What it is and what the ratings mean
The Siemens 3RU7146-4MB0 is a SIRIUS Size S3 thermal overload relay rated for 100 A operational current with a Class 10 trip characteristic. Class 10 means it will trip within 10 seconds at 7.2× the set current — fast enough to protect standard induction motors during a stall or locked-rotor condition without nuisance tripping on normal start-up. The thermal bimetallic design responds to motor heating, not just instantaneous current, so it tracks the actual thermal state of the windings. The relay mounts directly onto a SIRIUS S3 contactor (the same frame size), forming a compact motor starter package on the DIN rail. The front panel carries an IP20 rating — safe for finger contact in an open panel — while the terminal area is IP00, meaning the wiring zone expects enclosure protection. Screw-type terminals on both the main and auxiliary circuits accept 2× (0.5…1.5 mm²) solid or 2× (0.75…2.5 mm²) stranded, torqued to 4…6 N·m on the mains and 0.8…1.2 N·m on the auxiliaries.
Deployment context
In a motor control center or panel, this relay sits between the contactor and the motor — it senses current on all three phases and opens the contactor's hold-in circuit on overload. The auxiliary contact set includes a normally-open and normally-closed pair; the DC-13 rating of 1 A at 24 V means those contacts can switch PLC inputs or relay coils in a 24 VDC control loop without derating. The -20 to +55 °C operating range covers most indoor industrial environments, and the 0…90 % non-condensing humidity tolerance suits unsealed panel interiors. Mounting position is flexible: the relay can be rotated ±135° on a vertical surface or tilted ±45° front-to-back, which helps when fitting into tight enclosures or angled subpanels. A 6 mm air gap to grounded parts at the side is required — standard for 1 000 V rated insulation. The 21 W typical power loss at rated current means the enclosure's thermal budget must account for that heat, especially when multiple starters are ganged.