Configuring Redundant S7-300 Power Supply with SITOP Modules

Overview

The SIMATIC S7-300 system power supply (PS-307) is a single-point-of-failure component when the controller drives a process where downtime is not acceptable. Unlike the SIMATIC S7-400H which supports hot-standby CPUs and redundant system power supplies natively, the S7-300 platform has no internal PSU redundancy. Reliability must be built externally by paralleling two PS-307 supplies through a SITOP decoupling/redundancy module and feeding both AC feeds from independent sources or a single source wired through two branches.

This reference covers the field-proven method of adding a SITOP PSE202U redundancy module (Siemens part number 6EP1961-3BA20) between two PS-307 supplies and the S7-300 backplane. The same architecture applies to any 24 V DC load where two identical Siemens power supplies must operate in parallel without back-feeding each other.

Prerequisites

• Two identical Siemens PS-307 power supplies (e.g., two 6ES7307-1EA01-0AA0 or two 6ES7307-1KA02-0AA0). Mixing part numbers, output ratings, or firmware revisions is not permitted for parallel operation.
• One SITOP PSE202U redundancy module sized to the load current: 6EP1961-2BA00 (5 A) or 6EP1961-3BA20 (10 A).
• Two independent 120/230 V AC feeds (preferred) or a single AC feed split into two protected branches.
• Wire ferrules, ring lugs, DIN rail, and 24 V DC load cabling rated for the sum of both PS outputs.
• STEP 7 (V5.5 / V5.6) or TIA Portal (V15.1 or later) for fault diagnostics configuration via the signal contact.
• Multimeter, current clamp, and STEP 7 online diagnostics for commissioning.

S7-300 PS-307 Specifications

The PS-307 family converts 120/230 V AC to 24 V DC and feeds both the backplane bus (for CPU and signal module internal logic) and the 24 V DC load terminals of the I/O. Three output ratings cover most S7-300 architectures:

Each PS-307 powers the backplane of the S7-300 station to which it is mounted in slot 1. For redundant operation the rack contains only one PS-307, and the second PS-307 mounts in a separate location (or in slot 1 of a second S7-300 station in the same project). The two 24 V DC outputs are then combined downstream through the SITOP redundancy module.

Redundancy Architectures Compared

Three architectures are used in industrial plants. Each has trade-offs in cost, voltage drop, and diagnostic capability.

The SITOP PSE202U MOSFET design is preferred over discrete diodes because the voltage drop is approximately 50 mV versus 500 mV, which keeps the load voltage inside the 24 V DC ±5 % tolerance under high load. The integrated signal contact also provides a dry-contact closure that can be wired to a digital input on the S7-300 for alarm generation.

SITOP PSE202U Module Details

The SITOP PSE202U manual (entry ID 18123560) describes the redundancy function and terminal layout. Two key specifications drive selection:

• 6EP1961-2BA00 – 5 A continuous, 24 V DC ±5 %, two inputs (In1, In2), one output (Out), one signal contact per input, screw terminals.
• 6EP1961-3BA20 – 10 A continuous, 24 V DC ±5 %, two inputs, one output, two signal contacts (one per input), push-in terminals, status LEDs.

Choose the redundancy module so that its continuous current rating is greater than or equal to the worst-case load current of the S7-300 station, including the inrush of digital outputs and analog loops. A 50 % derating rule applies: if the load is 7 A continuous, select the 10 A module rather than paralleling two 5 A modules.

Step-by-Step Wiring Procedure

1. Mount the components. Snap both PS-307 supplies and the PSE202U onto a 35 mm DIN rail inside the cabinet. Allow 30 mm clearance above and below each device for convection cooling. The PSE202U mounts vertically; do not mount horizontally as the derating curve assumes natural convection upward.
2. Wire AC inputs. Connect L, N, and PE of PS-307 #1 to branch circuit 1 (CB1). Connect L, N, and PE of PS-307 #2 to branch circuit 2 (CB2). For maximum availability, run branch 1 from UPS source A and branch 2 from UPS source B (or one from mains and one from a generator-backed bus).
3. Connect PS outputs to PSE202U inputs. From the 24 V / 0 V terminals of PS-307 #1, run two conductors (red, blue) to In1 (+ / −) of the PSE202U. Repeat from PS-307 #2 to In2 (+ / −). Use 4 mm² conductors for runs up to 3 m on the 10 A module; longer runs require voltage-drop calculation: V_drop = 2 × I × L × ρ / A_m².
4. Connect PSE202U output to load. From Out (+ / −) of the PSE202U, run the 24 V DC supply to the S7-300 backplane's external 24 V terminals (if present) and to the load-side 24 V terminals of each SM. For a single-S7-300 station, this is the 24 V DC feed that normally comes from the PS-307's 24 V output.
5. Wire the signal contacts. The PSE202U provides one normally-open contact per input that closes when that input is healthy (within regulation). Wire contact 1 (In1 OK) to digital input I0.0 of the CPU. Wire contact 2 (In2 OK) to digital input I0.1. This gives one DI per PSU for alarm/SCADA.
6. Configure the digital inputs in STEP 7. In HW Config, mark I0.0 and I0.1 as "fault" inputs (wire-break monitor). Create a tag PSU1_OK = I0.0 and PSU2_OK = I0.1. A fault on either triggers an OB82 (diagnostic interrupt) and can raise a process alarm.
7. Power up in sequence. Close CB1 first. Confirm PSE202U "In1 OK" LED is green and I0.0 reads TRUE. Close CB2. Confirm "In2 OK" LED is green and I0.1 reads TRUE. Both supplies should now share the load within ±10 %.
8. Verify load sharing with a clamp meter. Measure the output current of each PS-307. With identical units and identical cable lengths, the currents should be within 10 % of each other. If one PS supplies 90 % or more of the load, swap the supply positions (In1 ↔ In2) to rule out cable-length asymmetry, then verify again.

