Siemens 3SK1121-1AB40 Advanced Safety Relay

The Siemens 3SK1121-1AB40 is an advanced safety relay belonging to the 3SK1 series. This compact unit is designed for monitoring emergency stop circuits, safety doors, light curtains, and two-hand control devices. It provides two safety-related output channels and features high flexibility via configurable input logic.

Input and Output Specifications

The module has 4 safe inputs (2 redundant channels for E-stop, plus 2 additional inputs for start/reset or dual-channel light curtain). Outputs consist of 2 normally open (NO) safety contacts (each 5A at 250V AC) and 1 auxiliary NC contact. The safety contacts are forced-guided and positively driven. Outputs are tested for welded contact detection.

Configuration and Modes

Using DIP switches on the front, the user can select operating modes: single-channel E-stop, dual-channel E-stop with or without cross-circuit detection, automatic/manual reset, and start-up test. The module supports both active high and active low sensors. It also includes a delay-off function (0 to 10 seconds) via an adjustable timer.

Safety Integrity

Rated for category 4 / PL e (EN ISO 13849-1) and SIL 3 (IEC 62061). The 3SK1121-1AB40 has built-in cross-circuit detection and redundancy monitoring. The safe isolation voltage is 250V AC. It achieves a proven mean time to dangerous failure (MTTFd) of over 250 years.

Power and Installation

Supply voltage is 24V DC (±20%). Power consumption is 2.5 W. Dimensions: 100 x 22.5 x 120 mm (width 22.5 mm). It mounts on a 35 mm DIN rail, conserving panel space. The terminals are screw-type with 4 mm² capacity. Operating temperature: -25°C to 60°C, suitable for harsh environments.

Diagnostics and Status

Three LEDs indicate power (green), channel 1 status (yellow), channel 2 status (yellow), and fault (red). A flashing pattern on the fault LED helps diagnose wiring errors or cross-circuit conditions. The module also provides a test pulse output for checking sensor wiring.

Compliance and Certifications

Certified by TÜV, UL (US and Canada), and CSA. Compliant with EN 60947-5-1, ISO 13849-1, and IEC 61508. The unit is suitable for use in machine tools, packaging machinery, and conveying systems.

Applications

Industries such as automotive, food processing, and woodworking rely on the 3SK1121-1AB40 for safety-related stop and guard locking functions. Its slim design allows integration into existing control cabinets.

Conclusion

The Siemens 3SK1121-1AB40 safety relay provides reliable emergency stop and door monitoring with PL e capability. Its compact size, configurable inputs, and diagnostic LEDs make it a versatile safety component.