Temporary Remote I/O Cable Replacement Causes an Unexpected Issue

During a recent Modicon remote I/O service call at a large oil refineries tank farm which had been having intermittent communication, an unusual problem was discovered. The system had been having communication drop-outs intermittently for several weeks. During that time, several F connectors, taps, J890s and sections of trunk cable had been replaced attempting to fix the problem. Unfortunately, none of these parts repaired the problem. Delta had serviced this system several years ago and is familiar with the system layout. The system is a very large system with over 5000 feet of RG-11 cable. It is a splitter type system. Delta suggested utilizing a set of fiber optic modems to isolate the two legs of the split system to increase the signal levels and assist in any future troubleshooting. Delta was commissioned to design and build a panel containing the necessary components. It was completed and shipped to the client. They mounted the panel and notified Delta that they were ready for the unit to be installed and certified. Once on-site, the Delta engineers set out to first discover the issue that initially caused the failure. A TDR was placed on the system and immediately an anomaly was noticed on the first section of trunk cable. This cable was installed overhead on telephone poles utilizing messenger cable. There was no time to properly hang the replacement cable before production had to be resumed, so the cable was installed in a temporary fashion by attaching it to a chain-link fence that was nearby. New connectors were installed and the cable connected to the system. The fiber optic panel was connected as well. The system was then ready to be certified. The TDR test was completed, then the loss measurement and attenuator tests were to be completed. Both of these were being done in an alternating fashion to save time. As the testing progressed, it was noticed that some of the test data was out of specification. The remote drops were still having retries and drop-outs. The TDR was used to reinspect the cable structure and all was found to be correct, with the exception that there was a small amount of AC on the data stream. Since the routing of the temporary cable was different from the original cable, electrical noise was suspected. A ground was installed on the tap at one end of the new cable. The AC disappeared and the errors were gone as well. Testing was resumed and all was found to be within specification. The system was returned to full production with no errors. In the process of replacing the defective cable, and choosing to install a temporary cable in a different routing location, noise was inadvertently introduced into the system. Fortunately the Delta engineers are experienced in these systems and quickly located the introduced problem.

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