>internet:europa@avnet.co.uk
>>On page Z-6, you show a 70 amp fuse from the starter to terminal B on the
>>Alt. Should that supplied by NSI for us Subaru guys? Looking in
>>Aircraft Spruce's catalog, they don't offer much of a selection.
>>Klixon for around $80. I have discussed this with other builders and they
>>aren't aware of this requirement. Is this item really necessary?
>>If this is a question that should be put on the KitfFox list, please do.
>>
>>Thanks
>>Jim Rose
"Big" fuses can be a problem in homebuilts . . . by "big" I'm talking
30 amps an up. We've been working the fat-fuse issue with several builders
in both permanent magnet (rotax) and externally excited (NSI) alternators.
I'll offer the following to answer your question: For fuse requirements
50 amps and up, the CLASSIC approach in airplanes calls for ANL series
devices mounted on insulated posts supported by molded, insulating bases.
The objection ANL fuses is size of the base which is also expensive.
The fuses aren't real expensive but combined with the mounting base,
they're really not very attractive in terms of dollars and space
required. Bussman makes a JJS and JJN series fuse with
some interesting features. Beginning with 70 amp devices and larger,
the fuse cartridge is terminated on each end with healthy copper tabs
with holes in them! Hmmmm . . . how about bolting wires to the
fuse and covering it with heatshrink? Turns out that this works
pretty well and provides a compact (1.5" diam x 3" long) assembly that
simply ties into a wire bundle. Problem is that the smallest fuse
in the series is 70 amp. Okay, let's use 4AWG wire (rated for 72 amps)
to wire up 35, 40, 50, 55 -and- 60 amp alternators. It's agreed that
4AWG wire is plenty big for a 35 amp machine but it's not too big; the
4AWG/70A combo finds it's way onto most of my system diagrams.
You can get the JJN/JJS series fuses from local electrical supply houses
that sell to electricians. There are a number of brands of equivalent
fuses . . . the guy behind the counter should be able to cross it to
what ever brand he stocks. Also, B&C offers a kit consisting of the
fuse, bolts, nuts, 4AWG terminals and 1" heatshrink for finishing off
the installation.
We've been trying a smaller, waterproof, in-line fuse holder (Bussman
model HFB) for protecting the main feedwire from a Rotax 912 alternator
system. The alternator would like to be wired with 10AWG which is
mechanically too big for this fuseholder. I've supplied this holder
to about a dozen Rotax 912 builders . . . most went together with
no problems but a couple have had problems with the relatively stiff
10AWG wire and the semi-flexible fuseholder. I've been looking into
the "fusible link" technology used on cars. Turns out the so called
fusible link is just a piece of copper wire . . . about 4AWG steps
smaller than the wire it protects. In our application, we'll take about
8" of 14AWG and slice it onto the end of the 10AWG alternator feedwire.
Ordinary terminals and butt splices are used to hook everything together.
Now, in the very rare instance this wire does get fused, we need to be
sure it does it in an orderly fashion. I propose to insure an acceptable
"failure" by sleeving the fuse link with a fiberglass/silicon sleeving.
I've got some testing to do with this combination before I ship to
builders. The really nice things about this combination are resistance
to effects of age and the very low parts count. The technique uses
ordinary, low cost materials with excellent service capabilities. Watch
this space for future developments. In the mean time, hit up your local
wire-store for the 70 amp devices.
Bob . . .
AeroElectric Connection
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