SCHMERSAL SRB-NA-R-C.35/CH2A-24V Retrofit-Ready Safety Relay for SRB Series Control Systems
The SCHMERSAL SRB-NA-R-C.35/CH2A-24V is a dual-channel safety relay module engineered for seamless integration into existing SRB series safety circuits and legacy control panel architectures. Rated to Category 4 / PLe / SIL 3 per EN ISO 13849-1 and IEC 62061, this module delivers the highest functional safety classification available for emergency stop, safety gate, light curtain, and two-hand control monitoring applications. With a 24 VDC supply voltage and a compact DIN-rail footprint, it is the preferred drop-in replacement for discontinued SRB variants across a wide range of industrial automation environments.
Whether you are modernizing a legacy machine safety circuit, recovering from an end-of-life component failure, or upgrading a control cabinet to meet current EN ISO 13849 compliance requirements, the SRB-NA-R-C.35/CH2A-24V provides a verified, tested, and immediately deployable solution. SMARTNEXMSK maintains ready stock with a 12-month warranty and pre-shipment functional testing on every unit.
Upgrade Compatibility Table
| Parameter | SRB-NA-R-C.35/CH2A-24V | Retrofit Notes |
|---|---|---|
| Supply Voltage | 24 VDC | Verify existing PSU output; compatible with SCHMERSAL SRB-E-201ST and similar 24 VDC rail supplies |
| Safety Category | Cat. 4 / PLe / SIL 3 | Maintains or upgrades legacy Cat. 3 / PLd installations |
| Input Channels | Dual-channel (CH2A) | Retain existing dual-channel wiring; no rewiring required for CH2A-compatible predecessors |
| Output Contacts | 3 NO + 1 NC (relay) | Map to existing contactor coil circuits; confirm terminal block pitch (5.08 mm) |
| Mounting | 35 mm DIN rail | Direct replacement on standard TS 35 rail; no bracket modification needed |
| Communication | Hardwired safety circuit | No fieldbus dependency; compatible with PROFIBUS, PROFINET, and EtherNet/IP upstream architectures |
| Replacement Compatibility | SRB-NA-R-C.35 series variants | Verify suffix (CH2A = dual-channel auto-reset); cross-reference SRB-NA-R-C.35/CH2A-230V for AC-powered panels |
| Commissioning | Manual or automatic reset | Configure reset mode via terminal S33/S34; document in machine safety validation record |
| Warranty | 12-Month Warranty — pre-shipment functional test included on every unit | |
Retrofit Planning for Existing Automation Systems
Successful integration of the SRB-NA-R-C.35/CH2A-24V into a legacy control system begins with a structured pre-retrofit audit. Before removing the failed or obsolete module, engineers should document the existing terminal wiring layout, confirm the 24 VDC rail supply capacity — typically sourced from a SCHMERSAL SRB-E-201ST or equivalent DIN-rail power supply — and photograph the original wiring for reference during recommissioning.
In control cabinets where the SRB-NA-R-C.35/CH2A-24V is installed alongside a SCHMERSAL SRB-NA-R-C.35/CH2A-230V (for mixed AC/DC panel architectures), engineers must confirm that the 24 VDC and 230 VAC supply rails are clearly segregated and that the correct module variant is installed in each slot. Mislabeled or mixed variants are a common source of commissioning delays in multi-zone safety panel retrofits.
For systems using a SCHMERSAL SRB301ST or SRB-E-301ST as the primary safety controller, the SRB-NA-R-C.35/CH2A-24V typically functions as a subordinate expansion relay, monitoring a specific safety zone — such as a perimeter gate monitored by a SCHMERSAL BNS 260 coded safety switch or a light curtain with OSSD outputs. Confirm that the OSSD signal levels are within the module’s input threshold range before finalizing the wiring.
In I/O-intensive retrofit projects, the SRB-NA-R-C.35/CH2A-24V is often deployed in parallel with SCHMERSAL SRB-NA-R-C.35/CH2A-24V-ST variants or alongside MSR127TP-class modules from partner platforms, where the safety relay handles the hardwired safety loop while a separate PROFIBUS DP or PROFINET gateway module — such as a SCHMERSAL SRB-E-MSR-PB — reports diagnostic status upstream to the PLC. This architecture preserves the hardwired safety integrity while enabling remote diagnostics without modifying the core safety circuit.
