Almost always caused by having the probes in the wrong positions on the meter (like the current measuring inputs) or the meter on the ohms or current scale.

Now that you mention it, my son believes he made a mistake by inserting the black probe into the "10A" jack instead of the "COM" jack. So why does this cause a spark/arc?

Other questions remain as well. Thanks.

Your son should be able to research the situation online on his own and then clearly explain the errors he made and why what happened, happened.

Here's a hint: You don't confirm voltage at a switch by measuring from the top screw to the bottom screw. You would set up to measure ac volts with one VOM multimeter probe on the top screw and the other probe on a good ground.Then you do the same thing again probing the bottom screw while keeping the other probe on ground.

It sparked and smoked because he had several amps "trying" to go through the leads. The circuit breaker did not trip because even a dead short has to be in place for a minimum period to so that. He jerked the probe off pretty quickly in all probability. The internal fuse in the meter might have burned out and maybe not. I would not worry about the house cables. That is what circuit breakers and fuses are for. There is no reason to believe that the switch is damaged either.

Thanks for your replies. The only visual effect to the wiring was some black on the exposed copper connection on the hot wire which came off when he wiped it with his finger (yes with the power off ;^))

Even though the breaker didnt trip nor did the fuse in the meter burn out its quite impressive what it did to the old switch and the test lead though... melted metal and flying sparks!

We put it back together after leaving the cover off for about 6 hours and it seems ok so hopefully there is no issue with something smoldering or something.

"my son believes he made a mistake by inserting the black probe into the "10A" jack instead of the "COM" jack. So why does this cause a spark/arc? "

The ten amp position in most meters is a solid piece of wire from the 10 A black to the red jack.

There are often pair of small wires at a set distance on the larger conductor to allow the voltage drop over that small section of wire to be used to measure the voltage drop in the wire. This voltage drop is than scaled to indicate the current flowing in the wire.

The line is usually fused, so the 10 A input will likely no longer operate since the fuse inside the meter will have blown.
Find someone who knows how to use a multimeter to show him how, before he blows up more than just a meter or hurts himself