Can someone confirm what the "sand" in typical cartridge fuses is put
there for? I suspect it probably has to do with quenching a high current
arc before it eats everything up. I'm looking at a 60 amp 250 vac fuse
right now which is labled "Class K5, 50,000 amps internal rating" and
which spilled sand on me when it crumbled apart yesterday as I was
pulling it out of its holder.

If arc extinguishing is what the sand is there for, is maybe a chemical
reaction involved?

My focus on fuses right now is because I've been living at home for the
past 19 years with a 60 amp GE fused disconnect which has a proclivity
for having its contacts start heating up after about a year or two of
service protecting the circuit to a HVAC heat pump air handler with
auxillary resistance make up heaters in it.

The stationary switch contacts are just the flat sides of the fuse clips
on the input end of the fuses. (The other side of those clips are curved
to match the fuse end caps.) So, when the contacts start oxidizing, they
eventually get hot enough to melt the solder joining the fuse link to
the inside of the fuse end cap, the circuit opens, and the air handler
quits.

Along the lines of "Things which go away by themselves usually come back
by themselves.", if I disassemble the switch parts and dress all the
contacting parts clean, then apply a little Kopper-Shield to them, the
darn thing works like a charm for so long that I forget about it until
we wake up to "no heat" on a cold morning and I have to climb into the
attic to fix it. I even went as far as to buy a new set of GE guts for
the thing about 8 years ago, thinking maybe the original installer had
just gotten a dud. but the same thing happened a year or so later.

Yesterday I went out and bought a Square-D non-fused disconnect to
replace that GE fused disconnect. It looks like it's got much "tighter"
contacts in it.

I understand the safety importance of having a disconnect next to the
equipment, but as the circuit is protected by a dedicated 60 amp breaker
in the home's load center, and the load is primarily resistance heating,
I think that local fusing there is redundant.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"As long as there are final exams, there will be prayer in public
schools"