Troubleshooting F30021 Ground Fault on SINAMICS S120 Phase W CT

1. System Identification and Failure Pattern

The application is a steel bar guiding line consisting of four identical SINAMICS S120 drive cabinets commissioned in 2013. The drive that fails repeatedly uses Power Module 6SL3320-1TG34-1AA0. The part number that appeared in the original incident post (6SL3352-1AG34-1EA1) is a different Siemens catalog code; the field-confirmed nameplate on the failing block is the SINAMICS S120 built-in / blocksize Power Module, air-cooled, 400 V class. The Control Unit is a CU320 with a CIM (Communication Interface Module) providing the PLC interface, and the converter runs SINAMICS firmware V2.6. The motor is connected through approximately 200 m of motor cable with an output reactor (du/dt choke) at the drive output terminals. Motor duty is S1 (continuous rated) but the process duty is intermittent - 1.5 minutes stop every 3.5 minutes of run, giving a duty factor of roughly 50%.

The recurring fault signature is F30021 (ground fault) with r0069[5] exceeding the configured ground fault threshold. In 2020 three of the four identical cabinets failed with the same signature, and the failing phase is always Phase W. Replacement Power Modules received from the factory are observed to carry a physically larger current transformer in the Phase W position. The combination of same-phase recurrence, identical-cabinet reproduction, Hall-sensor offset drift measured in the failed LEM CT, and an apparent Siemens internal design change points to a population-level reliability finding, not a random component defect.

2. F30021 - Fault Code Definition and Trigger Path

F30021 is the SINAMICS ground fault message. The fault is raised by the internal ground fault monitor function block when the residual current computed from the three phase current transformers exceeds the threshold p0287[0] for longer than the time p0289. The fault text in the SINAMICS List Manual reads: "Ground fault detected - the sum of the phase currents exceeds the set threshold p0287[0] for longer than the time in p0289."

On a SINAMICS S120 Power Module the ground fault monitor is fed by three phase current transformers mounted on the AC busbars inside the Power Module. The CTs provide a secondary current proportional to the primary (line) current, and the firmware computes a residual as the vector sum of the three phase currents. In a healthy drive with no insulation breakdown and balanced phase currents the vector sum is essentially zero, limited only by CT gain tolerance and offset. Any DC or low-frequency residual above p0287[0] for p0289 ms trips F30021 and pulses the internal Safe Torque Off (STO) input of the power section, opening the IGBT gates.

The default p0287[0] in firmware V2.6 is expressed as a percentage of r0209 (the maximum converter current). The time parameter p0289 is in milliseconds. p0287[0] can be configured, but raising it to mask a recurring F30021 is the wrong response - the residual is the only warning you have of either a developing insulation breakdown or a CT drifting out of specification.

3. r0069 Parameter Structure on Firmware V2.6

r0069 is the "Phase current actual value" array. On SINAMICS S120 firmware V2.6 with a CU320 the indexing of r0069 is fixed for the relevant phase current view as follows:

r0069[5] in this firmware generation is the smoothed output of the ground fault monitor - the same signal that the F30021 comparator reads against p0287[0]. It is a read-only diagnostic in V2.6, which is why it does not appear in the free function block picker, the standard telegrams (1, 2, 3, 4, 5, 6, 7, 9, 110, 111, 116, 118, 125, 126), or the standard PROFIdrive signal list. The supported ways to bring r0069[5] into the PLC are acyclic parameter access, a configured free PZD using p2051, or a STARTER/SCOUT trace export.

4. Root Cause Analysis - Why Phase W Fails Repeatedly

Given the data - same phase fails in three of four identical drives, the CT secondary current falls from a healthy ~33 mA to 5 - 9 mA when measured offline, the CT family is rated by LEM for "DC/AC currents up to around 4x drive nominal", no real insulation fault has been demonstrated on the motor or cable, and Siemens has visibly changed the CT in current production - the failure mode is thermal aging of the LEM Hall-effect CT at the Phase W location, accelerated by the specific thermal, mechanical, and electrical environment of this application. The most likely contributors, in order of weight, are thermal stress, common-mode voltage stress, mechanical resonance, and Hall-sensor offset drift.

4.1 Thermal Stress from Internal Layout

Inside the 6SL3320-1TG34-1AA0 Power Module, the three phase CTs sit on the AC busbars near the IGBT module stack. The W-phase CT is typically the most thermally loaded of the three because it is the closest to the heat sink of the bottom IGBT module (which carries Phase W on its positive or negative rail depending on the topology). In an S120 air-cooled blocksize Power Module the heat sink is shared with the rectification stage, so the W-phase CT sees a steady-state temperature several degrees higher than U and V under continuous load. With a 50% ED and a 1.5/3.5 minute cycle the CT is also subjected to constant thermal cycling rather than a steady-state soak, which drives solder-joint fatigue and Hall-sensor bias drift.

