In a message dated 98-01-02 17:20:57 EST, ingate@shiseis.com writes:
> Now I'm confused! Could Dan masters or Tom O'Malley please help out
> with this one? Bob, IMHO, the Lucas 35-amp and BUSS 35-amp fuses *should*
> be equivalent. A 35-amp *continuous* is one helluva fuse, and will
probably
> not be found in a GBC-style fuse (as is the BUSS). Such fuses are usually
> screwed into terminals to protect ignitions systems, etc, and they are
> enormous!
Shane:
I hate to be the bearer of bad news, but you can indeed get fuses up to at
least 30 amp in a glass case just like the lucas fuses. I am not sure what the
upper limit in that type is, but I know for a fact that 30 amp fast acting
fuses are readily available. Heaven forbid that you should have a 35 amp slo-
blo!
> I am sure there are a lot of people out there (like me) running
> non-Lucas 35-amp labeled fuses and perhaps we should'nt!
You are probably correct about a lot of people using the wrong fuse, and you
are certainly correct in that you shouldn't be.
> What sort of fuse should we use? Why dont fuse manufacturers label
> their product as being for peak or continuous ampage? Does this mean all
> cars use fuses that are not rated for continuous ampage, or is that a
> Triumph/Lucas glitch?
Let me answer your second question first, and then I'll try to answer the
others. Most manufacturers do list their fuses as being either fast acting or
"slo-blo" or some such, and label the package accordingly. Unfortunately, the
terminology is not real clear as to what all this means. If the box does not
say slo-blo, time delay, etc, you should assume they are fast acting. There
may be a way to tell by looking, but I am not aware of any. I throw away a lot
of perfectly good fuses just because I can't tell what type they are, and I
make it a practice to keep all fuses I buy in their original container till
they are used. I'll talk more about the differing types, and how they
function, a little later.
Before I get to the other questions, let me explain the purpose of fuses in an
automobile. There is a lot of misunderstanding about this. The purpose of
fuses is to protect the WIRING, and ONLY to protect the wiring. A fuse is NOT
meant to protect a radio, heater motor, lights, etc. It is not meant for that
purpose because it can't perform that function. It is entirely possible for a
radio, for example, to be damaged due to the heat of an internal short while
it is pulling less current than the fuse rating, and even while pulling less
current than it uses when played at high volume. A fuse can't protect a lamp,
because the only way to get more current through a lamp is for it to have
failed already. A fuse serves its purpose by offering itself up as a sacrifice
when a short occurs to save the wire from burning. It is much better to
replace a fuse than to replace burnt wiring, or, even worse, a burnt car! (as
an interesting aside, the National Electric Code, which governs most
electrical installations in this country, is published by the National Fire
Protection Association, rather than by an electrical engineering organization
as one might think)
How then should fuse sizes be determined? Basically, they should be sized
according to the current carrying capacity of the wires they are feeding. If
the wire is rated for 30 amps, then it is safe to use a thirty amp fuse. A 15
amp fuse should be used for a wire that is rated at 15 amp. Of course, a wire
that is rated at 15 amp will not burst into flames if 15.5 amps are ran
through it. Running 20 amps through it, though, for a long period of time will
probably heat it up enough to melt the insulation, and maybe even catch it on
fire. For a very short period of time, you might even be able to shoot 30 or
more amps through it without any problem, depending on how the wire is routed.
If it is in open air in a cool atmosphere, it will handle it better than it
would if it is in a wiring harness running close to the exhaust manifold. I
can tell you from experience that a 16 gauge wire, rated at about 10 amps,
will carry the total load of a TR6 (starter excluded), at night, without
burning, if you route the wire through the cockpit, away from the engine. In
an emergency once, I ran my car that way for about an hour. I held the wire in
my hand just to check the temperature, and it got quite warm. In a wiring
harness, it probably would have burned (the insulating effect of the other
wires and the harness wrapping would have held the heat in). Most wires in a
"modern" LBC, with the exception of the battery and main alternator cables,
are rated at 8 amp continuous, with a few rated at 5.75, and a very few rated
at 17.5.
