Troubleshooting Siemens 6ES5 955-3LC41: 3-Fan LED Shutdown Fault

Overview

The Siemens 6ES5 955-3LC41 is a SIMATIC S5 power supply module (Stromversorgung 955) designed for the S5-135U and S5-155U PLC chassis, with cross-compatibility to the EG 183U and EG 185U central-controller families. The unit accepts a strap-selectable AC 230V or AC 120V input and delivers a regulated 5V DC output at 18A continuous. Three internal cooling fans and a heatsink-mounted temperature sensor provide thermal supervision of the rectifier and output stage.

This article documents a recurring field fault in which all three fan monitoring LEDs illuminate steadily and the 5V output latches off approximately one second after power-up. It is structured as a field reference: supply identification, symptom interpretation, root cause ranking, bench diagnostic sequence, evaluation of the alarm-jumper bypass, and the practical replacement and verification path for S5-135U/155U operators.

Catalog Identification and Electrical Specifications

Confirm the module's MLFB (Maschinenlesbare Fabrikate-Bezeichnung) before any diagnostic or replacement action. The catalog number encodes the family, slot type, and electrical rating.

For chassis pinout, slot population rules, and LED semantics, refer to the SIMATIC S5-135U/155U System Manual. The manual is the only official Siemens document for the supply at block-diagram level. Locate the latest edition in Siemens Industry Online Support.

Symptom Profile: Steady Fan LEDs and 1-Second Shutdown

The reported fault has the following signature from cold start:

1. AC input applied. All three fan LEDs come on and remain steady (no flash, no sequence).
2. 5V output ramps up over a few hundred milliseconds.
3. Approximately one second after power-up, the supply drops the 5V output and latches off.
4. AC must be removed and reapplied (or the alarm reset by jumper change) to attempt a restart.
5. If the alarm jumper is reconfigured to disable the latch, the supply stays on and the 5V rail is stable.

The 1-second window is the diagnostic clue. Pure over-current and under-voltage trips occur in tens of milliseconds. A fixed ~1 s latch is the Siemens 955 supply's "external alarm not cleared" signature - the supply gives the fan tachometer and temperature sensor roughly one second to reach the healthy state, then latches off. The fact that fans spin freely and the temperature sensor reads healthy in-circuit is consistent with the alarm path failing, not the physical sensor failing.

Root Cause Matrix

Rank the suspects by field frequency. The first three account for the majority of steady-LED latches on supplies of this age.

Pre-Repair Bench Diagnostic Procedure

1. Power down, wait 5 minutes for bulk discharge, verify zero DC on the primary bus with a DMM.
2. Remove the supply from the chassis; place on an insulated bench with a grounded ESD mat.
3. Apply AC input through a 1A current-limited variac. Bring the variac up slowly. Note the input voltage at which the fan LEDs first illuminate and the point at which the output latches.
4. Probe each fan's tach output with a scope or frequency counter while fans are running. A healthy fan produces a 2-4 pulse-per-revolution square wave. Open circuit (DC level) or low-amplitude pulses indicate a failing fan Hall sensor.
5. Disconnect the temperature sensor connector. Measure resistance across the sensor pins with a DMM. Compare to the expected NTC resistance at room temperature; an open circuit on the sensor wiring reads as "hot" and latches the alarm.
6. Reconnect fans but leave the temperature sensor disconnected. Reapply AC. If the supply now stays up, the sensor path is the fault. If the supply still latches with sensor disconnected, suspect the alarm comparator or fan monitoring chain.
7. If the supply stays up with the sensor disconnected, bridge the sensor input with a resistor network equivalent to a healthy NTC at 25 deg C and recheck.
8. Measure 5V rail ripple under a 5A resistive load with a scope. Healthy: below 50 mV pk-pk. Above 100 mV: suspect output filter caps.
9. Measure ESR of the output filter bank. Replace any cap reading above 0.5 ohm at 100 kHz.

Test Equipment Required

Jumper Bypass: Why It Is Not a Repair

The 6ES5 955-3LC41 carries a configurable alarm jumper on the PCB. Field reports indicate that re-strapping the jumper causes the supply to ignore the alarm condition and remain in regulation. This is sometimes presented as a workaround. It is not a repair, and it should not be left in this state in service.

Use the bypass in two narrow cases only:

• As a temporary diagnostic to confirm the host system still functions with the supply forced on, before returning the unit to bypass-disabled configuration for repair or replacement.
• As an emergency measure to recover production while a replacement or repair is in transit, with cabinet ventilation upgraded and the chassis monitored for heat.

