You report in on the night shift to discover that a solvent day tank overflowed three times that afternoon. However, operators say the tank is properly supplying the process.

The notes from the responding tech include:

  1. Tested level switch by operating rocker. Meter shows voltage change when switch operates.
  2. Simulated open signal to tank fill valve, and it functions properly. Simulation was done by forcing logic in PLC.
  3. Unable to test further, due to time constraints.
With this information on hand, what should you do next?

The tech verified the system from the PLC through its output module and the final control element. The switch works, but he didn't verify anything from that point to the PLC. This leaves the wiring and PLC input module.

First, determine the expected voltage on that switch loop and verify it with a DMM. This means at the switch and at the module (L1 if non-isolated, L1-x if isolated).Once you've verified voltages, measure across the relevant fuse(s) to ensure an open fuse isn't causing this failure.

Let's assume this is a 120V loop. You can test upstream or downstream from the PLC input module. To test:

  • Downstream — simulate a 120V switching input at the PLC input module and watch the output change.
  • Upstream — put your DMM across the PLC input module terminals and watch for 120V to appear and disappear as your partner tech manually operates the flow switch.
From here, the list of potential causes is short. You should be able to quickly find the correct one.