What it is and what it does
The Siemens 3RU2136-4ED1 is a SIRIUS thermal overload relay — the part that sits between a contactor and the motor, watching current and tripping if the motor pulls too much for too long. It's a Class 10 device, meaning it will trip within 10 seconds at 7.2× the set current, which is the standard for standard induction motor starting — fast enough to protect the motor but slow enough to let it start. The relay is sized S2, the mid-frame in the SIRIUS family, and it's rated for a 15 kW motor at 400 V (AC-3e duty), with a rated insulation voltage of 690 V.
Ratings that matter for fit
The 3RU2136-4ED1 carries a 15 kW rating at 400 V, and also 18.5 kW at 500 V and 30 kW at 690 V — so it covers 400 V and 690 V motor circuits without needing a different frame. The auxiliary contact ratings are given for multiple voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, 1 A at 400 V — these are the contacts that signal the trip condition back to the PLC or annunciator. The relay dissipates 4.6 W per pole, which matters for enclosure heat rise if you're packing several in a small panel.
Mounting and wiring
This relay is designed for stand-alone installation — it mounts directly to the mounting plate or DIN rail, not piggybacked on a contactor. The main circuit terminals are screw-type, accepting 2x (0.5 to 2.5 mm²) solid or stranded wire, with a Pozidriv PZ 2 screwdriver tip. Mounting position is any, and the operating temperature range is -40 to +70 °C, with storage down to -55 °C. Temperature compensation is active from -40 to +60 °C, so the trip curve stays accurate across the working range.