The Siemens 3RU5146-4KB0 is a SIRIUS thermal overload relay with an adjustable current range of 57-75 A and Trip Class 10 protection. It mounts onto a S3-size contactor with screw-type terminals.
The 57-75 A adjustable range covers motor FLA from roughly 57 A up to 75 A. At the top end, that's a 37 kW motor on 400 V three-phase or a 50 hp motor on 480 V. The 75 A continuous rating is the thermal current — the relay must see that current through its power poles, so verify the main circuit wiring: the screw terminals accept 2x (10 AWG to 1/0 AWG) or single 2/0 AWG. Torque them to 4-6 N·m. The 16.5 W typical power loss is dissipated inside the enclosure — factor that into panel thermal calculations if you're packing multiple starters in a sealed cabinet. Trip Class 10 is standard for general-purpose induction motors. The thermal bimetal design is rated for -20 to +70 °C operating ambient and 0-100% RH.
This relay snaps onto a SIRIUS S3 contactor — no separate DIN-rail footprint needed. The fastening method is contactor mounting, meaning it plugs directly into the contactor's load-side terminals and is mechanically latched. The mounting position allows ±135° rotation from vertical and ±45° tilt front/back, so you can orient the starter assembly to fit tight enclosures. Required spacing to grounded parts at the side is 6 mm. Depth is 140 mm, width 70 mm, height 120 mm — the depth is the dimension that extends forward from the contactor face, so check your enclosure depth with the contactor plus this relay.
Sourcing note
This is a current-production Siemens SIRIUS part, widely used across European and North American panel builds. The 57-75 A range and Class 10 trip cover a common motor protection band. Same part, different passport: a genuine Siemens relay regardless of which authorized region it originated from.