What it is and what the key ratings mean
The Siemens 3RU2126-4NC0 is a SIRIUS-brand thermal overload relay, Size S0, designed for contactor mounting. Its CLASS 10 trip class means it will trip within 10 seconds at 7.2× the set current, which is the standard for protecting standard induction motors during start-up — fast enough to prevent winding damage from a locked rotor, but not so fast that it nuisance-trips on a normal acceleration profile. The relay is rated for a maximum operating voltage of 690 V. Its current-carrying capacity is voltage-dependent: it handles 3 A at 120 V, 2 A at 24 V and 230 V, 1 A at 400 V, and 0.75 A at 690 V — these are the auxiliary contact ratings, not the motor full-load current range. For motor protection, the relay carries power ratings of 15 kW at 400 V, 18.5 kW at 500 V, and 22 kW at 690 V, with corresponding FLA ratings of 28 A at both 480 V and 600 V. The main circuit uses spring-loaded terminals, accepting 2× (0.5 to 2.5 mm²) solid or stranded wire. Temperature compensation is active from -40 to +60 °C, keeping the trip curve stable across the panel's ambient range. The relay operates from -40 to +70 °C and survives storage and transport from -55 to +80 °C.
Sourcing and lifecycle
For compliance documentation, the relay carries a substance prohibition date of October 1, 2009, which aligns with the original EU RoHS directive. The MTTF under high demand rate is listed at 2,280 years, reflecting the solid-state reliability of the thermal bimetal mechanism in low-cycle applications.
Integration and mounting
The relay mounts directly onto the corresponding SIRIUS contactor (S0 frame) — no DIN rail required, no additional brackets. Fastening method is contactor mounting, meaning it clips onto the contactor's front face and connects via the integrated auxiliary switch. Mounting position is any, which simplifies panel layout. Dimensions: 45 mm wide, 102 mm high, 84 mm deep — the S0 footprint is compact enough for dense control panels. The auxiliary switch is integrated into the relay body, so there's no separate add-on block needed for the trip/alarm signal. The note "for message 'Tripped'" on the relay indicates that the integrated auxiliary contact changes state when the relay trips, providing a remote indication to a PLC or annunciator. Screwdriver shaft diameter is 3 mm, tip size 3.0 × 0.5 mm — a standard PZ1 or similar will fit the spring-loaded terminal cages.