When you are choosing the fuse based on the wire size, you have to keep in
mind that a17.5 amp wire may feed into an 8 amp wire, which in turn may feed a
5.75 amp wire. The fuse should be sized for the 5.75 amp wire, and not the
17.5 amp. This means that the fuses in a TR6 probably should be all sized at 6
amps or less, unless you have traced the wires and know for sure the sizes
involved. If you do this, however, I doubt that you could use the car for
blowing fuses. Fortunately, there is a lot of conservatism built into the wire
ratings.
For practicality sake, I would size the fuses to be just over the maximum
current draw for all loads fed by a given fuse. For a stock TR6, this would
be:
"RED" fuse - This fuse feeds all the tail, parking, marker, and dash lights,
and pulls less than 6 amps.
"PURPLE" fuse. - This fuse feeds the glovebox lamp, courtesy lamps, trunk
light, the horns, and the hi-beam flasher. With the exception of the horns and
the flasher, the load is less than 3 amps on this fuse. For a stock headlight,
flashing the hi-beams pulls about 9 amps, and the horns draw around 5 amps. If
you have quartz-halogen lights, the current will be more, but you really
should have them on a separate, un-fused, power source anyway (yes, I said
"un-fused" - that is not a typo!), and they should be relay operated. It is
very unlikely that you will have the doors, trunk, and the glovebox open while
blowing the horn and flashing the lights. Even if you should do this and blow
the fuse, the headlights, both hi and low beams, will still work, as they are
fed from another circuit. The flasher merely bypasses the headlight switch.
"GREEN" fuse - This one is a little harder to determine, as you will seldom
have all loads on at the same time, but the maximum load, with everything on,
is about 20 amps. This fuse supplies power to almost all of the loads that are
switched on with the key, the most notable exception being the ignition
circuit, which is fed directly from the key with no fuse.
My recommendation? 10 amp for the RED fuse, 15 for the PURPLE, and 20 for the
GREEN. Of course, if you are completely rewiring your car from scratch, making
your own harness as I did, then you can size the wires, distribute the loads,
and size the fuses as they should have been done in the first place. When I'm
asked why I used eight fuses, my reply is "because Triumph used 5 too few from
the factory." It should go without saying that you should always carry spares.
To simplify spares, you might want to use a 15 amp fuse for the RED circuit as
well, or a 10 amp for the PURPLE, and just don't honk the horn and flash the
lights!
This gets us then to the dual rating of the Lucas fuses. According to the
Haynes manual for the MGB, the 35 amp Lucas "...fuses are 17 amp current
rated, 35 amp blow rated." What does that mean? Well, I'm going to have to
take a guess, but I assume that over a long time period, 17 amps will blow the
fuse, and 35 amps will blow it pretty instantaneously. For an American fuse, a
20 amp fast blow will blow fairly quickly at 20 amps, while a 20 amp slow-blow
will take a while at 20 amps (not a great while - just long enough to ignore
short current spikes). Using Lucas markings, then, an American 20 amp slow
blow might be rated 20/40, or some such. A good application for a slow-blow
fuse is in a light circuit. The resistance of a light bulb is much less when
it is cold than it is when it is hot. When you first turn on a light, the in-
rush current is much more than the steady-state current after the lamp has
come to operating temperature. If you had 18 amps of lights, fed by an 20 amp
rated wire, protected by a 20 amp fuse, you might very well blow the fuse
everytime you turn the lights on with a fast blow fuse. There are several
other types of fuses available as well, but I am not a fuse expert, so I don't
know much about them, other than the differences have to do with the
time/current curves.
Once again, I've started talking and can't shut up. I hope this is of help to
you. Actually, I liked Tom's answer much better, especially the bit about
electrons traveling in the wrong direction! Take everything I've said with a
grain of salt, because, as I said, I am not a fuse expert.
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/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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