Glad you raised that point, Brian. In my Sunday school classes I ask the
question: "what does the fuse protect?" It's a revelation to all when I
give the right answer.
On our 240V distribution system, a melted cable can lead to a severe shock
hazard and I'm guessing that in the 110V centre-tapped system since the
shock hazard is so much less, the next major hazard is fire, so, I've often
thought that we are protecting against 2 hazards, each with a different
emphasis in each case.
In a 14V dc distribution system there is no real shock hazard (SELV), so the
remaining hazard has to be fire, so, a slow fuse is not a problem, since
heating effect in the wiring is so much slower.
Anyone care to comment/add?
David
On Jun 16, 2008, at 9:59 PM, <[EMAIL PROTECTED]>
<[EMAIL PROTECTED]
> wrote:
A regular fuse should blow at 2.6 x the rating within 30s, in other
words very slowly. It's only a short circuit that will normally blow a
new fuse. Have not seen any data on old fuses which might get metal
fatigue. There are faster fuses, but the fastest are rf transistors -
on three legs anyway...
Most people don't know that the purpose of a fuse or circuit breaker is
to protect the power distribution wiring, not the electronic component
itself. Wire will carry a surge without too much temperature rise. The
idea is that the fuse will blow or the breaker open before there is any
chance of damage to the wire.
This means that a fuse or breaker cannot protect your active devices. If
you want that level of protection you need something like a power supply
with fold-back current limiting.
Brian Lloyd
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