1 August 2006
Think of Power Before Design
By Robert C. Webb
The technologies available today in terms of transient suppression and system protection, from switching transients or lightning strikes, are pretty much the same as the ones we’ve always used. They include various forms of semiconductor clamps, gas discharge tubes, and appropriate fusing.
All these serve to limit the transient energy that might otherwise pass through your power supplies and damage or destroy your automation systems. Most qualified manufacturers incorporate the necessary features into their systems and can tell you the standard tests they have passed. Another consideration is the impact of large switching power supplies (such as variable speed drives) on the quality of the power going into your system.
When people don’t think it through carefully and put a new system in, they end up with outages and failures that were unexpected because new systems typically use fewer components to do more things—and are thus more vulnerable to loss of key support systems like power. Installation of fully redundant systems happens often, but if you don’t carry that redundancy through to the power supply, and you don’t put in adequate protection in case you lose the power, when the power goes out, you’ll lose everything.
Most systems designed for industrial plant operations have some sort of capability; you just have to decide as an engineer how much is enough. If you’re in a high lightning environment, you’ll specify a higher transient protection level. In power plants, even if you didn’t care about lightning, you have 12,000 horsepower motors starting and stopping, and high energy switchgear opening and closing, and that induces transients also. So you have to think not only about potential lightning strikes but about normal operation within your facility. You don’t want to have spurious operations of your control system because a pump stops or starts like it normally does.
You can have the most expensive and redundant control system in the world, with fully redundant I/O, controllers, and even control systems supplies. But if you haven’t done something to treat the power feeding your system properly, you have a common mode mechanism that will get you every time.
About the Author
Robert C. Webb, P.E., is a consultant in San Carlos, Calif., an ISA Standards and Practices Board Member, managing director of the ISA-SP67 Nuclear Power standards committee, and POWID executive committee member.