Resolving SINAMICS V20 F85 External Trip Fault on Startup

Resolving SINAMICS V20 F85 External Trip Fault on Startup

The SINAMICS V20 F85 external trip fault is one of the most frequently reported commissioning faults on the Siemens V20 basic line. The fault appears the moment the drive is energized because the firmware interprets an unconnected or mis-configured digital input as an external fault condition. This reference walks through the root cause, the parameter changes required to clear the fault, and the procedure to connect a motor PTC thermistor to the digital inputs for thermal protection.

F85 Fault Definition and Behavior

F85 is defined in the SINAMICS V20 parameter manual as "External Fault 1." The fault is software-driven, not hardware-driven: it is generated internally by the firmware whenever a digital input assigned to the external-fault function reads the wrong state. When the firmware raises F85 it cancels the IGBT gate signals (OFF2 reaction), opens the relay output, and logs the event in the fault buffer.

The fault behavior is consistent across the entire V20 range (FSA through FSE, 0.12 kW to 30 kW, 230 V 1AC, 230 V 3AC, and 400 V 3AC variants):

• Reaction: OFF2. The drive coasts to a stop, ignoring the configured ramp times.
• Acknowledgement: Power-on reset, P3981 = 0 (acknowledge all faults), or a rising edge on a DI configured with P070x = 9 (fault acknowledge).
• Default mapping: None. F85 is not assigned to any input on a factory-fresh V20. The fault only appears after the user explicitly maps a digital input to value 12 or 13.
• Firmware dependency: F85 is present in all V20 firmware versions from V1.0 through V5.0 SP3. The fault code is unchanged; only the fault value reported in r0949 has been extended in newer firmware.

Root Cause Analysis

When the V20 leaves the factory, parameters P0701 through P0704 are set to default values that do not include the external-fault function. The factory defaults for the digital input functions are:

• P0701 = 1 (ON/OFF1 on DI 1)
• P0702 = 12 (Reverse on DI 2)
• P0703 = 9 (Fault acknowledge on DI 3)
• P0704 = 0 (DI 4 disabled)

If the commissioning engineer changes any of P0701 through P0704 to value 12 or 13, the firmware begins monitoring that input on the next power-up. The two values select different contact logic:

• P070x = 12: External fault, normally-open (NO) contact. Closed contact = healthy state. Open contact = F85.
• P070x = 13: External fault, normally-closed (NC) contact. Open contact = healthy state. Closed contact = F85.

When commissioning offline (no motor connected, no control wiring) the engineer typically leaves the digital input terminal floating. For a NO contact, a floating input is read as "open," so the firmware raises F85 immediately on the first scan after power-up. This is the most common F85 root cause during bench setup.

A second class of F85 source is the PTC thermistor input. When P0610 (motor temperature sensor response) is set to trip and P0601 selects a PTC, an open PTC (cold motor) is healthy but a hot or shorted PTC generates a thermal fault. The exact fault code depends on the firmware version: V20 firmware 4.0 and later raise F85 for DI-mapped PTC trips, while firmware 3.x and earlier raise F11.

A third class, which corresponds to the symptom "the drive also trips the mains," is reverse wiring. If the AC supply has been landed on the motor output terminals (U, V, W) instead of the line terminals (L1, L2, L3, PE), the rectifier section sees line voltage on the DC bus. The upstream protection device (MCB, RCCB, or fused disconnect) trips within one to two line cycles. Reverse wiring is uncommon but it does happen, and the V20's internal protection does not always disconnect quickly enough to save the rectifier diodes.

Prerequisites

Before clearing the F85 fault, verify the following:

1. The drive is properly grounded. PE (protective earth) must be bonded to the cabinet star point and to the supply ground bus.
2. The mains supply is connected to L1, L2, L3 (three-phase versions) or L1, N (single-phase versions) and not to U, V, W.
3. The motor U, V, W conductors are routed to the drive output terminals and the cable shield is grounded at the drive end only.
4. The motor frame is bonded to PE via a separate conductor run alongside the motor cable.
5. The BOP (Basic Operator Panel) is plugged in or the V20 Smart Access Wi-Fi module is connected. Without one of these, parameter changes cannot be made and F85 cannot be acknowledged.
6. The upstream disconnect is locked out and tagged out before opening the drive enclosure for terminal inspection.

Step-by-Step Resolution

Step 1 - Verify the Wiring

Before changing any parameter, confirm that L1, L2, L3 are at the line terminals and U, V, W are at the motor terminals. Use a multimeter on continuity mode and trace each conductor from the upstream breaker to the drive terminal. Reverse polarity at this stage can damage the drive; if L1, L2, L3 are found on the motor terminals, the drive must be replaced because the rectifier section has been exposed to the supply for at least a brief period.

Step 2 - Identify the Digital Input Driving F85

Read the value of P0701, P0702, P0703, and P0704 on the BOP. Scroll through the parameters by pressing the up/down arrow keys on the BOP. The parameter currently assigned to value 12 or 13 is the input that is generating F85.