Topology Diagram

The following SVG depicts the field wiring of the redundant architecture. Both PS-307 units feed the PSE202U inputs; the single PSE202U output feeds the S7-300 station and any auxiliary 24 V loads.

Configuration in STEP 7 / TIA Portal

The PSE202U does not appear in the S7-300 hardware catalog because it is not an S7 module; it is a passive power component wired as a digital input source. The only software configuration required is on the S7 CPU side:

• In HW Config (STEP 7 V5.5/5.6) or Device Configuration (TIA Portal), open the DI module that owns I0.0/I0.1 and assign symbolic names PSU1_OK and PSU2_OK.
• Place the assignment: I0.0 → PSU1_OK (TRUE = healthy), I0.1 → PSU2_OK.
• Insert an FC or OB1 segment that combines both signals into a process alarm. Example ladder logic:

  A       "PSU1_OK"
  A       "PSU2_OK"
  =       "PS_REDUNDANT_OK"
  
  AN      "PSU1_OK"
  S       M 50.0          // PSU1 lost
  
  AN      "PSU2_OK"
  S       M 50.1          // PSU2 lost
  

• Trigger OB82 on loss of either supply by configuring the DI channel as "Diagnostic interrupt on wire break" and wiring the PSE202U signal contact to use the same pair.

Verification and Commissioning

1. Measure bus voltage. With both PSUs healthy, measure 24 V DC at the S7-300 backplane +24 V terminal. Value must be 22.8–25.2 V.
2. Measure load sharing. Clamp each PS-307 output. Difference between the two must be less than 10 % of total load current.
3. Force a fail-over. Trip CB1 while CB2 remains closed. Verify:
   • PSE202U "In1 OK" LED turns red within < 50 ms.
   • Signal contact 1 opens and I0.0 reads FALSE in the CPU diagnostic buffer.
   • S7-300 station remains in RUN with no OB85 / OB122 events.
   • Voltage at the S7-300 backplane remains within regulation (drop < 100 mV).
4. Restore and re-test. Close CB1. Verify "In1 OK" LED returns to green and I0.0 reads TRUE. Trip CB2 and repeat. Document the results in the commissioning report.
5. Long-duration test. Run for 72 hours under full load. Log PSE202U internal temperature (if model supports) and backplane voltage. Any deviation larger than 50 mV suggests cable asymmetry or a marginal PSU that must be replaced.

Troubleshooting Matrix

Source-Origin Context Notes

The S7-300 platform is described in the Siemens Industry Online Support portal. The PS-307 family is documented under the SIMATIC S7-300 Automation System / Power Supplies section. The SITOP PSE202U redundancy module manual referenced in field discussions is entry ID 18123560 in the Siemens support database, which contains the operating instructions in five languages including English.

The general principle of installing a system power supply for SIMATIC controllers (applies to S7-1500 / ET 200MP but the bus-feed topology is conceptually similar) is described in the TIA Portal manual collection. The S7-300 equivalent uses the PS-307 in slot 1 of the rack rather than a separate load-current supply.

Field-Proven Caveats

• Two PS-307 supplies from different production batches can show load-sharing imbalance because internal reference voltages vary by ±2 %. Always source both from the same shipment.
• The S7-300 CPU does not differentiate between a 24 V loss on the backplane and a missing AC input at the PS-307. Add the PSE202U signal contacts if you need to distinguish these failure modes in the alarm log.
• If the S7-300 station uses load-voltage 24 V from the PS-307 (no separate load supply), the redundant 24 V output of the PSE202U must be wired to both the backplane and the load terminals. Forgetting the load terminals leaves the I/O modules un-powered after the redundant PSU takes over.
• The S7-400H redundancy model is not interchangeable. Do not attempt to migrate S7-300 redundancy by substituting an S7-400H station; the backplane architecture, addressing, and STEP 7 libraries are different.
• PS-307 supplies do not support hot-swap. Always de-energize both AC feeds before removing a supply from the rail.