You reported in on the nightshift to discover that a solvent day tank sometimes overflows and sometimes runs dry. The consequences alternate between solvent spills and ruined product batches. You found extensive notes from the dayshift, showing methodical testing in the entire process loop, including point to point continuity tests. The I/O modules work properly, and simulated signals to the control valve provide the correct response.
With this information on hand, what should you do next?
This is an intermittent problem, not a steady-state one. Look again at those point-to-point wiring checks. Were these done with a DMM? Although a DMM is a fantastic troubleshooting tool, it does have limitations. One of those is it runs on a 9V battery — which means it can't test insulation.
Suppose the signal wiring runs through a high temperature area, and over time its insulation has deteriorated. You may have intermittent problems due to cyclical current leakage to ground. Problems will persist until the insulation breaks down completely and you get a dead short.
The insulation of any wiring in the system may be undergoing progressive deterioration. How do you check for it? You use an insulation resistance tester. How do you fix it? Replace the wiring, of course. How do you prevent a recurrence? Rerouting for temperature protection is a good first step, but it's not just heat that can cause insulation damage. Improper bonding can result in insulation punctures when voltage potentials reach flashover stage. In addition, look at the facility's transient voltage protection system.
