On 9 Sep 2002, at 22:04, David Brodbeck wrote:

> On Mon, 9 Sep 2002, Jack wrote:
>
> > But an even more fundamental purpose of fuses is to prevent fires,
> > regardless of wire size.  Can there be a fire without drawing enough
> > current to damage the wire?  You bet!  The probability of that happening
> > can be reduced (but not eliminated) by reducing the fuse size to the
> > minimum necessary.
>
> If you're gong to worry about this, I have something more serious you may
> want to think about.  A careful look at the wiring diagram in my Bentley
> seems to indicate that, at least on my '82 Diesel, much of the wiring has
> no fuse protection at all!  This includes the fuel solenoid and 'ignition'
> switch circuit, and the entire instrument cluster.  They're all wired off
> the battery side of the fuse panel.  I wonder if putting in a master
> fusible link would be a good idea?

Could be.  At least to minimize the unprotected circuits.  I'd guess that
as long as the fusible links are reliable & don't cause problems of their
own, it could be a good mod. to make.  But perhaps there's historical data
that says the unprotected circuits rarely if ever cause problems.  I don't
know.  As always, there will be trade-offs between risk, reliability and
cost.  Automotive electrics is not my specialty, so I'll defer further
comment.

> > Someone on this list, a month or two ago, described a situation where he
> > discovered a defective wire terminal connection in his vanagon (fuse
> > block?).  The crimp connection was bad (high resistance) and was heating.
> > Ifirc, it was getting close to where it could have started a fire.  The
> > circuit fuse had not and may not have blown until after a fire was
> > initiated (and a true short developed), but by then the fire may have been
> > self-sustaining.
>
> I don't think a fuse will save you there, until something serious starts
> to happen.  The high-resistance connection, even as it heats up, will
> cause *lower* current than usual to be drawn.  The fuse will think
> everything is peachy-keen.

Possibly.  Depends on how long it takes for the carbonization or
ionization current paths to reach a ground (or unintended loads) and to
begin creating a shunt current (as opposed to a series current).  That
shunt current is what will take out the fuse, hopefully before a self-
sustaining fire has developed.  The closer the fuse is to the normal load,
the quicker it will go.  But you're right, there's no guarantee.

[David, are you a ham?  (I'm guessing N8SRE is your call, but I've been
out of it so long, I'm not sure.)]

//Jack Cook (W6SGA)
'85 GL Original owner

>
>              _ _
>   __ _ _   _| | |  | David M. Brodbeck (N8SRE)              Ypsilanti, MI
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