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