Applicable Systems
MNS-MCC
Symptoms
The Modbus RTU communication to the motor protection relay is lost. The HMI shows Comm Fail and the last known current value is frozen. The relay itself continues to protect the motor.
Possible Causes
- Loose RS-485 termination resistor or incorrect bus topology
- EMI coupling into the communication cable routed near power cables
- Incorrect baud rate or parity setting in the relay
- Failed communication port on the relay or PLC gateway
Troubleshooting Steps
- Step 1: Check the RS-485 wiring A/B polarity and verify the 120 Ohm termination at both bus ends
Signal integrity is restored; oscilloscope shows clean differential signal - Step 2: Reroute the communication cable away from power cables (>300 mm separation) and verify shield grounding
Communication error rate drops to zero - Step 3: Compare the relay baud rate, parity, and station address with the PLC program settings
All parameters match exactly (typical: 19200 baud, even parity, address 1-247) - Step 4: Swap the RS-485 cable with a known-good cable and test communication
Communication restored immediately with the replacement cable
How to Reset
Reset the communication alarm on the HMI by acknowledging the fault after restoring the physical link. The relay protection functions remain active regardless of communication status.
Common Replacement Parts
- 1SFA619550R1071: ABB M101 motor protection relay communication module (Clip-on module for M10x series relays)
Frequently Asked Questions
- Q: Does the motor lose protection during a comm failure?
No. The relay operates autonomously. Only remote monitoring and control are affected.
- Q: What is the maximum RS-485 cable length?
Up to 1200 m at 9600 baud with proper termination and shielding. Higher baud rates require shorter runs.
Source: ABB MNS-MCC Technical Manual, Edition 04/2022
Category: Electrical