Murrelektronik 857781 Retrofit-Ready PSU for MPS Control Systems

Murrelektronik 857781 Retrofit-Ready PSU for MPS Control Systems: Seamless Compatibility for Legacy Automation Upgrades

The Murrelektronik 857781 is a 24VDC DIN rail-mount power supply unit from the MPS series, engineered to deliver stable, regulated DC power to industrial control panels, PLC racks, and distributed I/O systems. As legacy automation infrastructure ages and original power supply modules reach end-of-life, the 857781 has become a preferred retrofit solution for engineers tasked with modernizing control cabinets without full system replacement. Its compact DIN rail form factor, wide AC input range, and robust output regulation make it directly compatible with a broad range of existing panel layouts and control architectures.

When planning a retrofit or spare-part replacement involving the Murrelektronik 857781, engineers must verify several critical parameters before commissioning. Output current capacity must be confirmed against the total load of connected devices — including the CPU module, communication processors, I/O expansion modules, and any HMI panels drawing power from the same 24VDC bus. Undersizing the power supply in a retrofit scenario is a common cause of intermittent faults and unexpected shutdowns during production.

Upgrade Compatibility Table

Retrofit Planning for Existing Automation Systems

A successful retrofit using the Murrelektronik 857781 begins with a thorough audit of the existing control cabinet. In many legacy installations, the original power supply feeds not only the PLC backplane — such as a Siemens S7-300 rack or an Allen-Bradley SLC 500 chassis — but also external 24VDC loads including solenoid valves, proximity sensors, and indicator lamps. Before removing the old unit, document all terminal connections and photograph the wiring layout. This step is critical when the original installation predates modern labeling standards.

In systems using distributed I/O architectures — for example, Murrelektronik MVK Metal or Cube67 field bus nodes — the 24VDC supply must maintain tight voltage regulation under dynamic load conditions. The 857781’s regulated output ensures that field bus nodes receive stable power even during motor start transients or simultaneous I/O switching events. Engineers retrofitting Murrelektronik MICO Pro intelligent current distributors alongside the 857781 will benefit from coordinated overcurrent protection across the 24VDC distribution network.

For control cabinets integrating Siemens ET 200SP distributed I/O or Phoenix Contact Inline I/O modules, the 857781 provides a reliable upstream power source. When the retrofit also involves replacing a legacy PROFIBUS DP network with PROFINET, the 24VDC supply must support the additional load of PROFINET switches and media converters added to the cabinet. Similarly, if the project includes upgrading from a Siemens S5 or S7-200 controller to a modern S7-1200 or S7-1500 CPU, the new CPU’s power consumption profile must be factored into the total load calculation before selecting the 857781 output current variant.

Backplane and rack compatibility is another key consideration. In modular PLC systems using Siemens S7-300 mounting rails or Rockwell Automation 1746 SLC racks, the 24VDC supply feeds both the backplane bus and external I/O field power. The 857781 can serve as the primary 24VDC source for the entire cabinet, provided the total connected load — including any Murrelektronik 9000-41034-0300300 or similar terminal block assemblies — remains within the rated output current. Always perform a load calculation before finalizing the retrofit design.

Programming cable connections and HMI panel power must also be accounted for. Siemens TP700 Comfort or similar HMI panels drawing 24VDC from the same bus add to the total load. During the retrofit, temporarily disconnecting non-critical loads and bringing them back online sequentially allows the commissioning engineer to verify stable voltage at each stage. Murrelektronik’s own MSC series surge protection devices are recommended at the 24VDC output to protect downstream electronics from transient overvoltages during switchover.

Downtime Control During System Migration

Minimizing production downtime during a power supply retrofit requires careful pre-planning and a structured switchover procedure. Before the scheduled maintenance window, prepare the Murrelektronik 857781 by bench-testing it at rated load using a resistive load bank. Verify output voltage stability, ripple, and response to step load changes. This pre-shipment functional test — included with every unit we supply — confirms the module is ready for immediate installation.

During the switchover, the original PLC program logic should be backed up to a programming device or TIA Portal project file before any hardware is disturbed. For Siemens S7 systems, use STEP 7 or TIA Portal to perform an online backup of the CPU memory, including retentive data blocks and hardware configuration. For Allen-Bradley systems, use RSLogix 5000 or Studio 5000 to save the controller project. This ensures that if the CPU loses power unexpectedly during the swap, the program can be restored without re-engineering.

Where a redundant 24VDC supply is available, use it to maintain power to critical I/O circuits while the primary supply is replaced. In systems without redundancy, coordinate with the production team to schedule the switchover during a planned shutdown. Once the 857781 is installed and wired, restore power gradually — first verify no-load output voltage, then reconnect loads in sequence, monitoring the output voltage at each step. Field devices such as proximity sensors, encoder power supplies, and communication modules should be the last to be reconnected, after the PLC CPU and I/O modules have confirmed normal operation.

After commissioning, perform a full I/O check against the original HMI screen layout to confirm that all field signals are correctly mapped and displayed. Document the retrofit with updated wiring diagrams, terminal labels, and a commissioning report. This documentation supports future maintenance and reduces troubleshooting time if issues arise later in the system’s lifecycle.

Retrofit Support FAQ

Q: Is the Murrelektronik 857781 a direct replacement for my existing MPS series power supply?
A: The 857781 is designed as a retrofit-compatible replacement within the Murrelektronik MPS series. Before ordering, confirm the output current rating of your existing unit matches the 857781 specification, and verify that the terminal pitch and DIN rail mounting dimensions are compatible with your panel layout. Our technical team can assist with cross-referencing legacy part numbers.

Q: What wiring checks are required before commissioning the 857781?
A: Inspect all AC input and DC output terminal connections for correct wire gauge, insulation condition, and torque to the manufacturer’s specification. Verify polarity on all DC output connections before energizing. Check that the protective earth terminal is correctly bonded to the cabinet earth bar. Re-label all terminals to match updated wiring diagrams before closing the cabinet.

Q: How do I verify compatibility with my existing PROFIBUS or PROFINET network?
A: The 857781 is a passive 24VDC power supply with no protocol dependency. It is compatible with any control system that operates on a 24VDC bus, including PROFIBUS DP, PROFINET IO, Modbus RTU/TCP, and DeviceNet architectures. Ensure that the total 24VDC load — including communication modules, I/O nodes, and HMI panels — does not exceed the rated output current of the 857781.

Q: What does the 12-month warranty cover, and is pre-shipment testing included?
A: Every Murrelektronik 857781 unit we supply undergoes a pre-shipment functional test covering output voltage accuracy, load regulation, and basic protection function verification. The 12-month warranty covers manufacturing defects and functional failures under normal operating conditions. Warranty claims are supported with replacement dispatch within standard lead times. Contact our team at sales@smartnexmsk.com for warranty registration and claim procedures.

© 2026 SMARTNEXMSK. All rights reserved.
Original Source: https://smartnexmsk.com
Contact: sales@smartnexmsk.com | +86 18259474341