4.2 Forced Continuous Fan Operation - A Field Patch That Masked the Real Issue

Field engineering has wired the block fan to run continuously (originally fan-on-demand from the firmware thermal model). This modification is consistent with the diagnostic conclusion above: the original fan control was inadequate to keep the W-phase CT below its maximum rated ambient. Forced-air cooling alone is not enough - the heat input to the W-phase CT is high enough that the CT is being run at or above its maximum rated temperature for long durations regardless of fan state.

4.3 Electrical Stress from the 200 m Motor Cable

With 200 m of motor cable and an output reactor, the high-frequency common-mode voltage on the motor terminals during PWM switching is the dominant electrical stress on the CT insulation. The output reactor limits the dV/dt at the motor terminals but does not eliminate common-mode current, which returns through the bearing, the cable shield, and the PE path. The CT secondary winding sees the high-frequency common-mode voltage coupled through inter-winding capacitance; over time this ages the insulation and can also cause partial discharge in the encapsulation of the LEM CT. LEM closed-loop CTs use a compensation winding on the same bobbin as the Hall sensor; any partial discharge in the encapsulation will shift the Hall sensor offset and degrade the secondary current accuracy.

4.4 Mechanical Resonance of the CT Assembly

The Phase W position in a SINAMICS S120 blocksize Power Module is on the end of the busbar stack farthest from the cabinet mounting surface. Mechanical resonance of the busbar + CT assembly under the magnetic forces of the AC current (twice-line-frequency force at 100/120 Hz) can fatigue the CT mounting, the CT's internal lead dress, and the solder joints to the CT's terminal block. Three cabinets failing in the same Phase W in 2020 is more consistent with a design-level mechanical/thermal weakness than with a random component defect.

4.5 Aging of the Hall Sensor Element

The LEM CT specified here uses a closed-loop (compensated) Hall-effect design. In this design the Hall sensor measures the flux in a small gap in the magnetic core, and a secondary compensation winding is driven by an amplifier to null the flux to zero; the current in the compensation winding is the CT's output. The Hall sensor element itself drifts with temperature and time, and the LEM datasheet specifies a maximum primary-current-equivalent offset over the operating temperature range. If the CT is operated at the upper end of its temperature range for extended periods, the offset can exceed the ground fault monitor's p0287[0] threshold even though no actual ground fault exists. The 5 - 9 mA residual measured offline on the failed CT (with no primary current) is the smoking gun: the Hall sensor offset has shifted by 15 - 27% of nominal, well beyond the LEM datasheet limit.

5. LEM Closed-Loop Hall-Effect CT Construction and Failure Modes

The LEM closed-loop Hall-effect current transducer used in the SINAMICS S120 ground fault path consists of:

• Magnetic core with a small air gap in which the Hall sensor is mounted
• Hall sensor element (InSb or GaAs die) glued to the gap surface
• Compensation (secondary) winding wound on the same bobbin, driven by an internal amplifier to null the gap flux
• Internal power supply regulator (typically ±15 V or +24 V, depending on the variant)
• Terminal block or integral cable for primary bar connection
• Encapsulation (typically PUR or epoxy) for insulation and mechanical stability

Failure modes observed in industrial service for LEM closed-loop CTs in motor drive ground fault applications include:

6. The "Bigger CT" Design Change - Engineering Interpretation

The field observation that newly received replacement PMs are fitted with a physically larger CT is a significant diagnostic clue. Siemens does not publicize internal design change histories for individual component revisions, but a CT form-factor change in a current-production Power Module is the standard Siemens response to a population-level reliability finding. The most likely interpretations, ranked by probability, are:

1. Higher thermal class CT - the replacement CT has a higher maximum ambient temperature rating (e.g. 105 °C class instead of 85 °C class) or a wider operating temperature range. This directly addresses the thermal stress in Section 4.1 and is the most likely interpretation given the field data.
2. Higher continuous current rating - the new CT is rated for a higher primary current (e.g. 5x nominal instead of 4x nominal), giving larger safety margin against the brief current limit touches at 500 A.
3. Different Hall sensor technology - the new CT may use a different Hall element (e.g. GaAs instead of InSb) with better long-term offset stability at elevated temperature.
4. Improved encapsulation - the new CT may use a partial-discharge-resistant encapsulation to address the common-mode stress in Section 4.3.

