Repair time!
My Japanese friend who runs a ryukan (small Japanese hotel) and some AirBnB houses got this bit of Japan-specific electrical test gear that needed repair.
It had panel-mount shrouded male banana jacks and the cables had female inline sockets for some weird reason...
Couldn't source replacements for them (3 of 4 were broken with age and use), so I swapped them with female shrouded banana sockets from Cal-test and male shrouded banana plugs from Stabuli.
It does the usual earth-leakage breaker test, but also tests another bit of electrical safety gear that AFAIK is specific to Japan.
Basically, if you have a wooden house or structure and want to rent it out, you need a current transformer around the mains as close to where it meets the building as possible, that is then connected to a little box inside with an alarm.
It detects any imbalance in the power coming in compared to what's going out (perfect balance means zero current in the CT of course) and it will blast an alarm if there is an imbalance detected.
Why not just use an earth leakage breaker? Well that comes down to a peculiarity of cheap Japanese construction, and shitty electrical standards along with earthquakes etc.
Inside the exterior walls of wooden houses, steel mesh or chickenwire (sometimes galvanized iron sheeting) etc is used to give the outer layer of render/plaster something to bite to and hold the wall together in an earthquake. Even if the render cracks, the metal mesh prevents the render from flaking away from the wall and hitting you on the head. Then a bit of caulking has it sealed back up again.
Now, imagine what would happen over a couple decades of earthquakes if a cable were to wear through and make electrical contact with that mesh inside the wall...
Imaging if that wire was the main feed from the meter to the switchboard, i.e. completely unprotected by breakers or fuses.
The mesh, if given a path to earth (likely), will heat up like a toaster and burn your wall from the inside out and you wouldn't even know until it's too late.
This CT safety device will blare an alarm to alert you of this fault.
The tester I just fixed is used to simulate a fault to test the operation of this safety device.