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Posted by Julie Gruenholz

Upgrading your obsolete control system - Why and how to ground your ungrounded control system before a system upgrade

January 10, 2022

Upgrading your absolete control system is a helpful 7-part blog series addressing the important technical asspects of upgrades and written by an industry professional with over 25 years of experience.

Here are links to the other blogs in this series:

6 Questions to Ask When Upgrading an Obsolete Control System While Keeping Your Plant Running

KISS and 5 Other Items to Consider when Choosing to Upgrade Your PLC Platform

Replacing Your Obsolete Control System. Step 1 – Replace the SCADA

Replacing Your Obsolete Control System.  Step 2 – Upgrade the PLC Hardware

Replacing Your Obsolete Control System.  Step 3 – Key Considerations when choosing the new SCADA/HMI.

Upgrading your obsolete control system - Why and how to ground your ungrounded control system before a system upgrade.

A note for Mechanical Engineers… Don’t be afraid of the PLC programmer or the control system software. 

8 steps for plc5 to controllogix conversion

Why and how to ground your ungrounded control system before a system upgrade.

This is not a new debate… Should you have a grounded or an ungrounded control system?

The first control system I ever upgraded was pretty much ungrounded. It was 110VAC relay logic with a couple resistors trying to hold the center at ground. The original design (installed in 1973) was selected because 55 volts to ground was considered intrinsically safe. In order to install a PLC in 1991, the system had to be modified so it was a solid 120VAC to ground. The PLC dictated that all of the IO and the entire control system share a common ground. As our plant electricians embarked on this ground relocation process, without interrupting the integrity of the system, they found that many unintentional shorts had developed over the years that had gone unnoticed. This opened my eyes to the fact that it is better to have a known potential to ground on all devices rather than an unknown floating potential that may or may not keep your system from shutting down.

Later when I became a contractor and worked on many sites that have a 24VDC control system, I was introduced to the fact that many engineers believe that an ungrounded 24VDC system is the way to go. On systems like these, undetected shorts to ground can develop. One ground is fine, but if you get two grounds, your control system can operate erratically and the resulting system can be almost impossible to troubleshoot.

Always tie your 24VDC negative to the panel 120VAC ground!

Build redundancy and appropriate fusing into your system to detect shorts immediately. Do not rely on a floating potential to mask problems. A known failure is always better than an unknown floating potential that can cause erratic behavior. I encourage you to ground your ungrounded system before upgrading your system to a new PLC. This might be painful and scary but knowing all your problems before moving forward is always the best strategy.

This blog answers question #5 of “6 Questions to Ask When Upgrading an Obsolete Control System while Keeping Your Plant Running.” I hope that you have enjoyed these blogs and please feel free to email me  with your comments or thoughts or reach out to us throug our 

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About the Author

Julie has left Hallam-ICS to pursue other endeavors, but her contributions to the company continue to be valued.

About Hallam-ICS

Hallam-ICS is an engineering and automation company that designs MEP systems for facilities and plants, engineers control and automation solutions, and ensures safety and regulatory compliance through arc flash studies, commissioning, and validation. Our offices are located in Massachusetts, Connecticut, New York, Vermont and North Carolina and our projects take us world-wide.

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Topics: Process Control and Plant Automation

Julie Gruenholz

By Julie Gruenholz

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January 10, 2022

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