One of my friends reacted to the previous post with the explanation about what happened and how to prevent this. Learning every day!
Since I work with many physicists from various disciplines, I learned from one of them—a metallurgist—that tin becomes liquid when subjected to pressure. So, if you pre-solder a wire (as you are doing) and then secure it in a terminal block with a screw, you end up deforming the soldered joint. Over time, this leads to sparking—exactly the problem you are experiencing. You can prevent this by not pre-soldering the wire before inserting it into the terminal block; instead, use a wire ferrule (a small metal sleeve placed over the wire). You crimp this onto the wire, and then you can safely secure it in the terminal block. Alternatively, you could simply solder two wires together without using a terminal block, which would also avoid the issue.
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