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SECTION 8 – UNIT OPERATION
LOAD LIMITING
Load Limiting is a feature that prevents the unit from
loading beyond the desired value. 2 and 4 compres-
sor units can be load limited to 50%. This would allow
only 1 compressor per system to run. 3 and 6 compres-
sor units can be load limited to 33% or 66%. The 66%
limit would allow up to 2 compressors per system to
run, and the 33% limit would allow only 1 compressor
per system to run. Five-compressor units may be load
limited to 40% (1 compressor per system runs) or 80%
(up to 2 compressors per system) are permitted to run.
No other values of limiting are available.
There are two ways to load limit the unit. The first is
through remote communication via an ISN. Load limit
stages are sent through YORK Talk on pages 9 and 10
of feature 54. Page 9 is stage 1 load limit and page 10
is stage 2 load limit.
A second stage of load limiting the unit is accom-
plished by closing contacts connected to the Load Limit
(XTBC1 – terminals 13-21) and PWM inputs (XTBC1
– terminals 13-20). Stage 1 of load limiting involves
closing the Load Limit input. Stage 2 of load limiting
involves closing both the Load Limit and PWM inputs.
The first stage of limiting is either 80%, 66% or 50%,
depending on the number of compressors on the unit.
The second stage of limiting is either 40% or 33% and
is only available on 3, 5 and 6 compressor units. Table
24 on page 158 shows the load limiting permitted for
the various numbers of compressors.
158
TABLE 24 - COMPRESSOR OPERATION LOAD
LIMITING
COMPRESSORS
IN UNIT
2
3
4
5
6
Simultaneous operation of Remote Load
Limiting and EMS-PWM Temperature
Reset (described on following pages)
cannot occur.
COMPRESSOR RUN STATUS
Compressor run status is indicated by closure of con-
tacts at XTBC2 – Terminals 25 to 26 for system 1 and
XTBC2 – Terminals 27 to 28 for system 2.
ALARM STATUS
System or unit shutdown is indicated by normally-open
alarm contacts opening whenever the unit shuts down
on a unit fault, locks out on a system fault, or experi-
ences a loss of power to the chiller electronics . System
1 alarm contacts are located at XTBC2 – Terminals 29
to 30. System 2 alarm contacts are located at XTBC2 –
Terminals 31 to 32. The alarm contacts will close when
conditions allow the unit to operate, or the fault is reset
during a loss of power, the contacts will remain open
until power is reapplied and no fault conditions exist.
FORM 150.72-ICOM6
ISSUE DATE: 12/29/2017
STAGE 1
STAGE 2
50%
-
66%
33%
50%
-
80%
40%
66%
33%
JOHNSON CONTROLS
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Troubleshooting
