Siemens SIRIUS 3RU Thermal Overload Relay

The Siemens SIRIUS 3RU thermal overload relay is the standard motor protection device in Saudi Arabia's SIRIUS motor starter assemblies, providing reliable protection against thermal overload, phase failure, and phase asymmetry in a compact unit that clips directly onto the corresponding 3RT contactor without additional mounting hardware or inter-device wiring. This direct-mounting interface — the 3RU snaps onto the 3RT's base in a single action and the main current path is made through the contactor's mechanical interface — is the feature that most distinguishes the SIRIUS system from conventional separate-mount overload relay installations, reducing assembly time on large MCC projects by up to 25% and eliminating the potential wiring errors associated with separate terminal connections.

The 3RU uses bimetallic thermal elements that provide ambient temperature compensation — when the ambient temperature in the panel rises above the calibration reference (20°C), the bimetallic element automatically adjusts its trip threshold downward, protecting the motor against the additional thermal stress imposed by high ambient conditions. This self-compensating characteristic is especially important in Saudi Arabia where panel temperatures in outdoor MCCs can reach 50°C or above during summer months, conditions where a non-compensating relay would allow chronic motor overloading. The 3RU also provides phase failure sensitivity that detects single-phasing within 3–10 seconds, tripping before the motor's thermal model accumulates sufficient heat to cause winding damage — a critical protection function given that phase failure is one of the most common causes of motor burnout in the Kingdom.

The 3RU2 current range spans 0.1 A to 100 A, covering motors from fractional-kilowatt single-phase auxiliaries through to 45 kW three-phase process motors, with Class 10A trip characteristic as standard for general industrial motor applications. Manual and automatic reset options allow facility operators to choose whether the starter locks out after a trip (manual — requiring operator attention before restart) or automatically resets after the thermal element cools (automatic — useful for remote unattended pump stations). Both reset modes are selectable on the installed relay without tools, providing operational flexibility across different site management philosophies.

Razz Middle East supplies 3RU overload relays in all current settings from 0.1 A to 100 A for immediate delivery throughout Saudi Arabia, matched to the corresponding 3RT contactor sizes for complete starter assembly supply to panel builders and industrial maintenance teams.