Siemens Thermal Overload Relay 24V DC

The Siemens Thermal Overload Relay (specifically the 3RU21 series which includes models compatible with 24V DC control voltage) is an electrical protection device designed to protect motors from damage caused by excessive current due to overload or phase failure. It provides current-dependent protection for motors with normal starting by monitoring the motor current and responding to temperature rises in the motor windings.

Here are the key features and working principles:

• It detects an overload or phase failure when the motor current exceeds a preset rated current.

• Heating elements inside the relay cause bimetal strips to bend in response to the current rise.

• The bending of bimetal strips triggers auxiliary contacts that switch off the load through a contactor (the contactor is separate from the relay).

• It has adjustable overload current settings (for example, 14A to 20A in some models).

• The relay includes a tripping indicator and can be manually or automatically reset after a fault.

• It operates with DC control voltages such as 24V DC for the coil.