Troubleshooting SINAMICS G120C PN F07807 Short-Circuit Fault on Conveyor Lines

SINAMICS G120C PN (PROFINET variant, part of the G120C compact inverter family) reports F07807 when the converter's internal monitoring detects a phase-to-phase short-circuit or a ground fault on the motor-side output stage. The fault is latched, the IGBTs are inhibited, and the drive enters a restart-inhibit state until the cause is cleared and the fault is acknowledged. On a conveyor carrying full liquor bottles, an uncommanded stop drops containers onto the belt and creates both product loss and a mechanical impact that can damage the drivetrain. Root-cause analysis is therefore mandatory; masking the fault with parameter changes is not a sustainable fix.

1. Fault Definition and Operational Impact

The fault text returned on the IOP-2 operator panel, BOP-2, Starter, Startdrive, and via PROFINET diagnostics is: "Drive: Short-circuit/ground fault detected." On the conveyor line under test, r0949 (fault value) reports the decimal value 12238, which the firmware uses to identify which phase and which current component triggered the event.

F07807 is treated as FAULT, not ALARM. The default reaction is OFF2 (pulse inhibit, motor coasts), which is exactly the behaviour that prevents further damage to the IGBT module but is also the reason the conveyor drops its load.

2. Decoding the Fault Value r0949 = 12238

The fault value is stored in r0949 as a fixed-point decimal integer. The encoding is split by digit position into a one-digit class field and a four-digit current component field:

For the conveyor under test, r0949 = 12238:

• Class digit = 1 → ground fault, with the dominant current component in phase U.
• Component digits = 2238 → current component in phase V = 2238 per mille of the ground-fault detector's internal reference. Per mille means parts per thousand, so the measured component is 2.238 × the reference. A value above ~1000 per mille normally indicates a hard ground path rather than benign capacitive leakage.

The reference used by the detector is not the motor rated current but a value scaled to the power unit's IGBT current capability. To extract the value in Starter or Startdrive, browse the expert list to r0949 and read it in decimal; the operator panel shows the same value as F07807 fault value.

3. Root Causes on Conveyor Lines

On a multi-motor conveyor the most frequent causes of F07807 are:

1. Insulation breakdown in motor windings. Moisture, contamination from product spillage, or thermal aging lowers insulation resistance until phase U leaks to PE.
2. Damaged motor cable. Crushing at the cable chain, abrasion at the cable tray, or pinch points at the conveyor frame expose the conductor to the shield or to ground.
3. Reversed cable terminations. Interchanging the line and motor cables at the drive terminal block is interpreted by the firmware as a motor-side short circuit on first enable.
4. Output-side switching. A contactor, MCCB, or hand-off-auto switch on the drive output opens or closes a live circuit. The transient current when the contactor re-closes onto a motor that is still rotating or still partially energised is detected as a short-circuit pulse.
5. Multiple motors connected to one drive. Combined cable capacitance and leakage paths can exceed the detection threshold during the test pulse.
6. PE bond missing or high-impedance. Without a low-impedance PE return path, capacitive ground current returns through the converter's measurement shunt and is misinterpreted as a fault.

4. Why ON/OFF Switching at the Motor Side Triggers F07807

The SINAMICS G120C inserts a short test pulse on each output phase at every switching cycle of the IGBTs. The pulse charges the parasitic capacitance of the motor and cable to ground; the resulting current is measured and evaluated against the ground-fault threshold. If the measured current exceeds the threshold, the firmware raises F07807 immediately and inhibits the pulses.

When a contactor between the drive and the motor is opened with the drive still enabled (no OFF1/OFF2/OFF3 first), the contactor interrupts current at an uncontrolled angle. The energy trapped in the cable and motor leakage inductance collapses into a high dV/dt event. When the contactor is later re-closed onto a rotating or partially energised motor, the inrush transient can look identical to a phase-to-ground fault, particularly on phase U because of the asymmetry between phase-to-ground capacitance inside the IGBT module. With r0949 = 12238, the firmware is reporting exactly that condition: an inrush on phase U with an unexpectedly high return through the phase V measurement channel.

The trip is therefore not a nuisance but a faithful report of a real hazard — closing a contactor on a live drive output is itself the cause.

5. Why Disabling Protection (p1901 = 0) Is Unsafe

The proposal in the original question — set p1901 = 0 to suppress the fault — must be rejected on three grounds:

1. Parameter identification. The SINAMICS G120C parameter list published by Siemens does not expose p1901 as a parameter that disables the short-circuit test pulse. The legacy p1901 from the MICROMASTER 4 family belongs to a different feature set and is not transferable to the SINAMICS G120C firmware. Writing to a non-existent parameter through Starter or Startdrive either returns an access error or creates an unused parameter; it does not silence F07807.
2. Manufacturer prohibition of output-side switching. The Siemens Application Example titled "Operation of switching elements at the inverter output" at support.industry.siemens.com / 1017401 explicitly forbids installing a contactor, MCCB, or isolating switch on the inverter output. Any switching device on the output must be coordinated with the inverter, and the drive must be inhibited before the contactor opens.
3. Compliance with IEC 61800-5-1. Adjustable-speed electrical power drive systems must provide short-circuit and ground-fault protection either internally or through coordinated external protection. Disabling the internal detection and not providing an equivalent external device violates the protective-bonding and fault-detection requirements of the standard.

