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Thread: Circuit protection for shunt wires

  1. #1

    Circuit protection for shunt wires

    I see no mention of this in the installation manual, but... The two wires from the shunt, being essentially connected to the power bus / battery, should each have some form of circuit protection installed at the shunt end, to either break the circuit or limit the current should one of the wires becomes shorted to ground.

    The common forms of circuit protection, fuses or circuit breakers, would do the trick. But they are big and bulky, and are more than should be needed in this application, because the current through those wires in normal operation should be miniscule. I'm thinking that a simple/cheap/elegant solution in this case would be to put a simple resistor in series with each wire at the shunt end, to serve as a current limiter, and should have no adverse effect on the normal functioning of the current measurement circuit.

    Since under normal conditions there is very little current flowing in those wires (Likely nanoAmps, and certainly no more than microAmps, I presume. How much, actually?), a series resistor of reasonable value (say, 1K Ohm) should have very little effect on the voltage seen at the EFIS (No more than millivolts). And if the resistors on the two wires are well-matched, and the leakage current of the two EFIS input pins is reasonably well-matched, then the voltage drop associated with the resistors should also have no effect on the current measurement. This is because it only introduces a common-mode voltage shift, i.e. same voltage shift on both wires, and the current measurement is a differential measurement between the voltages of the two wires.

    Rob, can you please comment on the above? Can you tell me what is the approximate worst case leakage current of those inputs? Can I go ahead and use a matched pair of resistors for circuit protection as I outlined above, or do you see any problem with this?

    Thanks,
    -Roee
    Last edited by rkalinsky; 05-11-2013 at 10:41 PM.

  2. #2
    The theory is that the wire acts as the fuse. Long as you are worrying about these things what about the wire from the battery to the master relay > starter relay > starter motor?
    Rob Hickman
    Advanced Flight Systems Inc.

  3. #3
    Rob,

    Thanks for the swift reply! Interesting point about the battery cable. But it's not quite the same thing.

    [edit: Re-reading my original post and your response, I was wondering if there was confusion about which shunt wires I was referring to. So to clarify: I was referring to the sense wires from the shunt to the EFIS, not the big power wires connecting to the shunt.]

    About the wire from the battery to the master solenoid, and from the master solenoid to the starter solenoid, you're absolutely right, those are unprotected. There is no practical way to protect them. But in terms of risk mitigation, they are 1) very short, 2) physically well protected and separated from other wires and structures, and 3) are forward of the firewall in their entirety, so the fire hazard of a short circuit event would be contained and pose no direct risk to the occupants in the cockpit. (And about the starter wire, on my airplane it actually does have short circuit protection by virtue of the electronics that control my starter relay. But that's a different story.)

    Back to the shunt sense wires: You're right that a hard short to ground will easily smoke that 22 AWG wire fairly quickly, acting as a fuse in some sense. But unlike a fuse, the conflagration of the wire would not be safely contained by a little glass tube or plastic blade. This wire is approximately 9 ft long, and makes its way from the lower right corner of the firewall across the engine mount to the upper left corner and back into the cockpit in a bundle with many other wires. There is no guarantee that the wire would only burn at a safe location, forward of the firewall at the shunt end. It could just as likely burn along its entire length, including on the cockpit side of the firewall, causing fire/smoke in the cockpit, igniting other adjacent wires in the bundle, etc. That's a scenario I'd prefer to avoid. And it's easily prevented with circuit protection at the shunt end.

    You did give me another idea though for simple and effective circuit protection, and that is to use fusible links. I.e., splice in a short section of much narrower gauge wire near the shunt end that would be the sacrificial section guaranteed to be the first to blow, and thereby protect the rest of the wire run. I think that's a reasonable solution.

    But I'd still be interested in your answers/comments on the resistor approach, if for no other reason than academic curiosity. Seems more elegant (nothing ever "smokes"), and I think it would work well in this application. Do you see any reason why it wouldn't?

    Thanks,
    -Roee
    Last edited by rkalinsky; 05-13-2013 at 01:48 AM. Reason: Clarification

  4. #4
    The resistors should work on the shunt wires, I don't think we have every had anyone try it. How are you protecting the wire from the master relay to you circuit breakers hot side? In my RV-10 it is about 12 feet long and unprotected.
    Rob Hickman
    Advanced Flight Systems Inc.

  5. #5
    Thanks, Rob. I may go with the resistors then. And if I find any problems with that, then I'll revert to fusible links.

    The big power feed wire from the master relay to the breaker panel, if left unprotected, is just about the scariest wire of them all. If ever there was a short to ground at the panel end of that wire... yikes! Hundreds of amps for a few seconds, potentially resulting in a major electrical fire in the cockpit. In my RV-7A that wire is protected with a 60A ANL fuse, located on the forward side of the firewall right next to the master relay.

  6. #6
    The wire acts as a fuse? I thought the purpose of the fuse was to protect the wire. Putting an ANL fuse on the wire from the master contactor is a good idea. I was just about to rewire my alternator as suggested in aeroelectric to go to the battery contactor instead of the main bus using an ANL fuse, so maybe I'll get two and add one to the main cable as well. I'm trying to switch my plane from a main/avionics bus to a main/essential bus with alternate feed switch. I do find it interesting that even in the aeroelectic diagram the essential bus wire from the battery has an ANL fuse, but the main doesn't??

  7. #7
    I see, the main wire doesn't have a fuse because it's after the master contactor, but the essential bus is before it so it needs a fuse. If you put a fuse on the master bus wire, you would want to use something high enough to prevent nuisance tripping, but not so high that the wire would melt before it tripped.

  8. #8
    This wire is approximately 9 ft long, and makes its way from the lower right corner of the firewall across the engine mount to the upper left corner and back into the cockpit in a bundle with many other wires. There is no guarantee that the wire would only burn at a safe location, forward of the firewall at the shunt end. It could just as likely burn along its entire length, including on the cockpit side of the firewall, causing fire/smoke in the cockpit, igniting other adjacent wires in the bundle, etc. That's a scenario I'd prefer to avoid. And it's easily prevented with circuit protection at the shunt end.
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