Issue
During installation and commissioning of a MNL-800, a component labeled MFC21 on the backplane was damaged. Is it necessary to replace the backplane?
It seems like the MNL-800 is still completely functional. It's reading all of its inputs and the program logic is controlling the outputs accordingly.
Environment
MNL-800
Cause
The issue is typically a ground loop between full-wave and half-wave devices or power supplies connected to the UIs and / or AOs. MFC21 is a Multi-function Ceramic protection capacitor / varistor component connected between the internal +15 VDC and earth ground (not common). This and the other 28 varistors on the base board provide various forms of protection. MFCs have burned out in situations that contain one or more transformers and a combination of full-wave / half-wave devices. Connections (common) between these devices can cause undesired ground loops (especially where transmitters are involved). It may also be related to how the transformers are wired (e.g. 24H / 24G or one side tied to earth ground). The MNL-800 is a full-wave power supply device.
Resolution
Review the power wiring of all connected I/O.
Replacing the backplane is ideal because it will provide proper protection for the next voltage spike.
Without a functional MFC21, the logic card may get damaged.
