Applicable Systems
FC-30
FC-51
FC-101
FC-202
FC-302
Symptoms
The drive shows a warning or alarm. The display shows Alarm 2, indicating a live zero error. An analog input signal configured for live zero (4-20mA) has dropped below the minimum live zero threshold.
Possible Causes
- Analog signal wire disconnected or broken
- Sensor or transmitter powered off or failed
- Analog input wiring loose at the terminal block
- Signal current dropped below 4mA due to sensor fault
Troubleshooting Steps
- Step 1: Check the analog input wiring at the drive terminal
Wire is connected and the terminal screw is tight - Step 2: Measure the current on the analog input with a multimeter in series
Current is between 4mA and 20mA in normal operation - Step 3: Verify the sensor or transmitter is powered and operational
Sensor is powered and producing a valid output signal - Step 4: If the sensor is faulty, replace or repair it
Analog signal returns to the 4-20mA range
How to Reset
To reset: 1) Restore the analog input signal to the valid range (4-20mA). 2) The alarm should clear automatically. 3) If not, press RESET. Caution: Configure the analog input to either trip or use a fallback value on signal loss, depending on safety requirements.
Frequently Asked Questions
- Q: What is a live zero signal?
A live zero means the minimum signal value (e.g., 4mA in a 4-20mA loop) represents 0%, not 0mA. This allows the drive to detect a broken wire: if current drops below 4mA, a fault is generated.
- Q: How do I configure the analog input for live zero?
In the drive parameters, set the analog input type to 4-20mA and enable the live zero detection. Set parameter 6-1x for the terminal and configure the live zero timeout and action.
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Source: Danfoss VLT FC-101 Design Guide, 2023
Category: PLC & VFD
Published: 2026-10-20
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