The following tip is from the ISA book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. This is Tip #20, and was written by Hunter.

I recently encountered a control cabinet on a client’s site that had 50 valve-limit switches wired to a single breaker. If any of the 50 shorted, the entire group tripped off line. Between this installation and several others like it, the technicians often spent hours and even days trying to track down a single field wiring problem while the process equipment sat idle.

Concept: The small amount of extra money required to individually fuse I/O with indicating fuse blocks will be quickly recovered through dramatically improved troubleshooting. Even current limited I/O can benefit from individual fuses and/or disconnects.

Details: Troubleshooting a system that does not have individually fused I/O can be one of the most difficult and time-consuming activities for a technician. A single fault in the field can take out all the points on a card and might take out the entire cabinet. In such a case, the technician usually starts lifting wires and continually resetting the breaker until he or she finally isolates the problem. This problem can be avoided if the cabinet design incorporates individually fused I/O and indicating fuses. If the cabinet incorporates these features, the problem is immediately obvious the moment the door is opened. (The blown fuse light is difficult to miss.)

Unfused I/O is common in third-party skid packages where the vendor is trying to reduce costs wherever they can. Eliminate this possibility by specifying that all I/O must be individually fused with indicating fuses in the bid package.

Watch-Outs: In an attempt to add fuses, avoid the temptation to use two-, three-, or four-high terminal blocks to save room. These blocks look wonderful on paper but are AWFUL when they are installed in the field. The technicians cannot even SEE the lower terminals, much less get their probes on them for voltage readings, and the cabling is an absolute nightmare. Beware of I/O cards that purportedly include individual fuses. Almost none of these cards use indicating fuses and some of them require the entire card to be removed in order to replace one fuse!

Exceptions: Some I/O cards utilize current limiting circuits that can sustain a field short without damaging the card. In such a case, fuses are unnecessary, but consider installing terminal blocks with a built in disconnecting plug. Such a disconnect provides an easy means for the technicians to take series current measurements or connect their handheld communicators.

Insight: The increased cost of using individual fuses will be quickly recovered by the reduced time to troubleshoot and resolve field wiring problems. One or two instances of bringing production back on line within minutes rather than hours (or days) will easily pay for the initial installation.

Rule of Thumb: Individual, indicating fuses and single-high terminals take up more room in the cabinet, but the benefits far outweigh the costs. Build this into your standard cabinet designs and be sure to specify it in your third-party skid package specifications.

About the Author
Gregory K. McMillan, CAP, is a retired Senior Fellow from Solutia/Monsanto where he worked in engineering technology on process control improvement. Greg was also an affiliate professor for Washington University in Saint Louis. Greg is an ISA Fellow and received the ISA Kermit Fischer Environmental Award for pH control in 1991, the Control magazine Engineer of the Year award for the process industry in 1994, was inducted into the Control magazine Process Automation Hall of Fame in 2001, was honored by InTech magazine in 2003 as one of the most influential innovators in automation, and received the ISA Life Achievement Award in 2010. Greg is the author of numerous books on process control, including Advances in Reactor Measurement and Control and Essentials of Modern Measurements and Final Elements in the Process Industry. Greg has been the monthly "Control Talk" columnist for Control magazine since 2002. Presently, Greg is a part time modeling and control consultant in Technology for Process Simulation for Emerson Automation Solutions specializing in the use of the virtual plant for exploring new opportunities. He spends most of his time writing, teaching and leading the ISA Mentor Program he founded in 2011.

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Hunter Vegas, P.E., holds a B.S.E.E. degree from Tulane University and an M.B.A. from Wake Forest University. His job titles have included instrument engineer, production engineer, instrumentation group leader, principal automation engineer, and unit production manager. In 2001, he joined Avid Solutions, Inc., as an engineering manager and lead project engineer, where he works today. Hunter has executed nearly 2,000 instrumentation and control projects over his career, with budgets ranging from a few thousand to millions of dollars. He is proficient in field instrumentation sizing and selection, safety interlock design, electrical design, advanced control strategy, and numerous control system hardware and software platforms.

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