The Mitsubishi TH-T1811A is a solid state overload relay from the TH-T series, classified under Relays and within the broader Circuit Breakers, Fuses & Protection top category. It's the piece that sits between your contactor and motor, monitoring current and tripping if the load starts pulling too much — protecting windings from sustained overcurrent without the mechanical wear of a thermal bimetal.
Solid state overload relays like this one use a current transformer and electronic circuit to sense motor current, giving a repeatable trip curve that doesn't drift with ambient temperature the way a bi-metal strip does. That matters on a hot panel or near a furnace — the trip point stays where you set it.