According to Jeff Wisnia :
Kurt Ullman wrote:
So, I open up the box following my wife telling me the motor isn't
working and that there was a zap sound. I find a rather large black,
sooty spot next to the on-off switch for the motor and the black wire
burnt and the spade thing that the black wire was attached to still
screwed into the switch, although as I mentioned the wire itself was no
longer attached.
Is this just a matter of switching out the switch or something
worse that I should maybe call in repair people for?? I can do the
latter, but that about exhausts what I am comfortable doing with
electricity.

It may just be that whoever attached what you call "that spade thing" to
the black wire didn't make a solid crimp and the joint developed high
resistance over time, heated up, arced enough to melt the metal and failed.

Try shutting off the breaker feeding power to the switch and connect the
black wire directly under the switch's terminal screw by bending a half
loop on the wire.

Clean the wire and the switch terminal and screw so they are bright and
shiny before attaching the wire to the switch, and tighten that screw
firmly. (Check the other switch screw for tightness while you're at it.)

Turn the breaker back on and see how things go. Report back with results.

Couple of random thoughts:

1) Are you using a regular household electrical switch to control
the pump?

2) Is this switch outdoors? How dampproofed is it?

3) is the crimp-on connector on solid or stranded wire?

Generally speaking, ordinary household switches shouldn't be used
to switch motors (= 1/4HP say) on and off routinely. As a power
interrupt fine (mine has an ordinary switch for that purpose, but
uses a timer with HP rating to control the pump cycle), but not for
controlling routinely.

Motors, particularly pumps and compressors are hard starts, and pull
a lot of amps momentarily. This erodes switch contacts, makes poor
connections, and can cause the switch contacts and/or terminals to
overheat.

If the switch shows any signs of burning, I'd recommend replacing
it, even if it seems okay.

If replacing, go to a proper electrical supply and ask for a
switch with a "HP rating" at least as big as the pump HP rating.

[I built a 1/2HP stationary belt sander. I'm using a toggle
switch with a 1.5 HP motor rating for on/off switching.]

Failing that, ask for a "spec-grade" switch (eg: Hubbell). It'll
cost considerably more than a cheap HD bulk pack switch, but should
still be $10.

2) Moisture can accentuate poor connections/accelerate contact
degrade. Make sure the box is sealed reasonably well - eg: use
an outdoor box (plastic or cast aluminum) with gasket seals - seal
where the cable enters the box.

3) Crimp connectors suck on solid wire. Pigtail it to a chunk
of stranded, or, fasten the wire directly under the screw without
a crimp.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.