A faster method is to read the fault buffer. Press the OK key on the BOP, scroll to r0947[0] (fault code, most recent), and r0949[0] (fault value, indicates which DI caused the fault). The fault value for F85 contains the bit-pattern of the digital input that triggered the fault:

Step 3 - Apply the Correct External-Trip Logic

For most installations the safe choice is P070x = 12 (NO contact, failsafe on wire break). Connect a jumper or a contact from the upstream device between the DI terminal and the 24 V common (terminal 9 on most V20 variants). A closed contact will hold F85 clear. If the application genuinely requires an NC contact, set P070x = 13 and wire the NC contact between the DI and terminal 9. An open NC contact holds F85 clear; a closed NC contact raises F85.

Step 4 - Power-Cycle the Drive

Parameter changes to P0701 through P0704 do not take effect until the next power-up. With the jumper in place, power down the drive, wait at least 30 seconds for the DC bus to discharge below 5 V, and power back up. F85 should clear on the first scan.

Step 5 - Acknowledge the Fault

If F85 persists after power-up, the digital input is still reading the wrong state. Acknowledge the fault by setting P3981 = 0, by toggling a digital input configured with P070x = 9, or by power-cycling the drive a second time. The V20 stores faults in non-volatile memory; a one-time acknowledge is sufficient to clear the current fault, but the fault remains in the r0947 history buffer with a timestamp.

Step 6 - Confirm the Run Command Path

Once F85 is clear, apply a run command from the configured source. The default run command source is P0700 = 2 (terminals), with P0701 = 1 (ON/OFF1) and P0702 = 12 (reverse). If P0701 has been reassigned to external fault, ON/OFF1 must be moved to a different input:

P0700 = 2           ; Run command source = terminal default
P0702 = 1           ; DI 2 = ON/OFF1
P0703 = 12          ; DI 3 = External fault, NO
P0704 = 0           ; DI 4 = disabled

The ON command and the external-trip must not be on the same digital input. The V20 firmware does not allow P070x to be set to "command + external fault" simultaneously on the same input; attempting to do so is silently rejected and the parameter remains at its previous value.

Digital Input Configuration Reference (P0701-P0704)

The table below lists the most common values of P0701 through P0704. Only values relevant to F85 troubleshooting and standard control wiring are shown; the full list runs to value 99.

When P070x is set to 29, the firmware switches the digital input into PTC mode. The input circuit changes from a 24 V logic-level read to a low-voltage resistance measurement; the V20 measures the resistance of the connected PTC and generates F85 when the resistance crosses the trip threshold (typically 3.5 to 4.0 kΩ for a DIN 44081 PTC).

Connecting a Motor PTC to the V20 Digital Inputs

The SINAMICS V20 supports motor temperature monitoring via a PTC thermistor in two ways:

1. Via the analog input (terminals 3/4): Set P0601 = 1 (PTC) and wire the PTC leads to AI+ and AI-. P0610 selects the response.
2. Via a digital input: Set P070x = 29 and wire the PTC between the DI and terminal 9 (24 V common). The drive performs its own PTC-style resistance check on the input.

The digital-input method is preferred when no analog input is available or when the PTC wiring is short (less than 5 m). The analog-input method is preferred for long cable runs because the analog input has better noise immunity.

For a typical three-phase motor with three embedded PTC thermistors (one per phase) wired in series, the total cold resistance is around 60 to 750 Ω. At the trip temperature (typically 155 °C for insulation class F), the resistance rises sharply to over 4 kΩ. The V20 firmware interprets the rising edge of resistance as a thermal trip and raises F85.

The wiring is straightforward:

Add a 2.7 kΩ pull-down resistor in parallel with the PTC if the cable run is longer than 10 m, to keep the input biased against noise pickup. Without a pull-down, capacitive coupling on long cables can hold the input in a high state and prevent the drive from reading the PTC resistance correctly.

After wiring the PTC, configure the firmware:

P0601 = 1           ; Sensor type = PTC
P0610 = 2           ; Response = trip with F85/F11 and warning
P0701 = 29          ; DI 1 = external trip via PTC

A trip on the PTC raises F85 (if the firmware version interprets the PTC input as external fault) or F11 (motor overtemperature). Verify the fault source by reading r0949[0] (fault value) after a trip event. A value with bit 6 set (0x0040) indicates a PTC trip; a value with bit 3 set (0x0008) indicates an external fault.

