Application Note

2 Fluke Corporation Better ways to troubleshoot automation and process control loops
The first step is to measure
the 4-20 mA signal, either by
breaking the loop connecting in
series with a DMM, or by using
a mA clamp meter like the Fluke
771 and verifying the loop cur-
rent value. If the loop current
measured is not as expected,
there are three likely causes:
broken/disconnected/shorted
wires, a bad loop power supply,
or faulty instrumentation.
If no problem is found in the
wires, use a DMM (or the 773
clamp meter) to check the loop
power supply. If the power
supply shows no output, use the
24 V loop power function of the
meter to substitute for it; if the
loop then works properly the
source of the problem is obvious.
If the wiring and the power
supply both check out, it’s time
to check the transmitter. If you
have a loop calibrator, process
calibrator or multi-function
clamp-on meter, use its mA
simulate mode to substitute for
the transmitter. If the loop per-
forms as requested, the problem
lies with the transmitter, if not,
it is elsewhere.
If a final control element
(valve positioner, etc.) is sus-
pected, use the mA source/
simulate mode on the Fluke
772/3 to feed a signal into it
while watching the local indica-
tor for a response.
Loop malfunctions
If the problem is not a dead loop
but an inaccurate one, likely
possibilities include a bad I/O
card on the PLC or DCS, or a bad
final control element (I/P on a
valve positioner, etc.). It’s usu-
ally best to start by doing a field
check of the transmitter, local or
remote indicator or final control
element.
For a final control element,
use a clamp-on meter to mea-
sure loop current and compare
the value to the local position
indicator on the valve or other
final control element. Relay that
information to the operator to
verify findings.
In the case of a measure-
ment loop, use the clamp meter
to measure loop current, then
check with the operator to see
how well the value indicated on
the control panel agrees with
the actual loop current. This will
give a quick check on the PLC or
DCS I/O card that handles that
particular loop. It’s also possible
to use the meter’s mA source/
simulate mode to send a known
signal to the control room; as
before, compare the value as
read by the operator to the
actual current in the loop.
Some loops show random
fluctuations or intermittent faults
that tend not to happen while
a technician is watching. The
solution here is to use a clamp
meter with a scaled mA output.
In this mode the meter measures
the current in the loop without
breaking the circuit, and pro-
duces an identical and isolated
mA output. Feed that output to
a DMM with a logging function;
by allowing the DMM to record
over time, any disturbance will
be recorded.
Field checks and plant
commissioning
Start by using a clamp-on loop
current meter like the Fluke 771
to check each loop for current
in a matter of seconds, without
disconnecting anything. If a
loop is not working, a multifunc-
tion clamp meter can also make
quick work of diagnostics. If cur-
rent is not present on some loops
go on to classic troubleshooting:
check the wiring, the power
supply, and the control system’s
I/O cards (by using the meter to
inject a signal into the I/O, then
contacting the operator to ask
what he sees. If the operator
agrees with what is being sent,
then there may be something
amiss with the transmitter—
either the transmitter itself or,
if this is a new installation,
perhaps miswiring, the sensor’s
input to the transmitter.
Checking DCS and PLC
I/O cards
The mA process clamp meter
can be used as an accurate
signal source to check the oper-
ation of input/output cards on
programmable logic controllers
(PLCs) and distributed process
control systems (DCSs). For
4-20 mA input cards, disconnect
the process loop and use the
meter’s mA source mode to feed
in a known signal value (4.0 mA
for zero, 12 mA for 50 %—using
the meter’s 25 % step function,
and 20.0 mA for 100 %) and
compare it to the value shown
on the operator’s readout.
Voltage input cards (1 V to 5 V
or 0 V to 10 V) are checked in
a similar way, using the meter’s
voltage source function.
Checking a valve
positioner
Milliamp clamp meters can be
used for periodic in-field checks
of electronic valve positioners
as part of preventive mainte-
nance programs. Accounting for
manufacturer-specific instruc-
tions, perform quick operational
checks using the Fluke 772/3 as
a signal source while observ-
ing the valve stem position,
mechanical position indica-
tors, or flow indicators as input
changes are made.
Mitch Stewart, Field Service
Manager, L2 Systems, tells of
using the 4-20 sourcing output
of a mA process clamp meter to
drive a control valve open and
closed when the process output
from the PLC wasn’t working.
“We disconnected the PLC’s
output at the control valve and
connected the [meter] up to the
control valve and ran it open
and closed to verify that the I/P
on the valve worked correctly,”
he explains.
The general method is to
set the meter to the 4-20 mA
source/simulate mode and
connect it to the input termi-
nals of the valve positioner. Set
the meter to output 4 mA and
wait for the positioner to settle;
then vary the current in small