BEWARE - why do extension cords melt?

[YarraRanges]

As an electronics technician I was taught that you must NEVER EVER tin the end of a wire with solder that is going to be held in a screw terminal.
Compressing the solder causes micro cracks in the solder where corrosion will eventually occur. Crystalisation can occur in these areas and multiple small diodes can be created by the corrosion causing heating and eventual failing of the termination.

Remember NEVER EVER tin a copper wire that is going to be held in a compression screw terminal!