To verify which of these is in play, compare the part number stamped on the new CT against the failed CT (they should differ) and request the CT datasheet from Siemens with the PM serial number. The part number revision (e.g. -1AA0 vs -1BA0) is the Siemens internal marker for this type of change.

7. CT Bench Test Procedure (Field Validation)

To verify whether a removed CT is the source of the F30021 rather than a real insulation fault, perform the following bench test on each phase CT before installing a replacement PM. This procedure does not require a calibrated current source - a healthy LEM CT in this family will give stable, repeatable results with no primary current.

1. Remove the PM from the cabinet. Disconnect the three phase CT signal leads at the CT terminal block. Label U, V, W.
2. Connect the CT to its specified supply voltage (±15 V or +24 V per the LEM datasheet). Allow 5 minutes for thermal stabilisation.
3. With zero primary current, measure the secondary current of each phase CT with a calibrated mA meter or mV shunt. A healthy CT should read 0 ± 0.5 mA at room temperature.
4. Apply a known primary current (e.g. 0.5x nominal via a loop of cable through the CT window if it is a through-hole type, or via the primary bar if it is a bar-type). Measure secondary current and compare to the LEM datasheet turns ratio. A healthy CT will be within ± 0.5% of the nominal secondary current at this primary current.
5. Reverse the primary current (flip the loop). The secondary current should reverse with the same magnitude. A failed CT will show a different magnitude in the two directions (this is the Hall-sensor offset visible as a non-zero intercept on the I-V curve).
6. Repeat the zero-primary-current measurement at +60 °C (heat the CT with a hot-air gun, do not exceed 85 °C). On a healthy CT the offset will stay below ±1 mA; on a failing CT the offset will climb to 5 mA or more - this is the drift that triggers F30021 in service.

8. Cable and Motor Insulation Verification

Before condemning the CT, the motor and 200 m of motor cable must be ruled out as the actual source of the ground current. Use a 1 kV megohmmeter and measure phase-to-ground insulation for U, V, W with the motor disconnected and the cable end isolated. Expected values for a healthy 200 m motor cable with output reactor:

If the insulation readings are all > 100 Mohm, the source of the F30021 is the CT, not the cable or motor. Proceed to replace the PM (or, if a CT-swap procedure is available from Siemens, swap only the CT). If any phase reads < 1 Mohm, the cable, reactor, or motor is the real source and must be addressed before the new PM is installed - otherwise the new CT will also fail.

9. Monitoring r0069[5] in the PLC as a Pre-Warning

Because r0069[5] is not exposed as a free BICO source in firmware V2.6, three approaches can be used to bring it into the PLC for a pre-warning threshold. Choose based on the required update rate and the PLC platform.

9.1 Acyclic Parameter Read via PROFINET / PROFIBUS (DPV1)

The cleanest method, and the one Siemens officially supports for diagnostic parameters, is acyclic parameter access using the standard PROFIdrive parameter channel. The PLC reads r0069 index 5 by issuing a read request to parameter number 0069 sub-index 5. Update rate is one acyclic read per PLC scan (typically 50 - 200 ms) which is more than adequate for a pre-warning function.

Example S7-1500 / S7-1200 Structured Text for an acyclic read of r0069[5] (using the SINA_PARA function block or a direct RDREC / WRREC call):

// Read r0069[5] (ground fault residual) from SINAMICS S120
// Hardware ID = PROFINET device ID of the SINAMICS CU320
// Parameter number = 0069, subindex = 5

#iReq := 1;                          // 1 = read request
#iParamNo := 69;                    // parameter 69 (r0069)
#iSubIndex := 5;                    // index 5 = ground fault monitor
#iAttr := 16#10;                    // attribute = value
#iNoElem := 1;                      // read 1 element

#iDWordCnt := 4;                    // 4 bytes for Float

// Call SINA_PARA instance DB, evaluate done / error after execution
// Scale: r0069[5] is in A; convert to % of p0287[0] for pre-warning
#rResidual_A := #oValue;            // oValue returned in A
#rResidual_pct := #rResidual_A / #rGroundFaultThreshold_A * 100.0;

IF #rResidual_pct > 70.0 AND #rResidual_pct < 90.0 THEN
    #bWarning := TRUE;              // raise operator warning
    #bTrip    := FALSE;
ELSIF #rResidual_pct >= 90.0 THEN
    #bWarning := TRUE;
    #bTrip    := TRUE;              // request controlled stop, schedule CT replacement
ELSE
    #bWarning := FALSE;
    #bTrip    := FALSE;
END_IF;

9.2 Mapping r0069[5] into a Free PZD (Faster Update)

For a faster update rate (one PLC cycle instead of one acyclic read per scan), r0069[5] can be mapped into a free PZD using the BICO interconnection p2051[x] = r0069[5]. Configure p0922 = 999 (free telegram configuration) and set p2079 to identify the source of each PZD word. The PZD containing r0069[5] will then appear in the cyclic I/O image of the PLC and can be read every 1 - 4 ms.

