What it is and what it does
The Siemens 3RU2136-4KB1 is a SIRIUS Size S2 thermal overload relay — the part that sits between the contactor and the motor to protect against overloads and phase loss. It's a CLASS 10A trip, meaning it will open the circuit within 10 seconds at 7.2× the full-load current setting. That's fast enough for most standard induction motor starts, but not so fast that it nuisance-trips on a normal acceleration curve. The relay is rated for a maximum insulation voltage of 690 V, and it can handle motor loads up to 55 kW at 690 V or 37 kW at 400 V. The integrated auxiliary switch signals a tripped condition — the note on the part says "for message 'Tripped'" — so the PLC knows exactly why the motor stopped.
Mounting and wiring in the panel
This relay mounts standalone — no DIN rail adapter required, just screw it down. The Size S2 footprint is 105 mm tall, 55 mm wide, and 117 mm deep, so it fits in a standard motor starter cubby. Main circuit connections use screw-type terminals (M6 bolts) and accept 2×(0.5…1.5 mm²) or 2×(0.75…2.5 mm²) solid or stranded wire. The screwdriver shaft needs to be 5–6 mm diameter with a Pozidriv PZ2 tip — keep that in the van. It can be mounted in any position, which helps when you're retrofitting into a tight panel. Temperature compensation is active from -40 °C to +60 °C, so the trip curve stays accurate across the operating range.
Auxiliary contact switching capacity
The integrated auxiliary switch can handle a range of control voltages. At 24 V it switches 2 A; at 120 V it handles 3 A; at 230 V it's rated for 2 A; and at 400 V it drops to 1 A. For DC control circuits, it manages 0.3 A at 60 V and 0.22 A at 110 V or 125 V. That covers most PLC input and relay coil loads you'd find in a standard motor control center.
Environmental and compliance
The relay operates from -40 °C to +70 °C, and can survive storage or transport from -55 °C to +80 °C. The substance prohibition date is October 15, 2014, which aligns with RoHS compliance requirements. The relay uses a thermal bimetallic overload release — no electronics to fail, just a straightforward mechanical trip that resets manually or automatically depending on the application.