Fri Sep 13, 2013 11:18 pm Post subject: Why does a master switch turn off devices?

I've never quite understood something about aircraft electrical systems and I'm hoping someone here can enlighten me.



I've always flown under the impression that a primary purpose of the master switch is to effectively connect/disconnect power from the electrical bus, which in turns provides for the flight instruments and other electrical devices in the cockpit.  However, my question is that since the alternator is typically connected on the bus side of the master contactor (not the battery side), why doesn't the alternator continue to provide electrical power to the bus while the engine is running when the master is off?  Or does it?  I have to admit to not ever having tried to switch off the master in flight, but I've been under the impression that doing so should turn off the electrical equipment - it's after all, a recommendation to turn off the master in a forced approach.



 	  Quote: 	
		  From the electrical diagrams I've seen, it would seem as though that switching off the master while the engine is running should do nothing other than disconnect the battery from the bus, which would not prevent the alternator from continuing to power electrical devices, but instead merely remove the battery's ability to absorb transients, etc.  Wouldn't the alternator continue to supply its own field current once it is already running, so long as the alternator switch remains on?


	


I think I'm missing something pretty basic here.  Any help would be greatly appreciated!



Dan

--

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Phone: 780-961-2213

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peter(at)sportingaero.com
Guest

Posted: Fri Sep 13, 2013 11:46 pm Post subject: Why does a master switch turn off devices?

I don't think you are missing anything, you've described what happens!



The alternator should continue to power the bus if the master is 

switched off (or fails) - its not recommended for the reason you identified.



If the alternator fed into the battery, and not the bus, failure of the 

master contactor would be a single point failure that would take down 

the whole system. That could be catastrophic in IMC with electric gyros.



Switching the master off before an emergency landing is possibly a 

through toward minimizing fire risk?



Peter



On 14/09/2013 08:17, Dan Charrois wrote:

 	  Quote: 	
		   



 I've never quite understood something about aircraft electrical systems and I'm hoping someone here can enlighten me.



 I've always flown under the impression that a primary purpose of the master switch is to effectively connect/disconnect power from the electrical bus, which in turns provides for the flight instruments and other electrical devices in the cockpit.  However, my question is that since the alternator is typically connected on the bus side of the master contactor (not the battery side), why doesn't the alternator continue to provide electrical power to the bus while the engine is running when the master is off?  Or does it?  I have to admit to not ever having tried to switch off the master in flight, but I've been under the impression that doing so should turn off the electrical equipment - it's after all, a recommendation to turn off the master in a forced approach.



 >From the electrical diagrams I've seen, it would seem as though that switching off the master while the engine is running should do nothing other than disconnect the battery from the bus, which would not prevent the alternator from continuing to power electrical devices, but instead merely remove the battery's ability to absorb transients, etc.  Wouldn't the alternator continue to supply its own field current once it is already running, so long as the alternator switch remains on?



 I think I'm missing something pretty basic here.  Any help would be greatly appreciated!



 Dan

 --

 Syzygy Research & Technology

 Box 83, Legal, AB  T0G 1L0 Canada

 Phone: 780-961-2213

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

decaclops(at)gmail.com
Guest

Posted: Sat Sep 14, 2013 8:41 am Post subject: Why does a master switch turn off devices?

The Cessna style split master switch also turns off the field which 

kills the alternator. Bob's Z figures often do this with a DPST switch.



Ed Holyoke



On 9/14/2013 12:17 AM, Dan Charrois wrote:

 	  Quote: 	
		   



 I've never quite understood something about aircraft electrical systems and I'm hoping someone here can enlighten me.



 I've always flown under the impression that a primary purpose of the master switch is to effectively connect/disconnect power from the electrical bus, which in turns provides for the flight instruments and other electrical devices in the cockpit.  However, my question is that since the alternator is typically connected on the bus side of the master contactor (not the battery side), why doesn't the alternator continue to provide electrical power to the bus while the engine is running when the master is off?  Or does it?  I have to admit to not ever having tried to switch off the master in flight, but I've been under the impression that doing so should turn off the electrical equipment - it's after all, a recommendation to turn off the master in a forced approach.



>From the electrical diagrams I've seen, it would seem as though that switching off the master while the engine is running should do nothing other than disconnect the battery from the bus, which would not prevent the alternator from continuing to power electrical devices, but instead merely remove the battery's ability to absorb transients, etc.  Wouldn't the alternator continue to supply its own field current once it is already running, so long as the alternator switch remains on?



 I think I'm missing something pretty basic here.  Any help would be greatly appreciated!



 Dan

 --

 Syzygy Research & Technology

 Box 83, Legal, AB  T0G 1L0 Canada

 Phone: 780-961-2213

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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