Troubleshooting "External Fault" on Your VFD

Is your drive stopping with an "External Fault" (F0006, EPF, or EXT)? Learn how to trace the signal back to your safety circuits, sensors, and PLC logic.

 

External Fault: When the Drive is Just the Messenger

When your VFD trips on an External Fault (often coded as F0085, EPF0, EXT, or F0006), the drive is usually perfectly healthy. It isn't tripping because of its own internal physics—it's tripping because you told it to.

An external fault is a programmed reaction to a signal received on one of the drive's digital inputs. The drive is acting as a "security guard," shutting down the motor because another piece of equipment in the system is screaming for help.

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Where Does the Signal Come From?

The drive is monitoring a specific terminal. If the voltage on that terminal drops (or rises, depending on logic), the drive trips. Common sources include:

1. Motor Thermistors (PTC/KTY)

If your motor has internal temperature sensors wired back to the drive's digital inputs, the drive will trigger an "External Fault" if the motor windings get too hot.

2. Emergency Stop (E-Stop) Chains

In many legacy systems, the E-Stop button isn't wired to a safety relay; it's wired directly to a digital input on the drive. If the button is pressed or a wire is loose, the drive stops.

3. External Safety Devices

• Vibration Switches: Common on large cooling tower fans.

• Pull-Cords: Found on long conveyor belts.

• Flow Switches: Used in pumping systems to prevent the pump from running dry.

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The Troubleshooting "Paper Trail"

Step 1: Identify the Input Terminal

You need to know which terminal the drive is looking at.

• Siemens: Check parameter p2106 (External Fault source). It will point to a digital input (e.g., r722.3 for DI3).

• Schneider/Altivar: Look at the [Fault Management] menu under External Fault.

Step 2: Check the Physical Signal

Once you know the terminal (let's say DI3), use a multimeter to measure the voltage between that terminal and Common (DICOM/24V).

• If you see 0V: The external circuit is open. There is a tripped sensor, a pressed E-Stop, or a broken wire.

• If you see 24V: The drive is receiving the "Healthy" signal, but may have a "latched" fault that needs a manual reset.

Step 3: Bridge the Input (For Testing Only!)

WARNING: Only do this for diagnostics. Never bypass safety devices for normal operation. To prove the drive is fine, you can temporarily jump a 24V signal directly to the designated digital input. If the "External Fault" clears, you have confirmed 100% that the issue is in the field wiring or the external sensor.

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Common Fault Codes by Brand

Brand	Code	Typical Meaning
Siemens	F0085 / F0006	External fault triggered via BICO (p2106).
Schneider	EPF1 / EPF2	External fault via digital input or communication bus.
ABB	7081 / EF	External fault signal received.
Allen-Bradley	F2	Auxiliary Input trip.

An External Fault is the drive doing exactly what it was programmed to do: listen to the rest of the machine. Don't waste time replacing the drive; grab your schematics, find the digital input assigned to that fault, and follow the wire until you find the open switch or the broken terminal.