What it is and what it does
The Siemens 3RU2126-4NJ0 is a SIRIUS thermal overload relay — a bimetallic, ambient-compensated device that protects motors against sustained overcurrent by tripping a built-in auxiliary contact when the thermal model reaches the trip threshold. It is a CLASS 10 relay, meaning it will trip within 10 seconds at 7.2× the full-load current setting, which is the standard speed for protecting standard induction motors during start-up without nuisance tripping. The relay mounts directly onto a contactor via the contactor-mounting fastening method, eliminating the need for separate panel wiring between the two devices. It is sized in the S0 frame, which determines the physical envelope and the range of contactors it mates with.
Key ratings and what they mean for fit
Rated insulation voltage is 690 V, and the relay carries AC-3e switching ratings up to that same voltage — meaning it can handle the inductive motor-switching duty cycle typical of contactor-fed loads. At 400 V it is rated for 15 kW motor output; at 500 V for 18.5 kW; at 690 V for 22 kW. These are the values that govern real motor protection sizing, not the headline current alone. The auxiliary contact integrated into the relay is rated for switching at various control voltages: 2 A at 24 V, 3 A at 120 V, 2 A at 230 V, and 1 A at 400 V. These ratings cover the typical PLC input or contactor coil signal that the trip message drives.
Where it goes in the panel
Mounts in any position, directly to the contactor — no DIN-rail bracket needed for the relay itself, though the contactor typically snaps onto a DIN rail. Dimensions are 85 mm deep, 45 mm wide, 85 mm high, so the relay adds about 45 mm to the contactor's width when side-mounted. The main circuit connection uses ring cable lugs (M4 screw); the auxiliary circuit uses the integrated screw terminals with a Pozidriv PZ2 tip, 5–6 mm shaft diameter. Operating temperature range is -40 to +70 °C, with temperature compensation active over -40 to +60 °C, so it can live in a ventilated enclosure near the motor without derating for ambient heat up to that limit.
Lifecycle and sourcing reality
Substance compliance per RoHS is indicated by a substance-prohibitance date of 10/01/2009, confirming the part meets the EU RoHS directive. No separate UL or CSA listing is recorded in the available data, but the SIRIUS family typically carries CE and UKCA for the European market.