SCHMERSAL SRB-NA-R-C.35/CH2A-24V Replacement for SRB Series

Product Overview

SCHMERSAL SRB-NA-R-C.35/CH2A-24V Replacement for SRB Series: Seamless Safety System Transition

The SCHMERSAL SRB-NA-R-C.35/CH2A-24V is a dual-channel safety relay module designed for machine safety applications requiring the highest levels of protection — Category 4, Performance Level e (PLe), and SIL 3 per IEC 62061. As legacy SRB-series installations age and original units become increasingly difficult to source, this module serves as the definitive replacement and upgrade solution for engineers managing safety circuit modernization across manufacturing, packaging, press, and robotic workcell environments.

Whether you are retiring an older SRB-E-201ST, replacing a failed SRB-NA-R-C.35/CH2A-24V in a running line, or consolidating a mixed safety relay cabinet into a standardized SRB platform, this unit delivers direct form-factor compatibility with the existing SCHMERSAL SRB footprint. The 22.5 mm DIN-rail housing matches the mechanical envelope of the predecessor series, meaning no panel cutout modifications, no new DIN-rail sections, and no rewiring of the terminal block layout — a critical advantage when minimizing machine downtime during a planned or emergency replacement.

The 24 VDC supply voltage aligns with the standard control voltage used across the majority of SCHMERSAL SRB installations, as well as the broader 24 V DC bus found in Pilz PNOZ, Sick i16, and Omron G9SX safety relay panels. This cross-compatibility makes the SRB-NA-R-C.35/CH2A-24V a practical drop-in candidate even in mixed-brand safety architectures where the control power rail is already established.

Compatibility Comparison Table

Seamless Upgrade Solutions

A successful SRB-NA-R-C.35/CH2A-24V installation rarely involves the safety relay alone. In practice, field engineers replacing or upgrading a safety relay module must also verify and often refresh the surrounding components to ensure the entire safety loop meets the required PLe / SIL 3 integrity level.

On the input side, the dual-channel architecture of the CH2A variant requires two independent signal paths from the safety device — typically an E-stop button with two NC contacts, a SCHMERSAL AZ 16 series safety gate switch, or a Sick M4000 safety light curtain with OSSD dual outputs. Verifying that the field device provides clean, synchronous dual-channel switching within the discrepancy time window of the SRB-NA-R-C.35/CH2A-24V is the first step in any retrofit commissioning sequence.

On the output side, the two NO safety contacts typically drive the enable coils of downstream contactors — commonly Siemens 3RT2 or Schneider LC1D series — which in turn control the power feed to servo drives, frequency inverters, or pneumatic valve islands. When the safety relay is replaced, it is good practice to inspect the contact wear on these downstream contactors and replace them if they show signs of welding or pitting, since a welded contactor contact will prevent the safety relay from detecting a fault condition on the next cycle.

For installations where the SRB-NA-R-C.35/CH2A-24V is integrated into a larger SCHMERSAL safety network, the module may be paired with an MSC series modular safety controller or connected via the SCHMERSAL AS-i Safety at Work bus. In AS-i Safety architectures, the SRB-NA-R-C.35/CH2A-24V can operate as a standalone hardwired node while the AS-i Safety Monitor — such as the SCHMERSAL ASM-E series — handles network-level diagnostics and zone management. Migrating from a purely hardwired SRB cabinet to an AS-i Safety topology is a common modernization path that reduces wiring complexity while maintaining PLe compliance.

The 24 VDC power supply feeding the SRB-NA-R-C.35/CH2A-24V should be a SELV-rated, short-circuit-protected supply. SCHMERSAL recommends a dedicated safety power supply or a PULS, Phoenix Contact QUINT, or Siemens SITOP unit with redundant output capability. If the existing power supply is undersized or aging, replacing it alongside the safety relay prevents nuisance tripping caused by voltage dips during contactor switching transients.

For programming and documentation, the SRB-NA-R-C.35/CH2A-24V does not require a programming cable — it is a hardwired, parameterless device. However, if the broader safety system includes a SCHMERSAL MSC or a Pilz PNOZmulti safety controller, the associated SISTEMA project file or PNOZmulti Configurator project should be updated to reflect the new module’s PFHD value and MTTFd data, which are available in the SCHMERSAL safety characteristic data sheet. This ensures the overall machine safety assessment remains valid after the replacement.

