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.