Real Master switch

[EricB]

The factory design on CT has a significant amount of current coming from the battery (unfused as I see it) to the ammeter shunt then the breaker on the lower panel.  The BAT  breaker then allows power to flow from there.  It seems to me we should be keeping full battery power on the "other" side of the firewall.  Using low current draw contactors (relays) which can be easily fused within the engine compartment would then switch on the higher power things avoiding the possible hazards of heavy electrical current in the cabin.  Has anyone done this?  Am I the only one who thinks its an idea worth considering?

 

 

[Roger Lee]

They are like this with a approx. 1800 around the world without any issues from it. Probably better to leave well enough alone. 

[Madhatter]

No aircraft does this, there is no point to it. Besides, relays fail.

[Garrett Gee]

1 hour ago, EricB said:

The factory design on CT has a significant amount of current coming from the battery (unfused as I see it) to the ammeter shunt then the breaker on the lower panel.  The BAT  breaker then allows power to flow from there.  It seems to me we should be keeping full battery power on the "other" side of the firewall.  Using low current draw contactors (relays) which can be easily fused within the engine compartment would then switch on the higher power things avoiding the possible hazards of heavy electrical current in the cabin.  Has anyone done this?  Am I the only one who thinks its an idea worth considering?

 

 

I am actually working on this on my airplane right now.  I don't like the long unfused line either.  It works as is, but that doesn't mean it is the best way.  I am installing a second backup battery, creating a E-bus, as well as a second alternator.  All will be fused and will have contactors forward of the firewall.  I also don't like the fact that if the starter contactor fails closed, there is NO way to kill the starter.  I am also moving the ammeter(s) to forward of the firewall as well.

Most certified aircraft have a battery contactor forward of the firewall, but I haven't seen many that have the main battery bus cable fused forward of the firewall.  The risk is just not high enough with those thick gauged short length wires.  I am fusing mine, but using fuses bigger than any draw I would ever need, and obvisouly won't fuse the battery to starter wire.

I also found during an inspection a couple years ago that the firesleeve that protects the wires going through the firewall had migrated, and the wire bundle had signs of chafing.  It isn't that hard to do a proper firewall passthrough with protection, but FD didn't do that on my airplane, so I am also changing that as well.  The fact that the main battery wire could have chafed, unfused, with no way to cut it off if it had shorted, really made me think.

[Eddie Cesnalis]

45 minutes ago, Garrett Gee said:

I am actually working on this on my airplane right now.  I don't like the long unfused line either.  It works as is, but that doesn't mean it is the best way.  I am installing a second backup battery, creating a E-bus, as well as a second alternator.  All will be fused and will have contactors forward of the firewall.  I also don't like the fact that if the starter contactor fails closed, there is NO way to kill the starter.  I am also moving the ammeter(s) to forward of the firewall as well.

Most certified aircraft have a battery contactor forward of the firewall, but I haven't seen many that have the main battery bus cable fused forward of the firewall.  The risk is just not high enough with those thick gauged short length wires.  I am fusing mine, but using fuses bigger than any draw I would ever need, and obvisouly won't fuse the battery to starter wire.

I also found during an inspection a couple years ago that the firesleeve that protects the wires going through the firewall had migrated, and the wire bundle had signs of chafing.  It isn't that hard to do a proper firewall passthrough with protection, but FD didn't do that on my airplane, so I am also changing that as well.  The fact that the main battery wire could have chafed, unfused, with no way to cut it off if it had shorted, really made me think.

Mine must have chafed and the result was a parasitic draw that would kill my battery in the hanger in about 2 days.  This was hard for the mechanics to find but easy for me to see with a meter.

[FredG]

I built a RANS S-20 with a Rotax 912 ULS and I designed the electrical system to be similar to the CTsw I already owned.  I ran a 12 AWG wire from the battery to the battery breaker on the panel.  That wire is protected by a 30 amp fuse mounted to the engine side of the firewall, very close to its origin at the battery so that a short sufficient to cause a fire (other than in the short run on the engine side of the firewall) would blow the fuse.  

I think it is a good practice to avoid having a large unprotected wire connected to the battery pass through the firewall to a device on the panel.  It can easily be protected with an appropriately sized fuse.  

I am not sure how anyone on this forum can know that there are "1800 around the world without any issues from it" is a statement of fact.  Also, I don't understand why there is no point to protecting the conductor from the battery to the panel.  If circuit protection is a good idea, why ignore this circuit?

 

[Tom Baker]

2 hours ago, Garrett Gee said:

Most certified aircraft have a battery contactor forward of the firewall, but I haven't seen many that have the main battery bus cable fused forward of the firewall.  The risk is just not high enough with those thick gauged short length wires.  I am fusing mine, but using fuses bigger than any draw I would ever need, and obvisouly won't fuse the battery to starter wire.

Of all the airplanes I work with a battery contactor, it is normally right next to the battery. The one exception I can think of is the Piper Pawnee. The only airplanes I have seen with the contactor forward of the firewall also have the battery forward of the firewall. Of these there is a high current wire that comes through the firewall to the buss to feed the electrical system. In my opinion adding a contactor is extra complexity without significant added safety. If I were to make any changes, it would be a simple fuse for the wire passing through the firewall, and possible a manual battery disconnect that can be pulled from the cabin. The Pipestrel that I work on has such a disconnect.

[Garrett Gee]

52 minutes ago, Tom Baker said:

Of all the airplanes I work with a battery contactor, it is normally right next to the battery. The one exception I can think of is the Piper Pawnee. The only airplanes I have seen with the contactor forward of the firewall also have the battery forward of the firewall. Of these there is a high current wire that comes through the firewall to the buss to feed the electrical system. In my opinion adding a contactor is extra complexity without significant added safety. If I were to make any changes, it would be a simple fuse for the wire passing through the firewall, and possible a manual battery disconnect that can be pulled from the cabin. The Pipestrel that I work on has such a disconnect.

I should have been a little more specific.  Airplanes that have a battery forward of the firewall typically also have the contactor forward of the firewall.  Should not have generalized most.  All of the Cessna, Diamond, and Cirrus aircraft we have in our training fleet have contactors and the battery foward of the firewall with the contactors very near to the battery.  That is how I am setting up my system.  Contactor for each battery, contactor for the starter, and each battery and alternator with a fuse.  Main power wires will then come into the cabin into a custom bus I designed fully protected and able to be disconnected. Now, yes, contactors can fail, just as relays, and anything else mechanical, but in 12 years and 3000 hours of flying, I have only seen 2 fail in our flight school (harsh environment).  In both cases, they failed closed (one starter, one battery).  Being able to kill the starter contactor with the master switch prevented a lot of other issues.