Signal isolators and terminal modules — such as Phoenix Contact MINI Analog or Weidmüller ACT series — are sometimes inserted between the SRB output contacts and the PLC digital input card to provide galvanic isolation and protect the PLC I/O from inductive kickback. If these isolators are present in the existing panel, they should be retained and their output polarity verified against the new SRB wiring diagram before energizing the circuit.

Retrofit Guidance FAQ

Q: Can I replace an older SRB-E-201ST with the SRB-NA-R-C.35/CH2A-24V without rewiring?
A: In most cases, yes. The SRB-NA-R-C.35/CH2A-24V uses the same 22.5 mm DIN-rail footprint and the same terminal numbering convention as the SRB-E series. You should verify the terminal assignment against the wiring diagram of your specific panel before energizing, but physical rewiring is typically not required for a like-for-like replacement.

Q: Does the CH2A variant support single-channel input wiring?
A: No. The CH2A designation indicates a mandatory dual-channel input configuration. If your existing safety device provides only a single NC contact, you will need to upgrade the field device to a dual-channel output type, or use a SCHMERSAL SRB-NA-R-C.35/CH1A variant which accepts single-channel inputs at a lower safety category.

Q: What is the maximum cable length between the E-stop and the SRB-NA-R-C.35/CH2A-24V?
A: SCHMERSAL does not specify a hard maximum cable length, but the input circuit resistance must remain within the specified range to ensure reliable contact detection. For long cable runs exceeding 100 m, use a cable with a cross-section of at least 0.75 mm² and verify the loop resistance against the SRB input threshold specifications in the datasheet.

Q: Is the SRB-NA-R-C.35/CH2A-24V compatible with AS-i Safety networks?
A: The standard SRB-NA-R-C.35/CH2A-24V is a hardwired standalone module. For AS-i Safety integration, SCHMERSAL offers the SRB-E-201ST/AS-i or the MSC series with AS-i Safety gateway functionality. The hardwired SRB can coexist in a panel alongside AS-i Safety nodes without interference.

Q: How do I verify the replacement is functioning correctly after installation?
A: After wiring, perform a full functional test: apply 24 VDC supply, confirm the power LED illuminates, actuate the monitored safety device (open the E-stop or safety gate), verify the output contacts open and the fault LED activates, then reset and confirm the output contacts close. Document the test result in the machine’s safety validation record.

Q: Can the SRB-NA-R-C.35/CH2A-24V be used with a Siemens S7-1200 or S7-1500 safety PLC?
A: Yes. The SRB-NA-R-C.35/CH2A-24V operates as a standalone hardwired safety relay and its output contacts can be wired directly to the digital input of any F-DI module on a Siemens S7-1200F or S7-1500F safety CPU. No special configuration is required on the PLC side beyond mapping the input signal to the corresponding F-address in TIA Portal Safety Administration.

Warranty Assurance

Every SCHMERSAL SRB-NA-R-C.35/CH2A-24V unit supplied by TOPNLMS is covered by a 12-month warranty from the date of shipment. Prior to dispatch, each unit undergoes a pre-shipment functional inspection covering supply voltage response, dual-channel input detection, output contact continuity, and reset circuit operation. Units that do not pass inspection are quarantined and not shipped.

In the event of a warranty claim, TOPNLMS provides a replacement unit or full refund within the warranty period. Customers are asked to retain the original packaging and provide a brief description of the fault symptom to expedite the claim process. Warranty coverage applies to manufacturing defects and functional failures under normal operating conditions; it does not cover damage resulting from incorrect wiring, overvoltage, or mechanical impact.

TOPNLMS maintains stock of the SRB-NA-R-C.35/CH2A-24V and related SRB series modules to support urgent replacement requirements. Standard lead time for in-stock units is 3–7 business days to most destinations, with express shipping options available for critical downtime situations. All units are shipped with original manufacturer packaging, a test report, and a certificate of conformity where applicable.

For procurement inquiries, technical compatibility questions, or urgent replacement support, contact the TOPNLMS team directly:

• Email: [email protected]
• Phone / WhatsApp: +86 18359293191
• Website: https://topnlms.com

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