Don't understand "electric plug was welded to itself" What welded to what?
Breaker didn't trip because you were using less than 30 amps and did not have a direct short To pass UL and NEMA testing a breaker must handle 130% of it's design current for 8 minutes, but a direct short for only 1 cycle. And there are 60 cycles in a second. So, just doing it's job.
Bill
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