Schmersal SRB-NA-R-C.21-24V Safety Relay Module

The Schmersal SRB-NA-R-C.21-24V is a compact safety relay module designed for monitoring emergency stop devices and safety interlock switches in industrial machinery. It features five enabling outputs and one monitoring output, providing reliable safety control in applications requiring high safety integrity. Operating on a 24 V AC/DC power supply, the module is suitable for safety circuits up to Category 4 according to EN 954-1 and PL e / SIL CL3 according to ISO 13849-1 / IEC 61508. The relay includes adjustable drop-out delay, auto-reset and cross-short recognition functions, and a compact 45 mm wide thermoplastic enclosure for DIN rail mounting.

Technical Specifications

The SRB-NA-R-C.21-24V operates on a supply voltage of 24 V AC/DC (-15%/+20%). Maximum power consumption is 4.0 VA, plus the monitoring output load. The relay has 5 enabling outputs: 3 normally open (NO) safety outputs and 2 time-delayed NO outputs. One monitoring normally closed (NC) output is also provided. Enabling contacts are rated for 230 V AC, 4 A resistive load (NO 43/44 and 53/54: 24 V DC/2 A DC-13, 230 V AC/3 A AC-15). Fuse protection requires a 4 A slow-blow fuse for enabling contacts and a 1.25 A internal fuse for power supply. The adjustable drop-out delay is programmable from 0 to 30 seconds, allowing timed stopping of hazardous motion. Selectable functions include trailing edge detection (for monitoring dual-channel devices), auto reset (permits automatic start after safety device resets), and cross-short recognition (detects wiring faults). Dimensions are 83 mm (H) × 45 mm (W) × 140 mm (D). Weight is approximately 480 g.

Key Features

• 5 enabling outputs (3 NO, 2 NO time-delayed)

• 1 NC monitoring output for PLC feedback

• Adjustable drop-out delay: 0 to 30 seconds

• Selectable auto reset or monitored manual reset

• Cross-short detection between input channels

• Trailing edge detection for dual-channel devices

• Category 4 / PL e / SIL CL3 safety rating

• Compact 45 mm DIN rail housing

• 24 V AC/DC supply with wide tolerance

• Internal fuse protection for power supply

Safety Functions

The SRB-NA-R-C.21-24V is used for monitoring emergency stop pushbuttons, safety gate switches, and other safety input devices. In dual-channel operation, the relay checks for simultaneous closure and detects cross-shorts (short circuits between channels) and ground faults. The drop-out delay function allows the safety outputs to remain energized for a user-defined period after the safety input opens, enabling controlled stopping before the hazardous motion is removed. The auto reset option automatically restarts the machine when the safety device returns to its safe state, eliminating the need for an operator reset button. However, manual reset is recommended for most applications to prevent automatic restart. The monitoring output provides a status signal to a PLC, indicating whether the safety contacts are closed or open.

Applications

The relay is suitable for safety applications such as emergency stop monitoring on machine tools, packaging equipment, printing presses, and conveyor systems. The time-delayed outputs are useful for applications where a braking action or damping of moving parts is required before the safety outputs are de-energized. The SRB-NA-R-C.21-24V is also used for safety gate monitoring on robots and hazardous area access points.

Installation and Wiring

Mount the relay on a 35 mm DIN rail. Connect 24 V AC/DC to the A1/A2 terminals. Wire the safety input device(s) to the appropriate input terminals (S11, S21, S12, S22). For dual-channel operation, connect both contacts of a safety switch between S11-S12 and S21-S22. For cross-short detection, use two independent cables. Connect the safety outputs (e.g., 13-14, 23-24) to the contactor coils. For the time-delayed outputs, connect the loads according to application. The NC monitoring output (L62) can be used to indicate relay status to a PLC. Set the DIP switches for operating mode (dual/single channel), reset type (auto/manual), and delay duration. After wiring, test the safety function by activating the emergency stop and verifying that the safety outputs de-energize within the set delay.

Diagnostics and Maintenance

The relay includes LED indicators for supply, input status, and output energised. A flashing fault LED indicates wiring errors (cross-short, open circuit, shorted reset button). The relay performs self-diagnostics at power-up and continuously during operation. If an internal fault is detected, the safety outputs de-energise and the fault LED illuminates. The relay requires no routine maintenance beyond periodic functional testing. Test the safety function annually or more frequently according to risk assessment. Replace the relay if it fails to open the safety outputs when the safety device is activated.

Environmental and Compliance

The SRB-NA-R-C.21-24V is CE, UL, and TÜV certified. It meets EN ISO 13849-1 (Category 4, PL e), IEC 61508 (SIL CL3), and EN 60204-1. The module is RoHS compliant. Operating temperature: -10°C to 55°C. Storage temperature: -40°C to 85°C. Humidity: 10-90% non-condensing. Vibration resistance: 2g (10-55 Hz). Protection rating: IP20 (requires installation in control cabinet).

Conclusion

The Schmersal SRB-NA-R-C.21-24V safety relay provides flexible safety monitoring with 5 enabling outputs, adjustable time delay, and selectable functions. Its Category 4 / PL e safety rating makes it suitable for demanding machine safety applications requiring high reliability and fault detection.