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Multiple Grounding Points Can Cause DCS Failures

Oct 1, 2000 12:00 PM, By John Paschal

If your facility has a modern distributed control system with a reading "offset" problem, the grounding system could be the cause.and the remedy.

One of the really great things about modern distributed control systems (DCSs) is once they're set up and operational, readings are steady and repeatable. In fact, they're so reliable that common programming relies on the DCS to serve as the watchdog over many processes with full authority to "trip" when values stray outside of predefined "windows" or maximum and minimum values. This is a valuable feature until the DCS trips the system due to a faulty value or, even worse, it trips at random times that are seemingly unrelated.

Recent history has shown multiple grounds can change the values the DCS computer reads, thus causing erroneous output values, that fall outside of operational limits. These multiple grounds can come from the most unexpected points.

Every DCS installation has a safety ground that's essentially the same as the static grounding system. The system also has a reference ground you can temporarily disconnect from the safety ground for troubleshooting purposes. However, you'll typically only find both connected together at the DCS computer bus. You won't find cable shields and shield-drain wires connected to ground at field junction boxes, because doing this causes a parallel path where circulating currents of varying magnitudes could flow. This situation also compromises cable shielding. Accidental grounds anywhere away from the main DCS reference grounding bus can cause circulating currents. These varying loop values can show up as valid data in the DCS computer as it crunches numbers, which leads to inaccurate results.

To produce shielded cables, manufacturers shield the twisted pair with a foil layer that's in contact with an uninsulated "drain" wire. They cover these three conductors with a jacket (sometimes with armor and an additional jacket). When terminating a cable in the marshalling cabinet, the jacket is frequently stripped at the cable gland where the cable passes through the sheet metal of the grounded marshalling cabinet enclosure. This leads to a myriad of twisted wire pairs and drain wires traveling vertically down into the marshalling cabinet section within the plastic wireways until each "peals off" to land on its appointed terminal block.

This type of configuration places the drain wires in intimate contact with one another at multiple locations. Normally, this arrangement wouldn't be a problem except that metal screws (whose screw heads are on the cable side of the plastic wireway) typically hold the plastic wireway into the marshalling cabinet. At this location, one drain wire touching a screw head electrically connected to the grounded marshalling cabinet leads to an offset voltage or circulating currents.

The only positive "fix" is to remove each drain wire from its terminal block and slide insulating spaghetti up to the point where the jacket ends in the gland. This simple solution insulates the drain wire from the gland, from other drain wires, and from the wireway mounting screws.

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