Dave Platt wrote:
In article .com,
wrote:
OK, here's a scenario for you.
Let's say (without loss of generality) that your equipment has a
1-ampere 250-volt fuse in it. This fuse will allow 1 amp of AC
current to flow, pretty much indefinitely, without blowing. If it's
in an equipment which has no motor and not a big power supply, it'll
probably be a "fast-blow" fuse which will pop quickly (within a few
hundred milliseconds) if the current drawn through it exceeds the
limit by a factor of 3 or more.
Let's say you replace this with a length of silver solder. This
solder is far thicker than a fuse filament, and will probably pass
15-20 amperes without melting and opening.
Now, let's say that a fault develops in the AC primary wiring of the
amp. Maybe a wire pops loose, maybe there's a hidden defect in the
power transformer (bad insulation), maybe somebody upstairs lets a tub
overflow and water starts dripping through the floor and runs down
into the amp, etc. Somehow, some portion of the AC-connected
circuitry develops an unwanted path to something grounded.
Let's say that it's not a complete "dead short" circuit. There's
still appreciable resistance between the AC "hot" and
neutral/ground... maybe a ten ohms or so.
This path is enough to allow 10 amperes of current to flow.
If your original fuse is still in the amp, the fuse will blow almost
instantly. Even if it's a "slow blow" fuse, an overload of this
magnitude will cause it to "pop" within a couple of seconds. Because
it's a 250-volt fuse, it's guaranteed not to arc-over inside the fuse
at this voltage. The AC circuit opens. Nothing bad happens.
If your silver-solder "fuse" is in the circuit, any of a number of Bad
Things can happen:
- If the silver-solder doesn't melt (and I suspect it won't), the
circuit fault will be drawing a full 10 amperes of current. That's
over 1200 watts of power, which will turn into heat *somewhere* -
probably inside the amp itself, where the ohms of fault-resistance
are. Your amp has now turned into a self-contained room-size space
heater, with all the heat in the wrong place.
It would not be surprising for your amp to catch fire after a few
minutes. Even if it doesn't, it'll probably heat up enough to be
ruined.
- If the silver solder _does_ melt, there will probably be a big and
messy "BZAP" and arc when it melts through, which will throw molten
solder around inside the amp chassis. At best, this will probably
leave solder-shorts in numerous places in the circuitry, resulting
in an amp which is not repairable at a reasonable cost. At worst,
the molten/vaporized metal might be sufficient to sustain the arc,
and you've got a 1200-watt "arc welder" burning inside your amp for
some amount of time.
Again, the risk of fire is nontrivial.
- If the short occurs to the chassis, and the fuse doesn't blow, then
quite a lot of current could be flowing on the chassis and the
ground wire in the power cord and in the house's ground wiring (and
could also flow through the grounded outer braid of your A/V coax
cables, into other A/V equipment, etc.). The high current can
create enough of a voltage differential between the chassis and
"true" ground to create a significant shock hazard, for anyone who
touches the chassis.
What it boils down to, really, is the idea that fuses are there for a
damned good reason. Fuses are *designed* to be able to interrupt high
voltage safely and reliably. There are national and professional
standards for them, and their use is legally required in AC-powered
equipment in most countries.
Replacing a fuse with a piece of solder is the equivalent of the old
"put a penny in the fusebox" trick that people joke about... but it's
a joke which has resulted in *real* fires and death in the past.
*REGARDLESS* of whether replacing a fuse in your amp with a piece of
silver solder (or jumper wire) makes it sound better to you (for
either objective or subjective reasons), doing so is risky. I really
doubt that *any* licensed electrical engineer or contractor would tell
you that it's a good idea, or would sanction you doing it.
But my local engineer or contractor HELPED me solder the jumper wire
across the fuse holder contacts? (Just kidding!). Thanks for taking the
time to explain in detail the many ways I might fry using solder for a
fuse bridge. Although honestly, I think your doomsday scenarios are a
little far fetched and unlikely to occur (ie.: were talking about
equipment thats over 25 years old; if there was a defect in production,
I think 25 years is enough to flush it out!), there's enough here for
me to have reconsidered the idea of operating the equipment without the
fuse. And thanks for showing the intelligence of saying the audibility
of the fuse may exist for objective reasons. That's more than I can say
for most respondents here, all of which have never conducted the same
experiments that I have.
--
Dave Platt AE6EO
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