In a message dated 98-10-02 16:27:44 EDT, ARhodes@compuserve.com writes:
> Maybe a 20 or 25 amp slow-blow would give a good match?
Tony,
Doing this would only make matters worse. The actuiall ratings for the Lucas
fuses is somewhere between 17 and 35 amps, but much closer to 17 amps. I'm not
sure what the actual rating is. Perhaps Ed Nunez could shed some light on
this? Are you there, Ed?
> It would appear that the fuses are rated too high for the installed
> wiring as delivered for the factory (17 amp continuous). I wonder
> why they did this when they would be creating a fire hazard. As you
> point out, the total draw on the trunk line would be greater than the
> 5 amps the terminal wires.
I'm curious myself as to why they did this. Economy alone wouldn't account for
it, I wouldn't think. One thing I've learned over the years - no matter how
simple something seems, when you really dig into it, it often becomes quite
complicated. Fuses are no exception. I can think of about three things that
factor into fuse design, other than the obvious:
1) By far the most common cause of blown fuses is errors on the part of the
mechanic. We've all done this - remove an electrical item without
disconnecting the battery first. As we work on the car, we push the wires out
of the way, causing them to short out to the chassis.
2) the second most common cause it moving a wrench or a screwdriver into
contact with the wiring.
3) Third is allowing the wiring to deteriate such that bare wire comes into
contact with ground when the car hits a bump.
Under those conditions, almost any reasonably sized fuse will blow instantly,
since a short circuit has zero resistance, theoretically producing infinite
current.
Contrary to what one might expect, a dead short is not the worst case
condition. The worst case is a "near" short, a condition where the wire is not
making a real good connection to ground, but has enough resitance to allow
more current to flow than the wire can handle, but not enough to blow the
fuse. Perhaps the manufacturers felt that the worst case scnerio wouldn't
happen? Of course, we know that it does, as we all know of cars that have
burned to the ground from an electrical fault.
> So, since you suggest 10,15 and 20 amp fuses, maybe these should
> be slow-blow types? Or maybe under rate the fuses some and then
> use slow-blows. I am concerned that even with the down-rated fuses
> that you recommend, it might still allow the smallest wires to get smoked.
It might be a good idea to use smaller fuses of the slow blow type. I just
wish I knew more about fuse ratings. The best thing to do, although for sure
not the easiest, is to rewire, using the correct size wiring, co-ordinated
with the fuses.
> I guess I will try to run short lengths of wires at high amperages to see
> whether the wire smokes or the fuse blows. I bet that the smoke limit
> for wiring is well known. I.E. 5 amp wire carrys 5 amps with no
> warming; when run at 10 amps continuous = hot but no melt;
> 15 amps = insulation melt under worst case scenarios.
Please do, and let us know the results. I would be most interested. A couple
of things to consider as you do your test. One is the environment that the
wires are in. A wire running close to the exhaust manifold, for example, would
already be hot before the short, so it would take less current to raise the
temperature above the melt point than a wire located under the dash.
Another is how the wire is situated in the wiring harness. If the wire is
buried deep in the harness, surrounded by other wires and the harness
wrapping, it will take less heat input to raise the temperature than an
individual wire by itself, as the other wires and the wrapping will insulate
the wire, keeping the heat in.
> Maybe the designers were using a different load limit when picking
> the fuses. Maybe they decided to let the smallest wires get close
> to melting before the fuse blows....
I wish I knew! Hopefully, some of the things mentioned above were considered,
and perhaps a few more that I am not aware of as well.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/index.html
'74 MGBGT---3000mile/year driver, original condition - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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