Troubleshooting Siemens 6SE70 O008 Inhibit-to-Start Fault on 6SE7036 Windlass Drives

The SIMOVERT MASTERDRIVES VC / MC family (6SE70 series) is widely deployed in marine winch and windlass duty where the converter is exposed to salt-air humidity, vibration, inductive DC-link transients, and noisy contactor logic. A recurring and often-misread condition on these drives is operating state O008 — frequently described by field engineers as an inhibit-to-start lockout. Unlike alarms (Axxx) and faults (Fxxx), O008 is a state, not a trip event, which is precisely why the main contactor can open and the panel can show "no fault, no alarm" while the drive refuses to release the inverter. This reference explains what O008 actually means on a 6SE7036-0EK60, why it can appear sporadically on a windlass, and how to systematically eliminate the root causes — from control-word DI bounce to the PSU2–IVI flat-ribbon cable, to optional K80 Safe Stop behaviour.

1. Problem Definition: Sporadic O008 on 6SE7036-0EK60 Windlass

Reported field behaviour on the 6SE7036-0EK60 (frame size E, typically 360 A three-phase rectified-class output) used in anchor windlass duty:

• Drive runs normally for 6–8 hours or more, then transitions to O008.
• Main input contactor drops out; drive remains powered on the electronics side.
• No Fxxx fault code and no Axxx alarm are stored in the fault buffer.
• OFF2 and OFF3 are not used (bits 01 and 02 of the control word are hard-wired high or unused).
• OFF1/ON (bit 00) and Inverter Release / Inverter Stop (bit 03) are wired to digital inputs through an interposing relay.
• At the moment O008 occurs, the ON signal is still present (bit 00 = 1).
• Issuing OFF1 (bit 00 = 0) transitions the drive to O009, after which the drive can be restarted.

The transition pattern is the key diagnostic clue: O014 → O008 → (after OFF1) O009 → restart. This sequence maps directly to control-word bit loss rather than a hardware trip.

2. SIMOVERT MASTERDRIVES 6SE70 Operating-State Reference

The 6SE70 platform exposes its current behaviour through the two-digit status word (r001 / r002 by default, parameter set by P654 and P655). The relevant states for the 6SE7036 windlass case are:

For the windlass case, the O014 → O008 transition (rather than O014 → O000) means a release condition is being revoked at the moment the drive is about to release. That condition is almost always control-word bit 00 (ON/OFF1) and/or bit 03 (Inverter Release/Stop) — wired in the affected cabinet through a relay contact that is bouncing or being de-energised by an upstream interlock.

3. Control Word Bit Mapping on 6SE70

The standard 6SE70 control word (P554 in the standard parameter set) is a 16-bit word assembled from freely-selectable binary sources (BICO). Default mapping with OFF2/OFF3 not used and DI-driven ON / Inverter Release:

The diagnostic giveaway: if bit 03 (Inverter Release) is momentarily lost while bit 00 (ON) remains high, the 6SE70 transitions O014 → O008 and stays in O008 until ON is dropped, even if Inverter Release is restored. This matches the reported field behaviour exactly.

4. Root Cause Tree: Why O008 Appears Sporadically

Five causes account for nearly every reported O008 on a 6SE7036 in windlass duty. The ranking reflects probability in marine installations:

5. Diagnostic Procedure: From Symptom to Root Cause

Follow this sequence before replacing boards. Each step uses a parameter on the 6SE70 that the engineer can read on PMU (parameter display unit) or export via DriveMonitor / STARTER.

5.1 Capture the State Word Trace

1. Connect DriveMonitor via serial (X104) or PROFIBUS.
2. Configure the trace function (P778 / P779 in older firmware, or the trace tool in STARTER) on the following signals:
   • r001 — status word 1 (16-bit, contains the Oxxx operating state in the high byte)
   • r002 — status word 2
   • r007 / r008 — active control word 1 / 2
   • P554 sources (especially the bit 00 and bit 03 DI sources)
3. Set sample time to 1 ms or 2 ms (the 6 ms used in the original incident misses sub-10 ms DI bounce).
4. Trigger the trace on a status-word edge matching the O008 code.
5. Run the windlass through repeated anchor drops. Capture at least three full cycles.

Interpretation:

• If bit 03 of the control word (Inverter Release) drops to 0 for any duration before O008 appears, the suspect is the DI wiring, the interposing relay, or the PSU2/IVI ribbon cable that carries the buffered DI signal back to the control electronics.
• If bit 00 of the control word (ON) drops to 0 simultaneously with the main contactor de-energising, the upstream interlock is forcing OFF1 — different problem; the drive would pass through O009, not O008.
• If no control-word bit change is visible at 1 ms resolution, suspect the PSU2 ±15 V supply dipping on the gate-driver / interface board side; the control word registers are correct but the inverter stage is internally inhibited.

5.2 PSU2 Voltage Checks

The PSU2 board on a 6SE7036-0EK60 supplies the control-electronics voltages, including ±15 V, +5 V, and the gating reference for the IGBTs. O008 will be latched if the ±15 V rail sags below approximately ±14 V on a transient basis.

