Description

Fault 21 in ABB inverters (particularly the ACS510. ACS550. and older ACS400 series) indicates a Hardware Fault specifically related to Current Measurement. This error triggers when the drive’s internal diagnostic system detects an inconsistency in the current feedback loop, often implying a failure in the current transformers (CTs), the measurement circuitry on the control board, or a short circuit in the power module (IGBTs). Unlike simple overload alarms, Fault 21 usually requires hardware inspection and potentially component replacement, as it signifies that the drive can no longer safely monitor motor current.

Application Scenarios

In a wastewater treatment plant, a critical lift station pump controlled by an ABB ACS510 suddenly tripped with Fault 21 during a storm. The operations team attempted to reset the drive, but the fault reappeared immediately upon start command. An onsite technician suspected a lightning-induced surge had damaged the current sensing circuit. Upon inspection, the IGBT module was found intact, but the operational amplifier chip responsible for processing the CT signals on the control board was fried. Replacing the control board (or repairing the specific op-amp circuit) resolved the Fault 21. restoring the pump within 2 hours and preventing a sewage overflow incident. This scenario highlights how Fault 21 often points to sensitive electronics damage rather than simple motor issues.