Diagnostic Flowchart

Verification Procedure

After completing the steps above, run the following verification sequence to confirm F85 will not reappear under normal operation:

1. With the drive powered and the BOP attached, read r0000 (drive output frequency) and r0021 (actual output frequency). Both should be 0 Hz with no run command.
2. Read r0052 (status word bit pattern). Bit 0 = 1 indicates the drive is in the "ready" state. If bit 6 = 1, the drive is in a fault state and F85 is still pending.
3. Read r0947[0..7] (fault history). The most recent F85 should be timestamped at the last power-up. If a fresh F85 is logged after the verification steps, the input is still reading the wrong state.
4. Apply a run command and observe r0021. The drive should ramp to the setpoint with the ramp time defined by P1120.
5. Trip the external input manually (open the NO contact or close the NC contact). The drive should respond within 50 to 100 ms by issuing OFF2 and logging F85 in the fault buffer.
6. Re-acknowledge the fault and confirm normal operation resumes.

The 50 to 100 ms response time is firmware-version dependent. V20 firmware 3.x has a 60 to 80 ms debounce on digital inputs; V20 firmware 4.0 and later reduced this to 20 to 40 ms. If the application requires a faster external-fault response (safety-rated systems), the V20 is not the correct drive family; the SINAMICS G120 with safety-integrated STO/SS1 is the appropriate choice.

Troubleshooting Matrix

Related Parameters and Status Words

Beyond P0701-P0704, several other parameters interact with the F85 fault. The most useful are:

• P0010 = 30: Save parameter set. Use this to commit changes before power-down.
• P0970 = 1: Factory reset. Clears all parameters back to defaults. Use this if the drive is in an unknown state and the parameter set has been corrupted.
• P0003 = 3: User access level. Set to 3 to expose service parameters including the fault history time stamps.
• P0014 = 21: Buffer mode for BOP. Set to 21 to allow parameter changes without entering run mode.
• P3981: Fault acknowledge trigger. Set to 0 to acknowledge all pending faults.
• r0947[0..7]: Fault code history. The most recent fault is at index 0.
• r0949[0..7]: Fault value history. The fault value identifies the specific source of the fault.
• r0052: Status word. Bit 6 = 1 indicates an active fault; bit 0 = 1 indicates the drive is in the "ready" state.
• r0054: Control word. Bit 7 = 1 indicates an active fault acknowledgement is pending.

Comparison with Other SINAMICS Families

The F85 fault exists across the entire SINAMICS family, but the parameter mapping and acknowledgement method differ:

The V20's approach (P0701-P0704 with values 12 and 13) is intentionally simplified compared to the G120 and S120 families, which use BICO routing for the external fault signal. The simplified approach makes the V20 easier to commission but limits the flexibility of the external fault path. If the application requires multiple external faults, a programmable reaction, or a safety-rated external fault, the V20 is not the right drive; the G120 with safety-integrated STO/SS1 or the S120 with PROFIsafe is the correct choice.

Edge Cases and Field-Proven Caveats

Several edge cases are commonly encountered in field commissioning and are not always covered in the parameter manual:

1. Single-phase V20 on a 230 V 1AC supply: The L1, N terminals are used. If L is connected to U and N is connected to V (a common wiring error when reusing a 3-phase cable), the drive will not fault on F85 but will run with reduced output and may eventually trip with F1 (overcurrent). Always trace the conductors individually.
2. Dual-motor application (PTC shared between two V20s): The PTC resistance is read by the master drive only. If both drives are configured for PTC (P070x = 29 on both), the second drive will read an open PTC and raise F85 immediately. Use a single drive for PTC monitoring and pass the trip signal to the second drive via a relay wired to DI 1 or DI 2.
3. Hoist application (external fault from upper/lower limit switches): Wire the upper limit switch to DI 1 (P0701 = 12) and the lower limit switch to DI 2 (P0702 = 12). The drive will raise F85 on either limit and stop the hoist with OFF2. The lower limit switch should also trigger a reverse run if the hoist is in a positioning mode; this requires a small external relay ladder, not a direct V20 parameter.
4. Modbus/USS-controlled drive with external fault: When P0700 = 5 (Modbus/USS) is selected, the on-board digital inputs are still active. A F85 from a DI will stop the drive even if the run command is coming from Modbus. The Modbus control word bit 7 (fault acknowledge) can clear F85 but cannot prevent it from being raised.
5. Battery-backed control circuit: If the V20 is powered down while a DI configured for external fault is held in the fault state, the fault is logged at power-down and re-raised at power-up. The fault must be acknowledged after the DI returns to the healthy state.

Safety Notes

• Never operate the V20 with the protective earth disconnected. The internal CM filter relies on PE for safe discharge of common-mode currents.
• If the upstream breaker trips when the drive is energized, do not reset the breaker repeatedly. Identify the root cause first. Repeated resetting of a faulted drive can cause catastrophic failure of the rectifier section and pose a fire risk.
• Do not connect 230 V control signals to the digital inputs. The DI inputs are 24 V logic only. Exceeding the maximum voltage of 30 V DC will damage the input optocoupler.
• The external fault path (P070x = 12 or 13) is not safety-rated. It is suitable for process faults and equipment protection. For functional safety, use a safety-rated VFD such as the SINAMICS G120 with safety-integrated STO/SS1.

Frequently Asked Questions