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Electrical Wiring
Doing research of electrical wiring for UL 350is.
I do want to stick to the manufacturer's recommendations as much as possible, however, also want to get inputs from experienced folks on some important aspects.
One thing that jumped at me from the UL sample drawing (below) is that the 12V bus in the center of the drawing is not isolated via a switch. This means with the master switch in the off position, I could still push the starter button and the starter would start! There might be a connection from the ECU to the starter that prevents this while the ECU is not switched on, but that still seems like an indirect control rather than a proper disconnection of the starter from the battery. Further, the ECU itself has a switch (presumable a simple panel toggle switch) but I don't quite understand why I would need to have the ability to turn off the ECU independently (including emergency).
My proposed modification to this involves 3 parts, see below.
1. The master switch in the UL diagram actually becomes my avionics master switch. This would allow me to quickly turn off all non-essential stuff in case of an emergency (e.g. suspected burning / fire under the panel)
2. Adding a master switch that isolates the main 12V busbar that feeds the fuel pump, ECU and starter button. That way, on the ground, when my new master switch is turned off, there is no way the starter can be turned on and no way the fuel pumps can be turned on (even accidentally, in case someone flips the individual toggle switches or presses the starter button). I can walk away peacefully from the plane after confirming that my master and avionics master switches are turned off. Granted this can also be assured by simply having the starter button as part of a key switch but somehow I would still feel uneasy that merely flipping the fuel pump toggle can turn it on while the master is off and the key is out.
3. Removing the switch that isolates the ECU from the 12V busbar. Basically, if the master switch is on (my proposed switch) then the ECU gets power. No need for further isolation. Fuel pumps will still have their individual switches (or a combined ON-ON toggle switch but that's a different issue).
Thoughts and suggestions?
Replies to This Discussion
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Permalink Reply by Abhishek Bawiskar on
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An interesting twist to this is that in the same manual, UL Power shows the starter button downstream of a master switch! In the snippet below, the 12V busbar is isolated by a master switch and the start solenoid is fed from this bus (through the starter button) that won't be live until the master switch is on.
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Permalink Reply by Ken Ryan on
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They seem to be leaving out the battery contactor (solenoid) that is usually included. As shown in the above drawing, all current (except what the starter motor draws) would have to flow through the master switch. Normally, the master switch controls another solenoid (often called the battery contactor) that is designed for higher currents. Also, the battery contactor is usually located as close to the battery as possible so that in the event of a forced landing, turning the master switch off opens the battery contactor and cuts power to the rest of the airframe. In the case of an electrically dependent engine, you need to be careful that turning off the master switch does not result in engine stoppage.
There is a resource called The Aero Electric Connection by Bob Nicholls that is a good resource. He has multiple schematics for different types of airplanes that have been thoroughly vetted (including for those with electrically dependent engines).
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That makes sense - that they left out the battery contactor in their overall drawing, which is shown as the master switch in their starter motor drawing.
Thanks for the reference; I've seen this name come up in a couple of forums. I'll be sure to check it out.
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