A question about 4-20 mA loop-powered devices was recently posted to
the "Automation & Control Engineering" group on LinkedIn. Jim Hausch, the
initial poster, was looking for advice on rules and best practices for powering
these devices. He asked: If there are only one or two loop-powered devices in
the system, I just parallel wire the loop-powered devices and that seems to
work OK. In one case, I used a separate small power supply for the single
loop-powered device. Are there any rules or best practices to follow for this?
Hausch
clarified that his parallel wiring reference relates to the use of one power
supply to provide 24V dc to multiple sensing devices in parallel, so that each
device will have its own connection to its own PLC input.
Noting that PLC 24V dc
supplies often have a low power budget, Terry Trewern, managing director at
Phoenix Engineering, says that this means these power supplies are not always
usable. He also cautions to beware of the difference between sinking and
sourcing.
"Another issue to take account
of is volt drop," Trewern says. "Long cable runs with shared power supplies can
leave you below the minimum 9V at the instrument."
Antonius van Breugel, an
industrial automation professional in Romania, added that loop-powered devices
can be wired in series as long as the total resistance does not exceed 600O.
Pointing out that there are
minimum voltage drops allowed by various manufacturers, Barret Davis, owner of
The AutoMate Co., offered his experience, saying "if you want to use a fuse in
the 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 quirky
and intermittent operation."
According to Davis, "The best
protection against surge is of course using metal conduit for the entire cable
run, preferably EMT or ridged conduit. Most people think that instrument cables
actually shield every frequency. In reality, they only provide attenuation at
frequencies above 1 megahertz."
Roy Gardner, a consultant with Foxboro, replied: "Generally
in the 4-20 mA process measurement family, almost all of the PLC manufacturers
have an input module that has the loop-powering capability built-in. The best
practice in my opinion would always be to use the integral power supply
capability of the control product itself. Any form of external power supplies
leave open questions related to reliability, robustness and error detection
related to the health of the power supply."
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