The "trip curves," time vs. current, for both slow-blow fuses and
thermal CB's show they both might perform similarly. It takes
enormous spikes of short duration (fraction of a second) to trip, like
300 amps for a 30A CB and somewhat less for the slo-blo fuse. For
surges of several seconds, as little as 150% times the rating.
What would be the cause of a 1/2-second, 300A spike in the startup
sequence? Maybe more likely is the effect of initial charging of the
battery after the starter extracts its toll. At least it's observable
with a cheap battery charger that's not current regulated (as the case
with the Rotax alternator regulator, vs. many mechanical regulators).
Fusible links are now common, but I recall Toyota has used a big
lug-type fuse or "fusible element," with rating of 2-3 times probable
alternator output. I assume they know what they're doing, and it's
possible that a rating too near the alternator's output can pop a CB
or simple fuse given the right conditions like the trips curves say.
Or a current limiter might be another way to go about it too? That
way a garden-variety fuse should be OK. As long as the chosen limiter
doesn't insert too much "cold" resistance, given the puny output of
the Rotax alt.
Regards,
>Fred F.
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