Schmersal SRB-NA-R-C.35/CH2A-24V Retrofit Safety Relay

Schmersal SRB-NA-R-C.35/CH2A-24V Retrofit-Compatible Safety Relay for Legacy System Modernization

The Schmersal SRB-NA-R-C.35/CH2A-24V is a dual-channel, 24V DC safety relay module from Schmersal’s established SRB series, designed for emergency stop monitoring, safety door interlock supervision, and two-hand control circuits in industrial automation environments. As legacy safety systems age out of manufacturer support cycles, this module has become a critical retrofit component for engineers tasked with upgrading control cabinets without full system replacement.

Whether you are migrating from an older Schmersal SRB-E series or replacing a discontinued PILZ PNOZ X-series equivalent, the SRB-NA-R-C.35/CH2A-24V offers a direct-fit solution with minimal rewiring. Its compact DIN-rail footprint, screw-terminal wiring interface, and standard 24V DC supply voltage make it compatible with the majority of existing control cabinet layouts found in automotive, food processing, packaging, and machine tool industries.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

Successful integration of the SRB-NA-R-C.35/CH2A-24V into a legacy control system begins well before the module arrives on site. Engineers should start by auditing the existing control cabinet for available DIN rail space, verifying that the 24V DC bus — typically supplied by a Siemens SITOP PSU100S or equivalent Phoenix Contact QUINT power supply — can deliver sufficient current headroom after the new relay is added to the load. In most retrofit scenarios, the existing power supply is already sized for the original safety relay, so a like-for-like swap presents no capacity issue.

Terminal wiring is the next critical checkpoint. The SRB-NA-R-C.35/CH2A-24V uses a screw-terminal interface compatible with standard 0.75mm² to 2.5mm² control wiring. When replacing an older Schmersal SRB-E-201ST or a PILZ PNOZ X2.8P, engineers should photograph the original wiring layout and cross-reference terminal assignments against the new module’s datasheet before disconnecting any conductors. Pay particular attention to the feedback loop (EDM) terminal, which is often wired differently across generations of safety relays.

For systems running on a Siemens S7-300 or S7-400 PLC with existing safety-rated I/O modules such as the SM 326 F-DI or SM 336 F-AI, the SRB-NA-R-C.35/CH2A-24V integrates seamlessly into the hardwired safety circuit without requiring any changes to the PLC program or F-CPU configuration. However, if the system uses a Siemens ET 200SP distributed I/O station with PROFINET communication, confirm that the safety relay’s output contacts are wired to the correct F-DI channel and that the F-address assignment in the TIA Portal safety program reflects the physical wiring.

HMI screens that display safety relay status — common in systems using Siemens TP700 Comfort or Allen-Bradley PanelView Plus 7 panels — may reference the relay’s reset acknowledgment bit. After module replacement, verify that the HMI reset button triggers the correct PLC output and that the safety relay’s manual reset input is wired to the same terminal as the original. Failure to confirm this step is a common cause of unexpected safety faults during commissioning after a relay swap.

In control cabinets that also house Schmersal AZM 200 solenoid interlocks or Schmersal BNS 36 position switches, the SRB-NA-R-C.35/CH2A-24V can be configured to monitor these devices directly on its dual-channel inputs, preserving the original safety architecture. If the cabinet includes a Murr Elektronik MICO+ 4.10 electronic circuit breaker for branch protection, ensure the relay’s supply branch is correctly assigned and that the breaker’s trip current is set above the relay’s inrush current at power-up.

For systems requiring I/O expansion during the retrofit — for example, adding a second emergency stop zone — consider pairing the SRB-NA-R-C.35/CH2A-24V with a Schmersal SRB-E-201ST expansion relay or a Wieland samos PRO COMPACT configurable safety controller to handle additional input channels without replacing the entire safety architecture. Signal isolators such as the Phoenix Contact MCR-SL-1-5 may also be required when integrating legacy 4–20mA sensors into the new safety circuit.

Downtime Control During System Migration

Minimizing production downtime during a safety relay replacement is achievable with disciplined pre-staging and a structured hot-swap protocol. Begin by pre-wiring the SRB-NA-R-C.35/CH2A-24V on a test bench using the original wiring diagram, and perform a functional test — including emergency stop actuation, reset acknowledgment, and output contact verification — before the scheduled maintenance window. This bench test, which typically takes 30–45 minutes, eliminates the most common source of extended downtime: discovering a wiring error only after the machine is de-energized on the production floor.

During the actual swap, follow a strict lockout/tagout (LOTO) procedure. Isolate the 24V DC supply to the safety relay branch, photograph the existing wiring, disconnect terminals in reverse order from the original installation, and install the new module before reconnecting. The entire physical swap for a single SRB-NA-R-C.35/CH2A-24V typically takes 15–25 minutes for an experienced technician.

After reconnection, power up the cabinet and perform a controlled functional test: actuate each emergency stop device individually, verify that the safety relay drops out correctly, reset the relay, and confirm that the machine’s enable signal is restored to the PLC. If the system uses a Siemens SINAMICS G120 or Allen-Bradley PowerFlex 525 drive with a Safe Torque Off (STO) input wired through the safety relay, verify STO release after reset before returning the machine to automatic mode. Document the commissioning test results and retain them as part of the machine’s safety file.

For multi-zone safety systems where replacing one relay could affect adjacent zones, coordinate the replacement sequence to maintain at least partial production capability. In some cases, a temporary bypass — implemented under a formal risk assessment and removed immediately after commissioning — can allow adjacent production cells to continue operating during the relay swap.

Retrofit Support FAQ

Q1: Is the SRB-NA-R-C.35/CH2A-24V a direct replacement for the Schmersal SRB-E-201ST?
A: The SRB-NA-R-C.35/CH2A-24V and SRB-E-201ST share the same SRB series platform and 24V DC supply voltage, but terminal pinouts and output contact configurations differ between generations. Always cross-reference the wiring diagram for both modules before installation. In most cases, minor terminal re-mapping is required, but no DIN rail or cabinet modification is needed.

Q2: Can this relay be used with both automatic and manual reset configurations?
A: Yes. The SRB-NA-R-C.35/CH2A-24V supports selectable reset modes. Manual reset is recommended for emergency stop applications per EN ISO 13849 to ensure deliberate operator acknowledgment before machine restart. Automatic reset may be used for guard door monitoring in lower-risk applications where the risk assessment supports it.

Q3: What commissioning tests are required after installation?
A: At minimum, perform a full functional test of each connected safety device (emergency stop buttons, interlocks, two-hand controls), verify output contact switching under load, confirm the feedback loop (EDM) is correctly wired and functional, and document the test results. For SIL- or PLe-rated applications, a proof test interval must also be defined and recorded in the machine’s safety documentation.

Q4: Does NINERMAS provide stock availability and warranty coverage for this module?
A: Yes. NINERMAS maintains inventory of the SRB-NA-R-C.35/CH2A-24V to support urgent retrofit and spare parts requirements. All units supplied by NINERMAS carry a 12-month warranty covering manufacturing defects. Units are functionally tested prior to shipment. For lead times, bulk pricing, or long-term spare parts program inquiries, contact our team directly.