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  #71  
Old 01-11-2019, 08:48 AM
leok leok is offline
 
Join Date: Apr 2013
Location: Clarkston, MI
Posts: 257
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Hey Ken,

Here is my thought process to answer your question on why I show the "switched battery buss" connected directly to the battery and fed through a DPDT "Master Switch" instead of through the master relay.

The key is that I have dual electronic ignition rather than magnetos. For that reason I needed a direct from the battery feed for one of the ignitions. No relays that could fail in flight. I also need the ignition to be off when the master is off, so no independent separate power switch to the ignition. Since the master will typically only be switched on/off on the ground, I will always know if there is a switch problem before I fly. It is highly unlikely for a switch to fail while in the on/off position without being cycled. When the master is off, I always know the ignition is dead/off.

The other loads tied to the "switched battery buss" are all loads I might want live with only the master on (i.e. prior to engine start) and also do not want going through the VPX (another possible failure point). I suppose you could make the argument that I could add another battery buss fed by the master relay moving all other loads there so the power did not flow through the switch. That would relieve some load from the switch contacts.

Anyway, as always I am willing to be schooled by others.
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  #72  
Old 01-11-2019, 09:55 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,863
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Hi Leok,

Forgive me if you've already dealt with this, but one thing caught my eye in that post: 'I also need the ignition to be off when the master is off'. Does the system include a 'smoke in the cockpit' procedure? Traditional action for smoke is typically 'master off' first (engine continues to run), then if smoke clears and comfort zone allows, try to find the culprit. Though troubleshooting is best left for ground ops.

If a typical pilot suspects fire in the cockpit and instinctively flips the master off, does your engine stop?

Charlie
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  #73  
Old 01-11-2019, 11:21 AM
leok leok is offline
 
Join Date: Apr 2013
Location: Clarkston, MI
Posts: 257
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Charlie,

I have done a lot of technical writing, product and production system design in my career and have found there will always be someone with a different perspective or greater knowledge in some area that will point out something you didn't think of. That is one of the reasons I put this thread out there ... well

You are correct. In the design as it stands (and is already mostly in place, but can be adjusted) there is no way to turn off the master and keep the big noisy fan going. And I agree master off, is one way of quickly killing all electrical quickly. I don't fly big high end stuff so I'm not sure if that would apply to more complex aircraft.
So now I need to ask myself is what I have acceptable? Much better to work through possible responses in my shop than in the air. This is one I dismissed (and perhaps should not have) since every circuit is protected by a fuse or electronic CB. What would be the response with smoke of unknown origin in the cockpit? Rather than trying to reason it out here, I appreciate the comment and will look hard at what I have to see if I should make some changes.
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  #74  
Old 01-11-2019, 11:38 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
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I just went through this and ended up with three busses and three switches. A master bus, an avionics bus and an ignition bus. In case of a fire or problem, the master goes off, leaving me avionics and ignition. If not solved, the avionics bus goes off, leaving only the ignition with power. In my case, each bus can be separately switched from Left, Right or both batteries: I am using relays/contactors.

I would not want to be without a way to kill everything except the ignition, being ignition dependant. &$*^ happens and you need a way to isolate things if you want to keep the fan spinning with dual EI.

Larry
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Last edited by lr172 : 01-11-2019 at 11:42 AM.
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  #75  
Old 01-11-2019, 12:39 PM
BMC_Dave BMC_Dave is offline
 
Join Date: Mar 2017
Location: Seattle, WA
Posts: 285
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I'm also going through the same planning process. I think I've settled on a dual bus, dual alternator, single battery system. Primary and endurance bus, with the e-bus only hosting all bits to keep the fan spinning (and pitot heat) and an alternate feed path to the battery. This does mean that an emergency requires you to close 'E-BUS ALT FEED' before turning off the master.

I'm considering the effects of leaving the relay-controlled alt feed closed in normal operations, allowing you to shut off the master with no effect to the engine. So far I can't think of any downsides to this but I'd be happy to hear what others think. ETA: http://forums.matronics.com/viewtopic.php?p=195860 Looks like this was already discussed and determined to be fine

Last edited by BMC_Dave : 01-11-2019 at 01:09 PM.
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