JOHNSON CONTROLS
76
FORM 100.50-NOM12
ISSUE DATE: 04/02/2019
SECTION 5 – SEQUENCE OF OPERATION
• A compressor system can also have a high pres-
sure unloading sequence if discharge pressure
transducers are installed on a compressor circuit.
Compressor Safety Chain
• The compressor safety circuit consists of:
a. High pressure cutout (HPCO) switch: one
per circuit
b. Manual motor starter/overload relay: one per
compressor
c. Compressor protection module: one per
compressor
1. Certain compressors, depending on the
HP, do not contain a compressor pro-
tection module. They only contain an
internal thermal overload in the motor
windings
2. The internal overload is not connected
to the compressor safety chain
3. When the internal overload opens, a
fault is not registered and the IPU board
still tries to run that compressor when a
cooling demand exists
• The safety switches that make up the compressor
safety circuit are wired in series
Compressor Safety Circuit Sequence of
Operation
• The IPU board monitors the status of each com-
pressor safety circuit.
• When any of the safety switches open, compres-
sor operation terminates immediately, and a com-
pressor safety trip occurs.
• Wiring of the compressor safety chain is shown
in
below and Figure 35
and Figure 36
(Comp Sys #1 shown). Refer to the
unit's wiring diagrams for other systems.
• At this time, the IPU board starts a 60 minute timer.
If the compressor safety circuit status remains
faulted/open for 60 minutes, a compressor safety
lockout occurs.
• When the faulted/open safety switch resets within
the 60 minute timer, normal operation continues.
• Once a safety faults/opens, the IPU board also
starts a separate 120 minute timer. When three
safety trips occur within that 120 minute window,
a Compressor Safety Lockout occurs.
• When the 120 minute timer expires and three
safety trips have not occurred, the timer resets.
• Approximate safety switch reset times are as fol-
lows:
a. HPCO switch: Approximately 30 seconds to
5 minutes to reset.
b. Compressor protection module: Approxi-
mately 30 minutes to reset.
c. Manual motor starter/overload relay: This
requires a manual reset of the overload relay
as well as the compressor circuit.
• A Compressor Safety Lockout requires a manual
reset through the IPU board.
LD28118
FIGURE 34 -
COMPRESSOR SAFETY CHAIN — WITHOUT COMPRESSOR PROTECTION MODULE