1. With the drive powered but inhibited, place a DMM (true-RMS, min-hold function) on PSU2 test points:
   • TP1: +15 V (±0.3 V tolerance)
   • TP2: -15 V (±0.3 V tolerance)
   • TP3: +5 V (±0.15 V tolerance)
2. Repeat while the drive is running at full load for ≥ 1 hour.
3. If TP1 or TP2 dips to ±14.0 V or below for any duration, the PSU2 is failing thermally or the load on the rail is excessive (a typical cause is a failing IVI board drawing excess current). Replace PSU2 first; if the new PSU2 also sags, replace the IVI board.

5.3 Flat-Ribbon Cable (PSU2 ↔ IVI) Inspection

The ribbon cable between the PSU2 and the IVI (Inverter Interface) board carries the ±15 V, the gating signals, and the inverter-status feedback. On windlass drives subject to cabinet vibration and humidity:

1. De-energise the drive, wait for DC-link to discharge (verify < 50 V on the DC-link test points).
2. Inspect the cable for chafe, oxidised contacts, or displaced seating in the connector.
3. Clean contacts with isopropyl alcohol and re-seat firmly.
4. If the symptom persists, replace the cable with a known-good unit from another operational 6SE70 of the same generation.
5. Apply a small amount of dielectric grease at the connector edges if humidity is suspected; do not contaminate the contacts themselves.

5.4 K80 Safe Stop Audit

The K80 option card implements Safe Stop (STO equivalent) by forcing OFF2 (control-word bit 01) low and the gate-pulse inhibit. The drive then sits in a Safe Stop condition indistinguishable from a normal user-induced O008.

1. Confirm whether the K80 is fitted (look for the K80 option label on the front of the electronics tray; consult the Siemens Industry Online Support portal with serial number for ordering data if uncertain).
2. If K80 is fitted, check alarm buffer for A017 (Safe Stop requested). A017 in the buffer is the smoking gun for K80 involvement.
3. Verify the 24 V supply at K80 terminals. A drop or interruption on the K80 24 V produces exactly the symptom reported: intermittent O008, no A017 if the drop is sub-baud and not registered, but the drive still locks out.
4. If the application does not require Safe Stop, the K80 can be disabled by removing the option card or by setting the configuration parameter that bypasses the function (consult the K80 commissioning manual for the firmware-compatible procedure).

5.5 DI Signal Integrity and Relay Contact Audits

If the control-word trace shows bit 03 bouncing but no PSU2 / K80 anomaly, the cause is the DI wiring path. The 6SE70 DI inputs on the customer terminal block (X101, X102 depending on the option card) are typically 24 V IEC 61131-2 type 1, with a guaranteed "1" threshold of 15 V and a "0" threshold of 5 V.

1. With the drive running, scope the DI terminals at the drive (not at the relay coil) using a differential probe or two-channel scope with a common ground at the drive PE.
2. Look for dip < 15 V on the Inverter Release input. Even a 1 ms dip is enough to latch O008 if it occurs at the wrong point in the start sequence.
3. Check relay contact bounce time. Mercury-wetted or solid-state relays have < 0.5 ms bounce; electromechanical relays with a coil-suppression diode only can show 5–10 ms bounce on release. If the relay is the source, replace with a sealed or solid-state unit.
4. Add an RC snubber (100 Ω + 0.1 µF, 630 V) across the contactor coil if contactor-pickup transient is the trigger.
5. Re-route the DI loom away from motor and contactor cables. Maintain a minimum 200 mm separation; cross at 90° if unavoidable.

6. Step-by-Step Resolution Path

Run the diagnostic sequence in order. Each step is non-destructive and reversible.

1. Capture the state and control word trace at 1 ms resolution over a full operating cycle. Identify which control bit, if any, dips at O008 entry.
2. Measure PSU2 ±15 V rails at TP1/TP2 with min-hold during a 1 hour loaded run. Replace PSU2 if any dip < ±14.0 V is observed.
3. Inspect and substitute the PSU2 ↔ IVI ribbon cable. A known-good cable from a sister drive is the fastest verification.
4. Audit the K80 option: confirm fitted/not fitted, check for A017 in the buffer, verify K80 24 V supply integrity.
5. Hard-wire the Inverter Release DI temporarily with a clean 24 V source, bypassing the interposing relay. Run for 8 hours. If O008 does not recur, the relay or its wiring is the cause.
6. Replace the interposing relay with a sealed or solid-state equivalent. Add an RC snubber on the upstream contactor coil.
7. Re-run the trace and verify O008 does not appear over at least three full anchor-drop cycles spanning a full work shift.

7. Verification and Commissioning Acceptance

After corrective action, perform a structured acceptance test before returning the drive to windlass service.

8. Spare-Parts and Documentation References

For ordering PSU2 boards, IVI boards, and ribbon cables, the 6SE70 spares are typically ordered by the option-code part number on the drive's MLFB and the position designation. Cross-reference against the serial number on Siemens Industry Online Support:

• Siemens Industry Online Support — MASTERDRIVES Compendium (manual collection for 6SE70)
• Operating instructions for SIMOVERT MASTERDRIVES VC / MC (6SE70) — safety, control-word, status-word, parameter structure
• Function diagram manual — state machine O000–O255
• K80 Safe Stop option manual — only relevant if the K80 is fitted and A017 is suspect
• STARTER / DriveMonitor commissioning manual — trace tool configuration

9. Frequently Asked Questions