Control system power and grounding forensic exam aids reliability

Before replacing an aging control system, the engineer willing to conduct a forensic examination increases the odds the new system will perform reliably. See power and grounding tips.


The engineer should do a forensic investigation of the existing system, which will prove invaluable when it comes time to engineering its replacement. Courtesy: Dave Harrold

Industrial automation and control technologies don't operate reliably for several decades by accident. Those persons willing to learn the secrets behind that reliability can ensure replacement systems perform equally well. This includes appropriate power and grounding.

With new cellphones, tablets, and computer platforms being introduced every few months, it's understandable to think that discussions about 15-, 20-, and even 25-year-old technologies is anything more than a history lesson. While that may be true about the technologies we personally use, things move a bit slower when it comes to industrial technologies.

In fact, it's not uncommon for young engineers at their first job to encounter industrial control systems named Provox (from Fisher, now Emerson), TDC2000 (Honeywell), IA (Foxboro, then Invensys, now Schneider Electric), RS3 (Rosemount, now Emerson), INFI90 (Bailey, now ABB), etc. Many of these system could be older than the young engineer tasked with replacing them.

The upside is that young engineers may be in the right place at the right time to be part of a major control system replacement project, which likely appeals to the technology geek for most. Before ripping and slashing that existing system however, an engineer should do a forensic investigation of the existing system, which will prove invaluable when it comes time to engineering its replacement.

Best-practices power and grounding

Hopefully, the forensic investigation will reveal that the installed system's power is optically or inductively isolated; grounded to a single point also known as a "star" ground (a design that minimizes ground loops); and there is proper separation of different cable types.

Engineers willing to do such a forensic examination might find the project daunting because it isn't the kind of thing that is generally taught in college. And if it were taught, it might not have seemed important or relevant at the time. Short of heading back to school and taking some electrical engineering classes, the company could hire an electrical contractor or control system vendor to conduct the forensic examination and prepare "as-built" documentation, but that's generally a tough sell to management. A more palatable solution is to do some research through books and other sources and compare the installed system with the descriptions and diagrams found through research.

Because time and costs are a factor, a forensic examination project might be turned over to a couple of co-op students or summer interns. The engineer can supervise while the interns document what's right and wrong about the power & grounding of the installed system.

Grounding tips for controls

Power & grounding tips for control systems include:

  1. Control system ac power should be supplied from a distribution system separate from other equipment and uses.
  2. The power source should be designed to accommodate initial inrush currents that can last up to 10 cycles.
  3. Control system ac power should be supplied through an isolation transformer or uninterruptible power supply (UPS).
  4. Control system ac ground should be established at or near the isolation transformer or UPS.
  5. Control system workstation ac power should be routed to a dedicated receptacle.

Dave Harrold is one of the authors of Control System Power and Grounding Better Practices. Harrold retired in 2009 after working for nearly four decades in the controls and instrumentation industry, including as a Control Engineering editor. Edited by Chris Vavra, production editor, CFE Media, Control Engineering,


Key concepts

  • Many control systems are out of date and in need of a replacement.
  • Engineers should perform a forensic investigation to determine what kind of control system should be installed.
  • Forensic investigations might be uncharted territory for engineers, and recruiting someone to assist may be necessary. 

Consider this

Forensic examinations can be used for additional engineering projects.

Control System Power and Grounding Better Practices is available from several sources including Elsevier Store and Barnes & Noble.

See additional stories from the author linked below. 

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