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System Performance with Advanced Microprocessor Controls
36
Physical Connections
Figure 6 shows the 4-20 mA output of LT750 connected to Analog Input #1 (41 and 42) on the exter-
nal inputs terminal strip. This strip is provided on units ordered with analog inputs. (If this strip is
not installed, there is a field installation kit available from your Liebert representative.)
The 4-20 mA output of the LT750 must be connected to the first analog input, as shown.
TB4 is the water detected relay output. It can be connected to any one of the four special alarm inputs.
TB5 is the cable fault relay output. It can also be connected to any one of the four special alarm
inputs.
Setup
(The following description assumes the wiring connections as shown above.) First, verify that special
alarms 1 and 2 are ENABLED to either WARNING or URGENT type. Do this by selecting VIEW/SET
ALARMS from the Main Menu. Then, select SETUP ALARMS. Follow the instructions on the display
to select the required type for CUSTOM ALARM #1 and CUSTOM ALARM #2 if not already set.
Next, select the alarm message for CUSTOM ALARM #1 and #2. From the Main Menu, select VIEW/
SET ALARMS. Then, select SETUP CUSTOM ALARMS. Then, select SETUP CUSTOM ALARM
TEXT. Define CUSTOM ALARM #1 to be CUSTOM 1. (CUSTOM 1 is the default message that will be
displayed if a message has never been programmed.) Next, select the text for custom alarm #2 to be
WATER UNDER FLOOR. Now, change the message CUSTOM 1 to LT750 CABLE FAULT. This is
done by selecting the CHANGE CUSTOM TEXT 1 menu item in the SETUP CUSTOM ALARMS
menu. Follow the instructions on the screen to change the message.
The slope and intercept values of Analog Input #1 are used to calculate the location of water. These
values should initially be set to zero. The default values are zero, but it may be a good idea to verify
those values. They can be viewed by selecting ANALOG/DIGITAL INPUTS from the Main Menu,
then SETUP ANALOG INPUTS.
See 3.4.6 - Setup Water Detect Floor Plan for more information.
Calibration
Calibration should not be required for most installations. The accuracy of this display is approxi-
mately 1%.
The display is calibrated by the slope and intercept values of Analog Input #1. The position of the
water is calculated from the analog output of the LT750 using this formula:
position = analog reading/full scale reading x (measured length + slope) + intercept
position is the distance from the LT750 to the position of the detected water.
measured length is the length of the cable which is calculated automatically when the layout is
defined. The units for these values are in floor tiles.
The intercept value read from Analog Input #1 is added to the measured position of a water indication
to determine which tile to highlight. For example, if water is displayed under the seventh tile but
determined to be under the fifth tile, set the offset value to -2 tiles. Use the intercept value to correct
errors close to the start of the cable.
Accuracy errors farther out on the cable should be corrected using Analog Input #1's slope value. This
value effectively adjusts the measured length of the cable. Increasing the effective length of cable will
increase the distance of the water and move the highlighted tile farther along the cable, and vice
versa. Unlike the intercept, which adjusts by the same amount for all locations on the cable, the slope
increases its effect for larger distances.
The best procedure to calibrate the cable would be to first simulate water close to the LT750, about
five tiles out. Adjust the intercept to get the correct reading. Next, simulate water five tiles from the
end. Adjust the slope to get the correct reading.