This breaker is a combined overcurrent and leakage device.  It had a fault where it wouldn't reset, and the issue was annoyingly trivial.

If you wonder why there's an awkward bit in the video it's because I left the camera running when I was exploring the circuitry.  The channels Patreons suggested I leave it in as a glimpse of what goes on in the jump-cuts.

Thomas Nagy's channel can be found at:-
https://www.youtube.com/channel/UC6EJTy6p58ZW16PEljSn4Qw

If you enjoy these videos you can help support the channel with a dollar for coffee, cookies and random gadgets for disassembly at:-
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