SINAMICS G120 F07801 Overcurrent Fault with Output Contactor

The Siemens SINAMICS G120 modular frequency converter family is widely deployed in pump, fan, conveyor, and compressor applications. In some legacy installations, a contactor is installed on the motor-side output of the VFD. When the same digital input (DI) that energizes the contactor is also wired to the ON/OFF1 command source of the drive, the system trips with fault F07801 (Drive: Motor overcurrent) on every stop attempt. This article documents the root cause, the affected firmware revisions, the underlying stop-command architecture (OFF1 / OFF2 / OFF3), and three field-proven remedies with full parameter configuration, commissioning steps, and verification.

1. Problem Definition

The reported configuration is:

• Drive: Siemens SINAMICS G120 with Control Unit CU240E-2 (or CU240B-2, CU230P-2, CU250S-2) and Power Module PM240-2, PM240P-2, PM250, or PM260.
• Output contactor: Installed between the VFD output terminals (U2 / V2 / W2) and the motor terminals (U / V / W).
• Command wiring: Coil of the output contactor is wired to the same digital input that is configured as the ON / OFF1 command source. On a default-configured CU240E-2, this is P0840[0] = r722.0 (DI0).
• Stop sequence: Stop is performed by removing the start command from the DI.

Symptom: Within 100–500 ms of the DI dropping out, the drive trips F07801 with the trace showing 200–400 % of rated motor current at the moment of contactor opening. The fault is recorded as OFF1 stop executed in the sequence control log, but the overcurrent threshold is breached during the dropout transient. No fault is recorded on the input supply side. The fault can be acknowledged and the drive restarted, but the trip repeats on every stop attempt. The drive does not trip on run-up or during steady-state operation, only during stop.

2. F07801 Fault Code Analysis

F07801 is defined in the SINAMICS G120 List Manual (function block diagram 8020, fault buffer 1) as Drive: Motor overcurrent. The full fault value, reaction, and acknowledgement are summarized below.

The fault is raised when one of the following conditions is detected by the Sensor Module / Control Unit firmware:

1. Motor phase current r0068 exceeds 200 % of r0207[0] (rated motor current, taken from P0305) for longer than the firmware-dependent integration time (typically 1–2 ms on firmware V4.7 SP3 through V5.2 SP3).
2. DC-link current r0070 exceeds the IGBT module limit defined by r0209[0] (rated power module current) × 1.6 (PM240-2) or × 2.0 (PM240P-2).
3. Short-circuit detection trips on firmware ≥ V4.7 when I_phase > 2.2 × r0207[0] is sustained for > 100 µs.
4. Ground-fault detection (F07802 shares the front-end filter) can be re-routed to F07801 on firmware V4.7 SP6 and later when parameter P0280[0] is set to 1 (current-controlled ground-fault monitoring).

The fault value field (r0949[0...63]) contains the following decoder:

Reading the fault value immediately after the trip tells you which detection path fired. On contactor-dropout trips the value is almost always 0x0007 (all three phase bits set) or 0x000F (phases + DC-link), which confirms a true external interruption rather than a winding or cable fault.

3. Root Cause: Contactor Dropout Under Load

When a single DI is wired to both the contactor coil and the ON/OFF1 command, the de-energization event is concurrent:

1. The PLC or operator removes the start command.
2. The DI transitions from 24 V to 0 V.
3. The contactor coil de-energizes. Mechanical dropout time for a typical 40 A AC-3 contactor is 10–25 ms; the contacts separate.
4. At the same time, the drive receives OFF1 on the same DI and begins a controlled ramp-down. The default ramp-down time is P1121[0] = 10 s (or whatever the commissioning engineer set).
5. While the contactor is opening, the drive is still in the active state, still driving PWM into the motor stator. The motor is still rotating and is still drawing magnetizing current.
6. When the last contact pole parts, the energy stored in the cable and motor leakage inductance L_σ is interrupted. The voltage across the parting contacts spikes to V = L_σ × di/dt. With L_σ ≈ 1 mH and di/dt ≈ 1000 A/s during arcing, the peak transient reaches 1 kV and the current measurement ADC saturates.
7. The firmware sees phase current above the 200 % threshold and raises F07801. Pulses are cancelled by the OFF2 reaction.

The condition is deterministic: every stop attempt, exactly the same sequence, produces the same fault. The fault is not a sign of undersized contactor or undersized cable; it is a sequencing bug.

