Loop Powering Best Practice

A question about 4-20 mA loop-powered devices was recently posted tothe "Automation & Control Engineering" group on LinkedIn. Jim Hausch, theinitial poster, was looking for advice on rules and best practices for poweringthese devices. He asked: If there are only one or two loop-powered devices inthe system, I just parallel wire the loop-powered devices and that seems towork OK. In one case, I used a separate small power supply for the singleloop-powered device. Are there any rules or best practices to follow for this?

Hauschclarified that his parallel wiring reference relates to the use of one powersupply to provide 24V dc to multiple sensing devices in parallel, so that eachdevice will have its own connection to its own PLC input.

Noting that PLC 24V dcsupplies often have a low power budget, Terry Trewern, managing director atPhoenix Engineering, says that this means these power supplies are not alwaysusable. He also cautions to beware of the difference between sinking andsourcing.

"Another issue to take accountof is volt drop," Trewern says. "Long cable runs with shared power supplies canleave you below the minimum 9V at the instrument."

Antonius van Breugel, anindustrial automation professional in Romania, added that loop-powered devicescan be wired in series as long as the total resistance does not exceed 600O.

Pointing out that there areminimum voltage drops allowed by various manufacturers, Barret Davis, owner ofThe AutoMate Co., offered his experience, saying "if you want to use a fuse inthe circuit, you must use a fuse with silver-coated ends. Otherwise, over time,you can get more voltage drop than the circuit can allow and cause very quirkyand intermittent operation."

According to Davis, "The bestprotection against surge is of course using metal conduit for the entire cablerun, preferably EMT or ridged conduit. Most people think that instrument cablesactually shield every frequency. In reality, they only provide attenuation atfrequencies above 1 megahertz."

Roy Gardner, a consultant with Foxboro, replied: "Generallyin the 4-20 mA process measurement family, almost all of the PLC manufacturershave an input module that has the loop-powering capability built-in. The bestpractice in my opinion would always be to use the integral power supplycapability of the control product itself. Any form of external power suppliesleave open questions related to reliability, robustness and error detectionrelated to the health of the power supply."