R
Rich Grise
- Jan 1, 1970
- 0
The interrupting capacity of a typical 1.25" Fast-Acting 3AG fuse at
250V is MUCH lower than at 125VAC. They will interrupt 10kA at 125V,
but only more like THIRTY-FIVE amps at 250V. Check out the data
sheets.
Put them in a situation where they can see thousands of amperes fault
current @240V and they can and often do EXPLODE, rupturing the glass
and sending fragments everywhere.
Of course the 20mm types are worse again.
That's why Fluke multimeters use a much larger fuse.
During failure of the 3AG type fuses, the melted metal ionizes inside
the fuse and allows an arc to form directly from one end cap to the
other, which has very low impedance (low voltage drop, but lots of
watts for such a small enclosed space).
I have a couple of fuse stories.
One is, I've seen 32V fuses used in a 240V circuit explode and blow
the cap off the holder, half-way across the factory floor.
One time, in the USAF, some guy in the shop ordered a 15A fuse, but
somebody made a typo in the stock number or something, because they
sent a 15A, 15,000V fuse. It was just like a standard 3AG, but it
was about 3" in diameter and about 12" long. The ends were copper,
and it was filled with sand. Nobody knew what to do with it, so we
took it apart. Inside, down the middle of the sand, was a ceramic
rod which looked like it had been extruded through an asterisk-
shaped die. Wrapped in a very loose helix around this (like,
looser than one turn per diameter) were five very fine silvery-
colored wires, the fuse element itself. The glass tube had about
about a 3/8" wall. It was quite fascinating. The sand, I guess,
was just plain old ordinary sand.
It was kinda kewl.
Cheers!
Rich