Siemens 3SK2122-1AA10 Safety Relay

The Siemens 3SK2122-1AA10 is a compact safety relay from the 3SK2 series, designed for emergency stop circuits, safety gates, and light curtain monitoring. It provides three normally open (NO) safety outputs and one auxiliary normally closed (NC) output. The relay achieves safety integrity up to PL e (ISO 13849‑1) and SIL CL3 (IEC 61508). Its small width (22.5 mm) makes it ideal for space‑constrained control cabinets.

Technical Specifications

Supply voltage: 24 V DC (19.2‑28.8 V). Power consumption: 1.8 W. Inputs: single or dual channel (with or without cross‑circuit detection). Safety output contacts: 3 NO (force‑guided), rated for 6 A at 230 V AC / 24 V DC (AC‑15, DC‑13). Auxiliary output: 1 NC, 2 A. Response time: ≤ 100 ms (from input to output). Release time: < 30 ms. The relay has 4 screw terminals for the reset circuit (automatic or monitored manual reset). LEDs indicate power, input status, and output energised. Dimensions: 22.5 mm (W) x 100 mm (H) x 110 mm (D). Operating temperature -25°C to +60°C. Protection rating IP20. The unit is DIN rail mountable (35 mm).

Key Features

• 3 safety outputs (NO), 1 auxiliary output (NC)

• Safe up to PL e / SIL CL3

• Single or dual‑channel operation

• Cross‑circuit detection for dual‑channel wiring

• Monitored manual or automatic reset

• LED status indicators for quick diagnostics

• Force‑guided contacts (EN 50205)

• Removable screw‑type terminals

Safety Functions

The 3SK2122-1AA10 is used to monitor safety devices such as emergency stop buttons, interlocking gates, and safety mats. In dual‑channel mode with cross‑circuit detection, it can detect short circuits between the two input channels or between a channel and 24V/GND. If a fault is detected or the safety device opens, the relay drops out immediately and disconnects the load. The monitored reset ensures that the reset button is released before the relay can re‑energize.

Configuration and Wiring

The relay is configured via DIP switches on the front. Switch 1 selects operating mode: S1 = OFF for single channel, ON for dual channel. Switch 2 selects reset type: OFF for automatic reset, ON for monitored manual reset (requires a feedback loop from the contactor’s auxiliary contacts). Inputs are connected to terminals S11/S12 and S21/S22. The safety outputs (13-14, 23-24, 33-34) are connected in series with the contactor coils. The auxiliary NC (41-42) can be used for PLC monitoring. For dual‑channel operation with cross‑circuit detection, use separate cable cores for each channel; do not share a common return wire.

Installation

Mount the relay on a 35 mm DIN rail. Connect 24 V DC to terminals A1 (+) and A2 (-). Ensure the power supply has sufficient current capacity (≥ 100 mA). The relay is not isolated between inputs and outputs; maintain proper separation from high‑voltage circuits. The terminals accept wire sizes 0.2‑2.5 mm². After wiring, set the DIP switches before powering up. Test the safety function by activating the emergency stop and verifying that all safety outputs de‑energize.

Applications

Typical applications include:

• Emergency stop monitoring on machine tools, conveyors, and packaging machinery

• Safety gate interlocking (with hinge or magnetic switches)

• Two‑hand control systems (with simultaneous monitoring)

• Light curtain evaluation (if the light curtain has OSSD outputs, a safety relay may be needed to interface)

Diagnostics and Maintenance

LED indicators: PWR (green) – supply OK; IN1 (yellow) – input channel 1 closed; IN2 (yellow) – input channel 2 closed; OUT (green) – safety outputs energized. If OUT is off but IN1/IN2 are on, check the reset circuit. If IN1 and IN2 have different states for more than 0.5 seconds, the relay detects a discrepancy and enters a fault condition (OUT off). To reset after a fault, remove the power for 10 seconds. The relay requires no periodic maintenance but should be tested annually by performing a function test of the connected safety device.

Standards and Certifications

The 3SK2122-1AA10 is CE, UL, and TÜV certified. It meets EN ISO 13849‑1 (Cat. 4, PL e), IEC 61508 (SIL3), and IEC 62061 (SIL CL3). It is RoHS compliant.

Conclusion

The Siemens 3SK2122-1AA10 safety relay offers a compact, highly reliable solution for common machine safety applications. Its flexible configuration, high safety integrity, and diagnostic LEDs make it a solid choice for machine builders and safety system integrators.