Wire zapping manifold shield
When sparks jump from a wire or other component,
IT IS BECAUSE there is something wrong
You could have a problem in that cylinder, IE burned valve, other lean condition
a broken/ damaged spark plug---they can lose continuity inside the ceramic
A bad (open) wire
All of the mentioned conditions --essentially an "open" removes the loading effect of the firing cylinder which allows the voltage to RISE and just like lightning, the spark voltage is now looking for the easiest path to ground.
It may be that the manifold/ shield being in poor shape overheated the wire and is the direct cause of the damage. If it rattles around and contacts the wire "in flight" might just be
If you want a real education, go out after dark, raise the hood, and run the engine. You'll soon see where the spark/ corona is "trying to go" to ground
If you have big enough nuts, running your fingers along the wires will soon give you an education, too. Not recommended with high energy, multiple discharge gear