DCA
Page 16
Setting the Dead Band.
See figure 1 for a graphic
representation of the DCA dead band. The dead band
may be thought of as an “area” around a trip point in
which the alarm will not reset.
In a scenario using a high-alarm DCA as an example,
the unit will signal an alarm at its trip point, and will
continue to signal an alarm as long as the input is
above that point.
Without an adjustable dead band, the relay will reset
when the input drops below the trip point. When
equipped with the AD Option, however, no reset will
occur until the input continues to drop to the dead
band setting, presumably some percentage of span
below the trip point.
To set the DCA dead band:
1.
Make sure dead band pots are turned fully
counterclockwise. Set trip points for installed
alarms as described in preceding section. Con-
nect ohmmeter to UPPER alarm section to set
UPPER dead band, or to LOWER section for
setting LOWER dead band.
2.
Apply input at the dead band setting level.
Typically, this will be lower than trip point for high
alarms, or higher than trip point for low alarms.
3.
Turn appropriate dead band pot slowly clockwise,
observing LED and ohmmeter.
4.
Set pot (stop turning) at exact point at which relay
changes state.
5.
Verify desired setting by varying input above and
below trip point, observing both LED and ohmme-
ter to confirm appropriate change of relay state.
Unit will “trip” at trip point setting, and will not
reset until input returns past dead band setting.
The DCA dead band can be expressed as a percent-
age of the unit’s factory-configured span.
For Example
:
A single, high-alarm DCA rated for 4-20 mA input is
required to have a 25% dead band around its 14 mA
trip point setting. To calculate the input level needed
to set the desired dead band, subtract 25% of 16 mA
(span) from 14 mA:
Trip point setting in mA = 14
– 25% of input span in mA = 16(0.25)
14 – 4 = 10
The result, 10 mA, is the level of input to be applied in
step 2. The DCA in this example will signal an alarm
condition when the input reaches or exceeds 14 mA,
but will not reset until the input drops below 10 mA.
Calibrating TT-equipped Units.
Lockable, multiturn
trip point adjustment pots on the front panel of each
TT-equipped DCA are set at the factory so that “0.00”
on the dial is equal to 0% of the application’s rated
input span. “10.00” is equal to 100% of span. The
accuracy of these settings is ±0.5% of the rated
span.
Calibration of DCA’s with this option consists of two
procedures. First, the internal, Full-Scale Adjust
(FSA) potentiometer(s) must be set (this involves
some disassembly of the STD DCA). Then, the unit
zero, trip points, and dead band (if so-equipped), must
be set.
Figure 13 shows the location of the STD DCA FSA
potentiometers. Figure 14 shows the location of the
pots on the PC-style unit. Refer to figure 10 when
disassembling the unit in STD housing, and refer to
figures 13 or 14 while executing the procedure.
NOTE
The following procedure assumes that the
DCA being calibrated has its “UPPER”
alarm section configured as a high alarm,
and its “LOWER” alarm section configured
as a low alarm. If this is not the case for
your DCA, interchange references to
“UPPER” and “LOWER”, as appropriate.
