Schmersal SRB324ST Safety Relay - 24 VDC, Version 3

The Schmersal SRB324ST (part number 101195504) is a compact safety relay designed for monitoring emergency stop circuits, safety gates, and light curtains in industrial machinery. It meets category 4 / PL e requirements per EN ISO 13849-1, providing fail-safe operation with dual-channel input monitoring and redundant output contacts.

Input and Output Specifications

Supply voltage: 24 VDC (range 20.4-27.6 V). Power consumption: 2.5 W. The relay has two safety input channels (dry contact or OSSD signals) that must be closed simultaneously. It provides two normally-open safety outputs (6 A each, 250 VAC/30 VDC) and one auxiliary normally-closed output for diagnostics. Outputs are forcibly guided to ensure safe switching.

Safety Functions

The SRB324ST monitors both channels for discrepancy, short circuits, and cross-faults. It features automatic or manual reset (selectable), and includes a start-up test that verifies correct operation before enabling outputs. The response time is less than 20 ms. The relay also supports feedback loop (EDM) for monitoring external contactors.

Diagnostics and Indication

LED indicators for power (green), input status (yellow), and output status (green). A red LED indicates a fault condition. The relay provides a transistor output for remote status signaling to a PLC.

Environmental and Mechanical Data

Housing: polyamide (UL 94 V-0), dimensions 22.5 mm x 100 mm x 90 mm (W x H x D). Mounting on 35 mm DIN rail. Operating temperature: -25 to +55°C, storage: -40 to +85°C. Protection rating: IP20 (terminal side).

Wiring and Connection

Screw-type terminals accept wire up to 2.5 mm². Shields are not required but recommended for long cable runs. The terminal layout follows standard safety relay pinouts.

Certifications

CE, UL, CSA, and TÜV approved. Compliant with EN 60947-5-1, EN 60204-1, and IEC 61508 (SIL 3).

Integration with PLC

The safety outputs are typically connected to safety-rated PLC inputs or directly to power contactors. The diagnostic output can be wired to a standard PLC input for remote monitoring of relay status.

Applications

Commonly used in packaging machines, press brakes, conveyors, and robot cells where operator safety is critical. It is also applied in woodworking machinery and injection molding equipment.

Installation and Maintenance

Installation requires proper grounding and adherence to wiring diagrams. Regular functional testing (as per machine safety regulations) is recommended. The relay has no user-serviceable parts; replacement is necessary if faulty.

Ordering Information

Part number 101195504 corresponds to the SRB324ST with 24 VDC and Version 3 firmware (enhanced diagnostics).

In summary, the Schmersal SRB324ST safety relay provides a robust, certified solution for implementing safety functions in PLC-controlled machines, ensuring operator protection and regulatory compliance.