Terminal block compatibility is a critical checkpoint. The SRB-NA-R-C.35/CH2A-24V uses a 5.08 mm pitch screw terminal. If the existing cabinet uses a Phoenix Contact MKDS 3 or Weidmüller WDU 2.5 terminal rail, no adapter is required. For cabinets with older 6.35 mm pitch blocks, a terminal adapter or rewire to the new pitch is necessary before installation.
Where the retrofit involves a PLC program update — for example, migrating from a Siemens S7-300 safety CPU to a Siemens S7-1500F with TIA Portal — the safety relay’s feedback loop (terminal Y1/Y2) must be mapped to the new F-DI input module. Verify that the F-DI module’s input filter time is compatible with the relay’s response time specification to avoid nuisance trips during the first power-on sequence.
Downtime Control During System Migration
Minimizing unplanned downtime during a safety relay replacement is achievable with disciplined pre-staging. Before the maintenance window opens, the replacement SRB-NA-R-C.35/CH2A-24V should be bench-tested against the original wiring diagram, with all terminal connections pre-labeled and the reset mode pre-configured to match the existing machine logic. This eliminates the most common source of extended downtime: on-site configuration uncertainty.
For production lines where a full shutdown is not feasible, a zone-isolation strategy allows the affected safety circuit to be de-energized independently while adjacent zones remain operational. The SRB-NA-R-C.35/CH2A-24V’s modular DIN-rail design supports this approach — the module can be extracted and reinserted without disturbing neighboring devices on the same rail, provided the supply rail is properly isolated at the zone breaker.
After physical installation, the recommissioning sequence should follow the machine’s original safety validation protocol: verify dual-channel input continuity, confirm output contact closure under normal operating conditions, test the emergency stop response time, and validate the manual reset function before returning the zone to automatic operation. All test results should be recorded in the machine’s safety logbook to maintain CE/UKCA compliance documentation continuity.
SMARTNEXMSK ships the SRB-NA-R-C.35/CH2A-24V with a pre-shipment functional test certificate, reducing on-site verification time and supporting same-day recommissioning for experienced maintenance teams. Units are available from stock with standard lead times of 1–3 business days for most destinations.
Retrofit Support FAQ
Q1: Is the SRB-NA-R-C.35/CH2A-24V a direct drop-in replacement for older SRB-NA-R-C.35 variants?
A: In most cases, yes. The CH2A suffix designates dual-channel automatic reset, which is the most common configuration in legacy SRB series installations. Verify the supply voltage suffix (24V vs. 230V) and confirm the output contact count matches your existing wiring before installation. No firmware or programming changes are required — this is a hardwired safety relay.
Q2: What wiring checks are required before powering up the replacement module?
A: Confirm terminal assignments for A1 (24V+), A2 (0V), S11/S12 (input channel 1), S21/S22 (input channel 2), S33/S34 (reset), and Y1/Y2 (feedback loop). Cross-reference against the original wiring diagram. Verify that no cross-wiring exists between the two input channels, as this will prevent the module from achieving Cat. 4 / PLe status and may trigger an immediate fault on power-up.
Q3: Can this module be used in systems communicating via PROFINET or EtherNet/IP?
A: Yes. The SRB-NA-R-C.35/CH2A-24V operates as a hardwired safety relay and is fieldbus-agnostic. It can coexist in control architectures that use PROFINET, PROFIBUS, EtherNet/IP, or DeviceNet for upstream PLC communication. Diagnostic status can be fed to the PLC via a separate gateway module if remote monitoring is required.
Q4: What does the 12-month warranty cover, and is pre-shipment testing included?
A: Every unit shipped by SMARTNEXMSK includes a pre-shipment functional test verifying input channel response, output contact operation, and reset function. The 12-month warranty covers manufacturing defects and functional failures under normal operating conditions. Units that fail within the warranty period are replaced or refunded. Contact [email protected] for warranty claims or technical support.
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