Very well stated, Tony. Picking fuse/breaker ratings with respect to
the device's nominal current draw may not allow for inrush current.
More pertinent to fast-acting fuses verses breakers, and often true of
motors and avionics -- on the test bench I've seen some spirited
inrush on things like transponders or avionics with DC-DC converters
for displays.
OTOH, if wire size selection has been conservative, I'd use caution in
placing too high a fuse/breaker rating on something that's really
expensive. Sometimes an avionics mfr. will specify even 16 ga. be
used on the 14V line, even though the box draws, say, 1.3 A. But if
the fault inside were to kick the draw to just below a 10-amp
fuse/breaker, you could be slowly cooking component$ inside that are
not initially defective and frying traces off a PCB board. 5-amp here
would make me feel better in this example, even less if the mfr. says
so (e.g., 3-amp for the King KT-76/78 xponder that draws 1.8 A max,
1.1 standby, but just maybe not a fast-acting).
Regards,
Fred F., A063
Tony Krzyzewski wrote:
>
> The thing to remember is that the fuse protects the wiring, not the device
> on the end.
>
> 1/ Work out the current draw of the device.
> 2/ Select a wire size that can handle the current
> 3/ Select the protection appropriate to the wire size.
>
> For the Europa use the following current ratings...
>
> Wire size Maximum Current
> 24 4
> 22 5
> 20 7
> 18 9
> 16 11
> 14 14
> 12 19
> 10 26
>
> Use a fuse equal to or lower than the maximum current rating. eg a 3A fuse
> would protect any wire size of 24AWG or larger diameter.
>
> The basic concept is that the fuse should blow before the wire.
>
> The table above is for 'short' wire runs. With the possible exception of
> position lights on the end of the wing tips there are no long wire runs in
> the Europa. For the position lights it would probably pay to go to the next
> larger wire size than is required. I am running mine with 18AWG.
>
> Hope this helps
>
> Tony
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