4. SINAMICS G120 Stop Command Architecture

SINAMICS drives implement three distinct stop functions, each with different behaviour toward the IGBT gates and toward the motor. Knowing which one is appropriate for an output contactor is the entire solution.

All three commands are level-triggered except ON, which is edge-triggered (rising edge on DI → ON). The drive will only run while OFF1, OFF2, and OFF3 are simultaneously at 1. The default of 1 for OFF2 and OFF3 is a BICO source that is permanently high; it must be held high by external logic.

Reference: SINAMICS G120 List Manual, section 7.4 (Control words / command sources) and function block 2520 (sequencer).

5. Solution 1 — Use OFF2 to Block Pulses Before Contactor Drop

The cleanest and most cost-free fix is to convert the single-wire start/stop into a two-wire arrangement in which OFF2 is forced low before the contactor coil is de-energized. The drive cancels its pulses within 250 µs; the contactor can then open with no current flowing. Typical sequence:

1. PLC sets OFF1 = 0 (start of stop sequence).
2. Drive ramps along P1121.
3. PLC monitors r0063 (actual speed filtered) and waits for r0063 < P2161[0] (default 0.5 rpm, parameter "off1 n_min"). Or the PLC waits a fixed time ≥ P1121 + 0.5 s.
4. PLC sets OFF2 = 0 by driving P0845[0] = 0 through r722.x, i.e. the DI that is wired to OFF2 is forced low.
5. Drive cancels pulses within 250 µs. Motor current goes to 0 within 1–2 ms.
6. PLC waits 50 ms for the current to decay fully, then opens the output contactor.

Parameter set for Solution 1 on CU240E-2 with firmware V4.7 SP3 or later:

Wiring change: move the OFF2 source from a virtual 1 to DI1, then wire DI1 to a separate output of the PLC that drops 50 ms after DI0. DI0 drives the contactor coil; DI1 drives the OFF2 input. The PLC software must enforce the 50 ms interlock.

6. Solution 2 — Two-Stage Control with Dropout Delay and Bypass DI

When the PLC cannot be reprogrammed (brownfield or vendor-locked) the sequencing can be done in hardware with a true leading control: a dropout-delayed contactor and a small latching relay wired such that OFF2 drops first and the contactor drops second. The standard scheme is:

Hardware parts list for Solution 2:

The PLC still sees only a single start/stop output. The drop-out delay timer 3RP1505-1BW30 (Siemens Sirius) gives an adjustable 0.05–10 s dropout delay; set it to 0.1 s. The auxiliary relay 3RH2911-1HA11 is wired with its coil in parallel with the contactor coil; its NC contact feeds the OFF2 input. The coil de-energization event propagates to OFF2 first (within 10 ms), then the contactor drops 100 ms later, by which time the drive is in pulse inhibit and current is zero.

7. Solution 3 — Eliminate the Output Contactor

The IGBTs of the SINAMICS G120 provide a galvanically isolated output when pulses are inhibited. The output terminals sit at DC-link midpoint potential, not at mains potential. If the only purpose of the output contactor is to provide lockable isolation for maintenance, the contactor can be removed and an input contactor (or the integrated STO / SS1 safety function of the CU250S-2) can be used instead. The relevant standards are:

• IEC 60204-1 §6.5.1 — Means of isolation shall be provided for every energy supply. Either the input contactor of the drive or the disconnect switch on the line side satisfies this requirement.
• NFPA 79 §6.2.3 — A single disconnecting means is permitted on the supply side of a drive; a separate disconnect on the motor side is not required.
• EN 61800-5-1 §5.2.4 — Electronic protective circuits that satisfy the isolation requirements are acceptable alternatives to electromechanical switching.

For an ATEX / IECEx motor where the drive is installed outside the hazardous area, the standard scheme is to use a SINAMICS G120 with the safe torque off (STO) safety function integrated in the CU250S-2 Control Unit, which is certified to SIL 3 / PL e per IEC 61508 / ISO 13849-1. In that case the output contactor is not required for safety isolation; the input contactor plus STO is sufficient.

If the application genuinely requires a motor-side disconnect (for example, a bypass circuit that connects the motor directly to the mains through a separate contactor for emergency operation at fixed frequency), the contactor must be interlocked with the drive such that the drive is in OFF2 / pulse inhibit before the bypass contactor is allowed to close. The standard bypass wiring is shown in the Siemens application manual "Bypass circuit for SINAMICS G120" (entry ID 91157568 in the Siemens Industry Online Support).

