How to Fix ABB ACS550 Fault F0021 Current Measure Fault Guide

Staring at F0021 on your ABB ACS550? Learn what the Current Measure Fault means, how to troubleshoot internal hardware, and when to call for a repair.

 

Solving ABB ACS550 Fault F0021: What You Need to Know

If your ABB ACS550 drive has suddenly tripped with Fault F0021, it can be a bit more stressful than a standard trip. This code stands for Current Measure Fault, and it usually points to an issue inside the drive’s "nervous system."

In this guide, we’ll break down what is happening inside the drive and the steps you can take to figure out if it's a temporary glitch or a hardware failure.

What is Fault F0021?

Every time you start or run an ACS550, it constantly monitors the current flowing to the motor using internal sensors (Current Transducers). Fault F0021 occurs when the drive’s control board detects that the current measurement circuit itself is not working correctly.

Essentially, the drive doesn't trust its own "eyes," so it shuts down to prevent any potential damage to the power electronics.

Common Causes of F0021

• The "Ghost" Glitch: A temporary electrical surge or EMI (Electromagnetic Interference) has confused the sensing circuit.
• Loose Internal Ribbon Cables: The connection between the power board and the control board is vibrating loose.
• Faulty Current Transducer (CT): The physical sensor inside the drive has failed.
• Damaged Control Board: The internal "brain" of the drive can no longer process current signals.

Troubleshooting Steps: The Practical Approach

1. The "Cold Boot" (Power Cycle)

Before you start taking things apart, try the simplest fix first.

• Turn off the main power to the drive.
• Wait for the display to go completely dark (usually takes 1-3 minutes for capacitors to discharge).
• Restore power.

If F0021 was caused by a minor logic glitch or temporary interference, this "cold boot" will clear it.

2. Isolate the Drive from the Motor

To confirm the fault is definitely internal:

• Disconnect the motor wires from terminals U2, V2, and W2.
• Try to start the drive without the motor.
• If F0021 still appears: The problem is 100% inside the drive hardware.
• If the drive runs fine: (You might get an "Earth Fault" or "Motor Phase Loss," which is normal without a motor), then you may have a severe short circuit in your motor or cables that is overwhelming the sensors.

3. Check the Internal Connections

Note: Only do this if you are qualified to work on high-voltage equipment!

• Open the drive cover and locate the gray or flat ribbon cables that connect the OMIO Control Board to the Power Board.
• Reseat these cables firmly. Over years of operation, thermal expansion and vibration can cause these to drift just enough to break the current signal.

4. Look for Signs of Contamination

Since many ACS550s live in dusty or humid environments, check for conductive dust or moisture on the circuit boards. A small "bridge" of dust across a current sensing pin can trigger an F0021 fault.

How to Reset Fault F0021

1. Correct the connection issue or perform a power cycle.
2. Press the RESET button on the ACS550 keypad.
3. If the fault is still present, the drive likely has a hardware failure in the current measurement circuit (usually a bad CT or board).

When is it Time to Replace the Drive?

Because the ACS550 is a legacy product, repair parts like current transducers or main power boards can sometimes be expensive. If the fault F0021 persists after you've checked the cables and cycled the power, it is often more cost-effective to upgrade to a modern ACS580 rather than attempting a board-level repair.

Summary

The ABB ACS550 F0021 fault is the drive’s way of saying it can no longer safely measure current. While it often signals a hardware failure, a simple power cycle or reseating internal ribbon cables can often save the day.