all originally built 1979.
circuit: circuit used only for a/c (single outlet). runs thru 20 breaker in panel , then
15 aml type sl fuse on each line (black, white) in nearby fuse box. then about 25 to 30 (possibly in concrete floor) to a/c putlet.i've noticed original 20a breaker has been disconn, and rewired to new 20a breaker in lowest positoin in panel, suggesting the original breaker failed.
a/c: originally slightly larger a/c. when choosing 2001 replacement (physically smaller) i think it had slightly larger cooling capacity.
this are "window" a/c installed per instuctions into original wall framing, siding, etc. 12 amp sticker.
it seems to blow fuses only in this drastic heat wave. after the a/c has been running overnight (much longer than 6 hours, possibly only after 10+ hours). two recent blows occurred only after cool, 10pm and midnight.
How compressor was cycling is unknown to me. a/c is decently qiuet, and no one was expecting fuse blow, so no one was trying to listen . How a/c may have been (or not) cycling depends on thermostat setting. and thermostat is just a numbered knob :)
i've noticed the brigtht solder-like coating seen thru "window" of *both* fuses turns oxidized looking gray. only one fuse blows, so i assume both got very hot, then one blows becuse sligthly more susceptible, or hit by additional tiny transient, or whatever chose that fuse.
only once did i *witness* the a/c stop due to fuse blow. unfortunately i don't know if the fuse had blown maybe 10 or 15 min before anyone noticed a/c had stopped.
the fuses were warm (not hot) the cover plate of fuse box was warm.
Box is 4 octagonal or 4 square (more likely but can't recall for sure). box might be deeper than standard but i don't think so (again as i recall from when invesigating earlier weird fuse behavior, not incl this post)
cover is flat, but with fuse sockets and flip cover over each socket. socket partly recessed into box, but hte covers also protrude from wall obnoxiously (approx 1 inch). hopefully that's enough descriptuon (if relevant) because i can't find pic on google images. :)
the sl sockets may be adapters, but the fuses screw in tightly.
any suggestions where to look?
have questions about facts or evidence missing from my description?
i see fuses in addition to breaker for a/c compressors so i assume the sl fuses ("slow blow") are economical method to give different type wiring protection than the regular household panel breaker (not designed for motorstart circuits).