Do not leave a bypassed unit in service. Either restore the alarm path or replace the supply.

Schematic Availability and Documentation Boundary

Siemens has not published a public component-level schematic for the 955 series power supply. The SIMATIC S5-135U/155U System Manual provides:

• Block-level functional description of the supply's role in the chassis
• LED semantics and front-panel layout
• Connector pinout at the backplane side
• Slot population rules

It does not provide component-level schematics, PCB layout, or component values. This is a deliberate documentation policy: the 955 is a live-mains product and the vendor restricts component-level service data to the Siemens repair center and authorized partners.

Where to look in the official Siemens documentation tree:

• SIMATIC S5-135U/155U System Manual - block-level reference. Available via Siemens Industry Online Support.
• S5-135U / S5-155U Operations List and Programming Guide - same portal, broader S5-135U/155U documentation set.
• Modern SIMATIC power-supply design patterns and functional descriptions - Siemens TIA Portal Documentation (reference material only; modern supplies are not drop-in replacements for the 955).

Operating a chassis on a misrepaired supply is the larger downstream concern. Misrepaired switch-mode supplies are a leading cause of bus fires and unexplained CPU stop events in legacy installations.

Repair vs. Replacement Decision

Donor Sourcing Checklist

When procuring a used 6ES5 955-3LC41, perform the following to avoid acquiring a unit in the same fault state.

1. Power on the donor unit on a test bench before purchase. Confirm all three fan LEDs extinguish within the ~1 s alarm window. Confirm the 5V rail holds at 5.00-5.10V under a 5A resistive load.
2. Confirm the donor was stored in a controlled environment. S5-era electrolytic capacitors degrade with shelf time; ESR rise after 20+ years of uncontrolled storage is common.
3. Recap any supply that has been in storage for more than ~10 years. Replace every aluminum electrolytic in the unit with 105 deg C low-ESR parts of equal or greater voltage rating and equal or greater ripple current rating.
4. Verify the AC input voltage strap is in the position matching your local mains. 230V and 120V straps are not interchangeable in operation.
5. Inspect the three fans for bearing noise. A noisy bearing is a 6-12 month failure indicator under continuous operation.
6. Verify the alarm jumper is in the factory (alarm-enabled) position. Replaced units that have been jumper-bypassed in the field defeat the supply's protection.

Verification Sequence After Repair or Replacement

Before installing the supply back in a live S5-135U/155U chassis, run the following sequence. Each step is a go/no-go gate.

1. Bench test with a resistive 5V load of 9A (50% of rated) for 30 minutes. Verify 5.00-5.10V at the load, ripple below 100 mV pk-pk.
2. Confirm the three fan LEDs extinguish after the ~1 s alarm window under load.
3. Scope the 5V rail during a CPU STOP-to-RUN transition with all I/O modules populated. Confirm no voltage collapse under inrush (drop must stay above 4.75V for at least 100 ms).
4. Install in the chassis with the CPU in STOP for 1 hour. Monitor the supply's heatsink temperature; should stabilize below 60 deg C in still air at 18A output.
5. Run the chassis with the CPU in RUN for 4 hours under a representative program and I/O load. Verify no spurious faults on the CPU or I/O modules. A failing supply typically presents as intermittent CPU STOP or I/O bus errors before presenting as a hard 5V drop.
6. Re-check the 5V rail under the final load. Voltage and ripple should match the bench numbers within 1%.

Preventive Maintenance for S5-135U/155U Power Supplies

For S5 systems that remain in service, schedule the following on the 6ES5 955-3LC41.

• Annual: measure 5V rail ripple under load. Above 50 mV pk-pk = plan a recap.
• Every 3 years: replace the three internal fans. Bearing failure on an aged fan is a near-certainty. Replace as a set to keep the thermal balance consistent.
• Every 5-7 years: full electrolytic recap of the supply, including primary-side bulk caps and secondary-side output filters. Use 105 deg C low-ESR parts.
• Every 5 years: thermal scan of the supply under load. Hot spots above 70 deg C on the heatsink indicate a developing capacitor or rectifier issue.
• Every 10 years: replace the temperature sensor as preventive maintenance. NTC sensors drift, and a drifted sensor is the second-most common cause of the steady-LED latch documented in this article.

FAQ