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Aug. 5, 2010
Mini-Fast Pigtails Installed More than 10 Years Ago Are Still In Service. Not One Has Failed. This, Due to a Defect or Fault in the Pigtail or Its Device Receptacle Counterpart
By John Rezabek, Contributing Editor

I'm going to speculate that cordsets—those pre-configured quick-connects for electrical terminations—had their roots in the military.

The old Amphenol connectors I used early in my career to connect level switches to the probes on rail cars were made of pot metal that must have been rendered from WWII armor. It took a little art and skill to solder up these guys, and I entrusted that task to a retired instrument man—and ham radio enthusiast—who had a good chance of avoiding cold solder joints. It worked out reasonably well, as the receptacle halves trundled the railways between Ohio and points south and still managed to function when they came home.

We still have applications for Amphenol connectors today, but in many plants the skills and materials—tried any lead-free solder?—are becoming scarce. Moisture, flooding and corrosive mists and vapors can make a specifying engineer anxious about the durability and reliability of such quick-connects, especially if the terminations, typically solder, are fabricated in the field. Plant instrument techs and electricians or tradesmen from the IBEW hall rarely get enough practice to be experts at soldering tinned copper to pot metal.

With the advent of buses like Profibus-PA and Foundation fieldbus, more people in the large process industries became interested in the quick-connect concept. A whole family of preconfigured and factory-terminated cordsets had been developed for switches in discrete parts manufacturing. Apart from the quick interchange of failed sensors, cordsets allowed more freedom of motion and adaptability to changing assemblies and calibration.  But a seemingly frail and unprotected cable was pretty foreign to the world of hazardous atmospheres, NEMA 7 enclosures and rigid, galvanized-steel conduit.

As an early adopter of bus technology, our plant determined that cordsets with mini-fast connectors could help to prevent maintenance activities from shorting out the bus. There were no commercial offerings for spur short-circuit protection at the time, and a factory-sealed quick-connect would ensure no bare copper was flailing around when an instrument was taken out of service. Transmitters with the mini-fast receptacles were a new concept for our supplier, who saw their explosion-proof rating rendered ambiguous by filling a conduit connection with a premanufactured receptacle-plus-pigtail. But most of our plants weren't pouring seals at every transmitter anyhow. We had been following NEC guidelines for non-incendive, non-arcing wiring and devices in Div. 2 areas for years with conventional instruments. Since our design didn't rely on NEMA 7 technology for containing an arc-induced flame front, we were comfortable with the customization of the device OEM's enclosure. For people installing in Div. 1/Zone 1 areas, suppliers like Turck now offer explosion-proof Minifast receptacles rated to maintain the OEM's NEMA 7 rating without a poured seal.

To my pleasure and to some amazement, most of the hundreds of mini-fast pigtails installed more than 10 years ago are still in service, and not one failed due to a defect or fault in the pigtail or its device receptacle counterpart.

We felt pretty clever when we found a DP cell that had been hooked up backward—low and high sides reversed—and could be flipped without rewiring. Our friends on the Gulf Coast must have felt even smarter when their IP67 bricks—fieldbus junction boxes fitted for these cordsets—were battle-tested by submersion in 10 ft of water. The weather, moisture and corrosion resistance of this technology has proven itself in humid climates.

Factory-soldered, sealed and tested pigtails for device connection can be taken a step further by specifying quick-connects for both the device and junction box ends. Maybe the brave and those in smaller, controlled environments will opt for a field-wirable connector to manage uncertain spur and trunk cable lengths, just like the old Amphenols. But most I've encountered opt for completely factory-manufactured and  sealed cables with connectors already attached. This results in some excess, which some installers choose to coil in the cable tray or at the field device. Users are cautioned to mind the minimum bend radius when coiling fieldbus cables. Some vexing network issues in the commissioning phase can be caused by bend radius abuse. 

John Rezabek is a process control specialist at ISP (www.ispcorp.com) in Lima, Ohio.

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