8. Parameter Configuration Reference Table

Complete BICO mapping for the three solutions above, suitable for direct transfer via STARTER (≥ V5.4) or TIA Portal (≥ V15.1 with HSP0185 / HSP0282 G120 V4.7 SP3+ or V17 with HSP0282).

9. Wiring Topology and Sequence Diagram

The diagram below shows the topology for Solution 1, the most commonly applied fix. The PLC has two outputs, Q0 and Q1, both sourcing 24 V. Q0 drives the contactor coil K1. Q1 drives DI1 on the drive, which is wired to P0845 (OFF2 #2). The interlocks are implemented in PLC logic (ladder / SCL) and not in the drive.

For a brownfield installation where the existing wiring drives the contactor coil directly from the start DI, the Q0 / K1 separation can be implemented by inserting a latching relay (Siemens 3RH2911-1HA11 + 3RT2916-1BA31 varistor suppressor module) such that the contactor is held closed by the latching relay and the latching relay is unlatched only after the drive confirms pulse inhibit. The drive confirmation is read back from r0899.0 (status word bit 0 — "Ready to switch on / pulses enabled" inverted) over PROFINET / PROFIBUS, or alternatively from a free digital output of the drive wired to r0899.0 via BICO.

10. Commissioning and Verification Procedure

Follow this procedure to verify any of the three solutions on a live drive. Tools required: STARTER (V5.4 or later) or TIA Portal (V15.1 + HSP0185 or V17 + HSP0282), the SINAMICS G120 Control Unit in question, a configured PC connection (USB or PROFINET), and a stopwatch.

1. Capture baseline. Connect STARTER / TIA Portal. Open the trace function and start a 2 s trace on r0068 (motor phase current), r0063 (actual speed), r0899 (status word), and r723 (digital input status, bit-mapped). Trigger on "status word bit 2 = 0" (drive in OFF2 / fault).
2. Apply Solution 1 / 2 / 3 parameters. Write the parameters from §8 into CDS0. Save with RAM to ROM (P0971 = 1) and back up the project.
3. Functional test — start. Issue a start command. Confirm r0899.0 = 1 within 50 ms of the start edge. Confirm r0063 ramps to the setpoint. Confirm r0068 does not exceed 150 % of rated motor current (P0305) during run-up.
4. Functional test — normal stop (OFF1 only). Remove the start command. Confirm r0063 ramps to 0 along P1121. Confirm the drive does not trip F07801. The contactor should remain closed throughout the ramp and only open 50 ms after the drive reaches n = 0.
5. Functional test — fast stop (OFF2 only). Force the OFF2 input low (DI1 = 0 in Solution 1, or auxiliary contact in Solution 2). Confirm r0063 drops to 0 within 250 µs. Confirm r0068 drops to 0 within 2 ms. Confirm the contactor is allowed to drop 50 ms later. Confirm no F07801.
6. Functional test — bypass / contactor-drop-under-load simulation. With the drive at n = 0 in pulse inhibit, force the contactor open directly (mechanically or by removing the latching relay). Confirm no F07801, no F07802, no fault. This proves the sequencing is correct.
7. Fault injection test. Temporarily re-route the contactor coil to the same DI as the start command (recreate the original bad wiring) and run one stop. Confirm the F07801 returns with the expected fault value (0x0007 or 0x000F). Restore the corrected wiring.
8. Document. Save the trace, the parameter backup, the wiring diagram, and the test record. The parameter backup is the *.dnx or *.par file written from STARTER / TIA Portal and is the official commissioning record.

11. Safety Considerations and Standards

The following standards and clauses govern this application. None of them strictly require a motor-side contactor; they do, however, require a documented means of isolation and a documented means of emergency stop.

If the output contactor is retained as a physical lockable disconnect (lockout / tagout — LOTO) per OSHA 29 CFR 1910.147, the following interlocks are mandatory:

1. The contactor must be interlocked with the drive such that the drive is in pulse inhibit (OFF2 / STO active) before the contactor is allowed to open. The sequencing in §5 and §6 implements this interlock.
2. The contactor must be a "disconnect" type (visible blade or visible flag) per NFPA 79 §6.2.2.
3. The LOTO procedure must be documented and trained. The drive's integrated STO is an additional safety function; it is not a substitute for LOTO.

12. Frequently Asked Questions