Application Note

2 Fluke Corporation Breaking new ground without breaking the loop
Milliamp loop sensor
measurement
The foundation of each model
in the 771/772/773 Milliamp
Clamp series is the ground-
breaking ability to troubleshoot
milliamp sensors inside crowded
control panels—without breaking
the circuit. In this scenario, our
technician notices that the static
pressure inside the supply duct
of a VAV system seems to be too
high.
•
A check of the BAS front-
end computer indicates that
the sensor is reading a per.
922 indicates that the static
pressure is actually 1.5” WC.
For these systems, 1’ WC is
considered normal.
•
The technician takes his
773 Milliamp Clamp out to
the control panel and finds
the wires that terminate the
static pressure sensor to the
controller. As in many of the
control panels in this building,
the wiring looks like cooked
spaghetti. The small jaws
of the 772/773 enable the
technician to clamp the exact
wires perfectly. He reads the
milliamp signal and compares
it to the listed range for the
sensor. The sensor is 0 mA to
20 mA and has a range of 0”
to 2” WC. It is currently read-
ing 15 mA. At 1” WC it should
be reading 10 mA.
•
The technician uses his laptop
and opens up the program in
the controller. He finds that
the programmer did not scale
the sensor properly in the
software, which caused the
error.
Damper and actuator
troubleshooting
Another controls technician
receives a too-cold complaint
from an office in the building.
The system is VAV with a hot
water reheat valve.
•
A quick look determines
that the valve is closed. She
connects her laptop to the
controller and uses it to com-
mission the controller. She
attempts to override the valve
open, but without success.
•
She disconnects the wires to
the actuator from the control-
ler and then connects to the
Fluke 773. She uses the meter
to send a 0 V dc to10 V dc
signal directly to the valve
actuator to make it open and
close. The valve actuator then
works perfectly.
•
Reviewing the control-
ler programming, the tech
determines that the reheat
valve was forced closed by an
improperly operating interlock,
which is then removed. After
this, the reheat valve works
properly and the space warms
up.
Troubleshooting VFDs
During a spell of hot weather,
all the occupants of a building
complain that it is too hot.
•
The chilled water pump
system uses a VFD, but its
operating pressure is too low.
•
The controller sends a mil-
liamp signal to the drive to
command it to the correct
frequency to obtain the cor-
rect system pressure. A tech
disconnects the system con-
troller, connects the Fluke 773
directly to the terminal block
of the VFD, and generates a
milliamp signal to the drive
to increase the speed. Despite
generating a 100 % signal to
the drive, it never comes up to
speed.
•
A check of the recorded fault
statuses of the drive indicates
that a circuit board has failed.
The manufacturer is contacted
and the faulty board replaced,
at which time the drive oper-
ates correctly again and the
chilled water system returns
to the correct pressure.
In these common scenarios, the
Fluke 772 and 773 meters are
already proving themselves in
the wide range of troubleshoot-
ing situations encountered in
today’s buildings. Every controls
technician will want one in their
tool bag.
Fluke Corporation
PO Box 9090, Everett, WA 98206 U.S.A.
Fluke Europe B.V.
PO Box 1186, 5602 BD
Eindhoven, The Netherlands
For more information call:
In the U.S.A. [phone] or
Fax [phone]
In Europe/M-East/Africa [phone] or
Fax [phone]
In Canada (800)-36-FLUKE or
Fax [phone]
From other countries [phone] or
Fax [phone]
Web access: http://www.fluke.com
©2010 Fluke Corporation.
Specifications subject to change without notice.
Printed in U.S.A. 1/2010 3610979A A-EN-N
Modification of this document is not permitted
without written permission from Fluke Corporation.
Fluke. Keeping your world
up and running.
®
Checking communication wiring voltages on automation system controls.