N546RV
Well Known Member
So I've been thinking about the Nuckolls E-bus setup, specifically the handling of having an alternate feed off an always-hot battery bus in case of a failure affecting the main bus. Bob's setup takes the same basic form everywhere I've seen it: the main bus feeds the E-bus through a diode, and the alternate feed is provided through a switch and/or relay (depending on expected E-bus loads).
The reasoning behind the diode between the main and E-bus is to prevent the E-bus from feeding the main bus and possibly popping a fuse/breaker. Bob's assertion seems to be based on an alternative scenario where two switches would be provided, and it would be incumbent on the pilot to ensure the main->E-bus feed switch was opened before closing the E-bus alt-feed switch, to prevent the aforementioned backfeeding.
However, it seems this issue can be fairly easily solved with careful switch choices, such that it's not possible to have both feed paths closed simultaneously. For example, if a five-pin relay is used for feed switching, the main bus could feed through the normally-closed contact, and the batt bus through the normally-open one. The choice of how to feed the bus is still controlled by a single switch throw, the only difference being that that switch position definitively disconnects one feed while connecting another one.
Example diagram:
This seems elegantly simple enough that I'm immediately suspicious that I'm missing some downsides. Some thoughts that come to mind in this vein:
Any other detractions I'm missing? Any thoughts on the concerns above?
The reasoning behind the diode between the main and E-bus is to prevent the E-bus from feeding the main bus and possibly popping a fuse/breaker. Bob's assertion seems to be based on an alternative scenario where two switches would be provided, and it would be incumbent on the pilot to ensure the main->E-bus feed switch was opened before closing the E-bus alt-feed switch, to prevent the aforementioned backfeeding.
However, it seems this issue can be fairly easily solved with careful switch choices, such that it's not possible to have both feed paths closed simultaneously. For example, if a five-pin relay is used for feed switching, the main bus could feed through the normally-closed contact, and the batt bus through the normally-open one. The choice of how to feed the bus is still controlled by a single switch throw, the only difference being that that switch position definitively disconnects one feed while connecting another one.
Example diagram:
This seems elegantly simple enough that I'm immediately suspicious that I'm missing some downsides. Some thoughts that come to mind in this vein:
- There's the possibility of the relay somehow failing such that both contacts close, thus possibly blowing the batt bus fuse and killing the E-bus in an emergency. Not sure what the likelihood of this is.
- If the relay somehow fails with both contacts open, the E-bus is now dead. This question seems to come down to the relative chance of failure between the diode and the relay.
- Switching feed paths means momentarily cutting power to the E-bus. This seems inconsequential since, when switching to the E-bus, presumably power has been lost already.
Any other detractions I'm missing? Any thoughts on the concerns above?