1. In STARTER / Startdrive, set p0922 = 999 (free telegram configuration).
2. Set p2079[0] = 0 for the first PZD (output from drive to PLC).
3. Set p2051[0] = r0069[5] to map the ground fault residual into PZD word 0.
4. Save and download. Power-cycle the CU320 for the new telegram to take effect.
5. In the PLC, configure the SINAMICS telegram as a custom length of 1 word (output only, no input required for this signal).

Note: the free PZD mapping may not be supported on all V2.6 firmware variants. If p2051 rejects the value with error F01910, fall back to the acyclic read in 9.1.

9.3 Setting a Pre-Warning Threshold

With r0069[5] available in the PLC, set a pre-warning threshold at approximately 70% of p0287[0] in the same engineering units. If the residual reaches 70% of the trip threshold, raise a non-latching alarm to the operator HMI; if it reaches 90%, request a controlled stop and schedule a CT replacement. This converts F30021 from a hard trip into a predictive maintenance event.

10. Mitigation, Prevention, and Spare Strategy

The following actions are recommended in order of priority:

1. Verify the new design on order - when ordering replacement PMs, specify the latest part number revision (consult Siemens with the cabinet serial number). The "bigger CT" version is the field-proven fix.
2. Add r0069[5] pre-warning - implement the acyclic read in Section 9.1 for all four cabinets. The pre-warning will give 2 - 6 months of advance notice before a hard F30021, based on the observed drift rate of 5 - 9 mA over 7 years of service.
3. Tighten preventive maintenance - include CT thermal inspection in the annual PM. Use a thermal imaging camera on the inside of the PM with the cabinet door open and the drive at full load; the W-phase CT area should not exceed 75 °C.
4. Check the forced-fan modification - the field wiring that runs the block fan continuously is masking a thermal problem. Either keep the forced-fan wiring in place until all four PMs are replaced, or revert to firmware-controlled fan operation once the new-design PMs are installed.
5. Add a du/dt filter with integrated common-mode core - the standard output reactor at 200 m of cable does not suppress common-mode current. A Sine-wave filter or a du/dt filter with an integrated common-mode core (e.g. Siemens 6SL3000-2DE32-6EA0 family) will reduce the common-mode stress on the CT and prolong its life.
6. Verify cable and motor insulation annually - 200 m of motor cable in a steel mill environment accumulates contamination. Annual 1 kV insulation testing (Section 8) is required to rule out a real insulation fault as the F30021 source.
7. Keep one PM as a warm spare - with a known repeat failure rate, a warm spare PM is justified. Use the bench test in Section 7 to verify the spare PM's CTs before placing it in service.

11. Firmware and Hardware Revision Considerations

SINAMICS S120 firmware V2.6 is end-of-life in Siemens' lifecycle roadmap and is no longer receiving functional updates. The V2.6 firmware has the ground fault monitor output exposed at r0069[5] (this is the index that the field is using), but does not expose it as a free BICO source. Newer firmware versions (V4.x, V5.x) may have moved the ground fault monitor output to a different parameter or exposed it as a free BICO source, which would simplify the pre-warning implementation in Section 9.

If a firmware upgrade is contemplated, the following must be considered:

• Parameter mapping changes between V2.6 and V4.x+ - a STARTER project upgrade will require manual re-mapping of BICO interconnections.
• Functional changes to the ground fault monitor - the algorithm and default thresholds may differ between firmware versions. Re-validate p0287[0] and p0289 after the upgrade.
• CU320-2 vs CU320 hardware - V2.6 firmware runs on the original CU320; newer firmware may require a CU320-2. Verify hardware compatibility before planning an upgrade.
• License and configuration memory - newer firmware may require additional CF card capacity and may need re-loading of the SINAMICS safety license.

For a high-reliability application like a steel bar guiding line, the cost-benefit of a firmware upgrade is rarely positive once the drive is stable in production - the engineering effort to re-validate every BICO interconnection is significant. The recommended path is to keep V2.6, implement the pre-warning from Section 9, and replace the PMs with the new-design version as they fail. The new-design PMs are backward compatible with V2.6 firmware.

12. Frequently Asked Questions