Quarantine your physical layer defects

June 23, 2020
Isolate and root out dodgy network installations that yield intermittent errors

A valve positioner had been consistently showing up in diagnostics, and the site’s ability to view its troubles over time helped convince maintenance to plan a replacement. So last fall they pulled and rebuilt the entire valve and replaced the positioner with a new one, including the latest Hall effect position feedback array. Everything checked out well on the bench and after the valve was reinstalled. But come spring, the communication “drops” began showing up in the event logs again. It was curiously intermittent—there would be no errors for days, and then another shift log would complain of fleeting alarms that typically cleared immediately.

The instrument tech was dispatched to check the terminal housing for signs of corruption or loose wires; he found the enclosure dry and terminals tight. Was the network itself corrupted? The site had obtained a new diagnostics tool to try out, a Pepperl+Fuchs FDH-1 (fieldbus diagnostic handheld), which they connected to the junction box in the field. The tests showed all communications were running perfectly, and even a couple shifts in history mode (where the tool gathers data continuously, or until its memory is consumed) revealed no defects. Ultimately, a work order was written to replace the fieldbus cable (a single 18 AWG twisted shielded pair), the spur from the field junction box to the positioner.

Electricians discovered a tortuous and lengthy path to the field junction box. The task would also require operations to run the loop on hand jack (a mechanical local override of the electro-pneumatic positioner). So, before undertaking the somewhat daunting task, they decided to “megger” the cable (test with high voltage) to see if the insulation was defective. While the test revealed no problem, a nearby fitting was disassembled to install a cordset. And there they found where the cable had been butt-spliced: a crimped connector had been used to connect and extend the conductors and shield. The butt splices are gone now.

The butt splice had functioned fine for at least a decade; a professionally installed splice should look not-much-different than terminals. Perhaps it was degrading the network in some subtle fashion for years, but it took 10-15 years of corrosion, moisture and vibration for the defect to create a nuisance, or so it was surmised (the comm errors stopped). This facility operates in a region where they’re fortunate to have experienced, professional electricians, but many places find it more of a struggle. Hidden defects in the physical layer can create perplexing issues, and many times those who inherit the infrastructure are discouraged, reverting to analog solutions. How are we to step through these conundrums and embrace the digital plant?

Basement guy 

I remember being introduced as a “basement” guy at the ARC Advisory Group Forum many years ago, and I suppose that means I’m engaged in the base of the pyramid, where the rubber meets the road, if you will. Looking after the physical layer—the cables, terminations and accoutrements of any digital network—is, like good hand hygiene, the foundation for sustaining a healthy system.

The current economic climate may lend itself to tidying up the basement. Diagnostics and event journals can be mined the old-fashioned way (by reading them) to reveal and prioritize bad actors. Write a work order to purchase torque screwdrivers, and check terminal tightness from the DCS to junction boxes to the end devices, bearing in mind the risks and potential consequences of a loose wire falling out or grounding out a current loop. The integrity, continuity, isolation and grounding of cable shields are crucial to digital communications, so taking the time to inspect and polish old installs will pay dividends in future digital endeavors.

If you’re a fan of Greg McMillan’s advice, you know he insists that final control elements be functioning reliably and repeatably before optimizing tuning or entertaining thoughts of advanced control. That’s a basement perspective, and the same principles should be applied to physical layer integrity, if we expect our digital plant to have a solid foundation.

About the Author

John Rezabek | Contributing Editor

John Rezabek is a contributing editor to Control

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