Randall wrote :
> > And are you going to buy proper Lucas replacement fuses, or try to
> substitute a
> > standard (eg Buss) fuse ? The rating methods are very different,
> so it makes a
> > big difference.
Drew asked :
> How are they different? I'm pretty sure my TR4 has standard
> auto-parts-store fuses in it. Not
> that it's actually running these days, but still...
Perhaps it IS time to revisit this topic, since it's an important one.
Like so many other things about our Triumphs, the British did things just a
little bit different than the Americans, including what the ratings marked on
fuses mean. The British Standard that Triumph used rated fuses by the current
that was guaranteed to make them blow "instantly"; while the American (and now
international) standard rates fuses by the current that they will carry forever.
Fuses work on heat, so a small overload may take a long time to build up enough
heat to melt the fusible element and "blow" the fuse. How warm the fuse was to
begin with plays a large role too. So the "carry" rating winds up being quite a
bit lower than the "blow" rating ... the difference is roughly 2 to 1 !
So, what this means to you is that, if you elect to use common American fuses
(Buss is one maker, there are others) in your Triumph, you need to use a much
lower rated fuse than what your Triumph documentation says to use. There have
been cases reported where a restored Triumph has caught fire from a short,
because someone used American fuses with the Triumph recommended rating marked
on them ... the short did not blow the fuse but instead set the wiring harness
on fire !
Here is a conversion chart, lifted from a long-ago Buss fuse catalog :
English Type Buss Replacement
50 amp AGC 30
35 amp AGC 25
30 amp AGC 20
25 amp AGC 15
20 amp AGC 10
10 amp AGC 7 1/2
5 amp AGC 3
Personally, I feel even this chart results in fuses rather larger than needed,
so for example I use an AGC 20 to replace the original "35 amp" fuse in my TR3A.
My conversion chart would go more like :
English Buss
50 25
35 20
30 15
25 15
20 10
10 5
5 3
Tim Buja and Glenn Merrell put on a good visual demonstration of the difference
in fuse ratings at VTR this year, where they first put in a "25 amp"
Lucas-equivalent fuse and applied enough load to blow it. Then they installed a
25 amp American fuse and applied the same load. The fuse did not blow, but
after just a minute or so the wire started visibly leaking smoke ...
BTW, "proper" fuses with the correct British markings are available from the
usual suspects.
Randall
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