JOHNSON CONTROLS
117
SECTION 5 – SEQUENCE OF OPERATION
FORM 145.05-NOM7
ISSUE DATE: 10/31/2019
5
If one of the safeties opens, the signal is lost. When this
occurs, the Unit Controller turns off the compressor.
The Unit Controller records the time it takes for the
safety circuit to reestablish the 24VAC safety circuit
to the Unit Controller. The time to reset is displayed in
the HISTORY buffer and is identified as “COMP STA-
TUS (A, B, C, etc. . . .) CLEAR TIME.” The length
of time it takes to reset is an indication of which safe-
ties opened. For example, the External Motor Protector
takes 30 minutes to reset. The High Pressure Cutout
usually resets in less than a minute. If time to reset is
greater than 60 minutes, the HISTORY buffer replac-
es the “COMP STATUS (A, B, C, etc. . . .) CLEAR
TIME” with “COMPRESSOR SYSTEM (A, B, C, etc.
. . .) TIME OUT.”
The safety circuit input is ignored when the compres-
sor is OFF. If the compressor is ON and the 24 volt
safety circuit input is lost to the Unit Controller for two
seconds, the Unit Controller turns OFF the compressor.
The compressor system is active again when the safety
circuit is reestablished. However, the event is stored in
the HISTORY buffer. During the time the safety circuit
is open, the User Interface displays the following mes-
sages:
• STATUS screen – “COMP SYS # STATUS –
SAFETY TRIP”
• COMPRESSOR SYSTEM # screen – “SAFETY
TRIP (1 or 2)”
• The HISTORY buffer stores and displays “COMP
# SAFETY TRIP (1 or 2)” depending if this is the
first or second trip
If the safety circuit opens three times in a 120-minute
window, the Unit Controller locks out the compres-
sor having the fault and prevent further operation of
the compressor until the system is manually reset. The
User Interface displays the following messages:
• STATUS screen – “COMP SYS # STATUS –
SAFETY LOCKOUT”
• COMPRESSOR SYSTEM # screen – “SAFETY
LOCKOUT”
• The HISTORY buffer stores and display “COMP
# SAFETY LOCKOUT”
The # symbol varies to match the compressor number
the data is recorded for (A, B, C, etc.)
Low Pressure Monitoring
The units use a Suction Pressure Transducer in lieu of
a Low Pressure Cutout. The Unit Controller monitors
the suction pressure in each active compressor system
and compares it to a SUCTION PRESSURE LIMIT
(90PSIG on this R410A unit).
Since the System A 25–60 ton compressor can vary the
capacity which could affect the suction pressure, the
Unit Controller calculates a 15 second rolling average
pressure. This average pressure is compared against
the SUCTION PRESSURE LIMIT.
Sequence of Operation
If the suction pressure falls 15 PSIG below the SUC-
TION PRESSURE LIMIT for 5 seconds, the unit turns
off the compressor and declare a “LOCKOUT – LOW
PRESS SYS (A, B, C, etc. . . .)” fault.
When the suction pressure falls below the SUCTION
PRESSURE LIMIT, the Unit Controller starts an in-
cremental timer and calculates a trip time based on an
algorithm in the Unit Controller.
As the suction pressure may change over time, the trip-
time is continuously calculated during which the trip-
time could increase with a rise in pressure or decrease
with a loss of pressure. If the incrementing timer grows
to a value greater than or equal to the calculated trip-
time value, the Unit Controller turns off the compres-
sor and declare a “LOCKOUT – LOW PRESS SYS
(A, B, C, etc. . . .)”
fault.
If the suction pressure rises above the SUCTION
PRESSURE LIMIT before the incrementing timer
overtakes the triptime, the timer is reset to zero and
does not start timing again unless the suction pressure
falls below the SUCTION PRESSURE LIMIT.
Compressor Lockout
If a compressor lockout has occurred, such as “LOCK-
OUT – LOW PRESS SYS (A, B, C, etc...)” fault, it can
only be reset using the PROGRAM key, COMPRES-
SOR SYS (A, B, C, etc...) and changing COMP SYS
(A, B, C, etc...) STATE from LOCKOUT to RUN.
The following procedure is an example for Fault Reset
for compressors, supply system, etc.
