In the annals of dangerous and evil electrical products, this fake untrippable circuit breaker surely ranks highly: "It looks the part, it's marked the part… it even feels the part. But it's not the part."
It is, in other words, just a big on-off switch. The wire in it would act as a fuse, but isn't intended as a fuse, and Big Clive guesses it would take 100 amps to melt it.
It was included with a generator bought online. A useful rule is to mistrust anything that came free with anything, and an even better rule is to mistrust anything that came with anything free.
It's also a reminder of my second-favorite Amazon product, the male-to-male mains power cord.