6. Systematic Diagnostic Procedure

Work through the procedure below before changing any parameter that affects protection:

Figure 1 — F07807 diagnostic flow for a G120C conveyor application.

1. Read r0949 and r0948. Confirm F07807 with the same fault value. Trend r0949 over several faults: a constant value points to a fixed hardware fault; a varying value points to a switching or contactor event.
2. Read the actual output currents. Use r0069[0..2] (phase currents, smoothed) and r0077 (torque utilisation). A pre-fault spike on phase U confirms the source phase reported by r0949.
3. Capture the trip environment. In Starter or Startdrive trace, record r0069, r0080 (torque), and r0949 with a sample time of 1 ms or faster. Align traces with the conveyor start/stop commands to correlate with the contactor.
4. Isolate the drive from the cable and motor. Lock out the line supply. Disconnect the motor leads at the drive terminal block. Measure insulation with a 500 V megohmmeter between each phase and PE, then between phases.
5. Megger the cable alone. Disconnect the cable at the motor junction box and repeat the insulation test. Look for a low reading when the cable is flexed through its normal service loop.
6. Verify the motor alone. Megger the motor windings. A reading below 5 MΩ on a 400 V class motor indicates moisture or winding contamination; a reading below 1 MΩ is a hard fault.
7. Check the terminal wiring. Confirm that line and motor cables have not been swapped. The G120C terminal block is labelled L1/L2/L3 on the input side and U/V/W on the output side; reversing them forces current through the IGBT body diodes on the output side and triggers F07807 immediately on enable.
8. Inspect the cable chain. For drag-chain installations, check the minimum bend radius and the chain's tension. Replace any cable with visible jacket damage.
9. Audit the output-side switching. Identify every contactor, MCCB, or disconnect on the motor side. Move them to the line side, or replace them with the inverter's own OFF1/OFF2/OFF3 commands.

7. Cable, Motor, and Insulation Test Methods

Use a 500 V DC insulation tester (megohmmeter) for 400 V class motors. Discharge the windings for at least 30 seconds after testing before reconnecting to the drive.

For very long motor cables (above 50 m), capacitive leakage can approach the ground-fault detection threshold. Verify that p0235 (motor cable length / power unit configuration) is set within the supported value for the connected power size. Where the cable cannot be shortened, install a sinusoidal filter or output reactor to limit dV/dt and to provide a defined leakage impedance.

8. Multi-Motor Operation Rules

If more than one motor is connected to a single G120C output, follow the rules in the Siemens Application Example on "Operation of several motors on one inverter" at support.industry.siemens.com / 14390055:

• All motors must share the same rated voltage and frequency.
• The sum of the motor rated currents must not exceed the drive's rated output current (with the typical derating of 80 % per motor).
• The combined cable capacitance must not push the ground-fault test pulse above its detection threshold. Where necessary, fit output reactors (du/dt filters) to limit the dV/dt and to provide a defined ground-fault impedance.
• Each motor branch must be isolated by an output reactor, not by a contactor. Switching a contactor downstream of a reactor is acceptable only when the drive is inhibited.
• The thermal model (motor I²t) is a single-block model; p0611 cannot be used to protect several different motors simultaneously. Use external thermal protection (PTC or PT100) at each motor.

9. Correct Stopping Procedure (OFF1 / OFF2 / OFF3)

After OFF1/OFF2/OFF3 has inhibited the pulses, the output stage is galvanically isolated from the DC link by the IGBT off-state. Only then is it safe to open a downstream contactor for service work. Power isolation at the line side must occur after the drive has stopped and the DC link has discharged; the discharge time is five minutes after line-side removal for the standard G120C frames.

10. Fault Acknowledgement and Restart

1. Remove the cause of the fault (clear cable short, repair motor winding, remove output contactor).
2. Acknowledge the fault buffer by a rising edge on control word bit 7 (STW1.7) via PROFINET, or by pressing the Ack key on the BOP-2 / IOP-2.
3. Wait for the DC-link pre-charge to complete; the drive signals readiness with ZSW1.0 (ready to switch on).
4. Issue a new ON command to enable the pulses and ramp the motor to the setpoint.
5. If F07807 re-trips with the same r0949, the fault has not been physically cleared; do not attempt repeated acknowledgements.

11. Preventive Maintenance Schedule

12. Frequently Asked Questions