Manufacturer: Johnson Controls
Description: 30sec 8/70# MR 3'CAP LUB CTL
Category: Refrigeration Controls
P28 Series Lube Oil Controls
with Built-in Time Delay Relay
The P28 and P128 Series Lube Oil Controls provide
dependable and economical oil pressure cut-out for
pressure-lubricated refrigeration compressors. The
field-adjustable pressure differential of these controls
provides compressor operation according to the
manufacturer’s specifications. The P28 and P128
controls operate by measuring the net lube oil pressure
and de-energizing the compressor if the pressure falls
below the differential setpoint.
Manual or automatic reset models are available with
factory set and sealed time delays of 30, 45, 60, 90,
or 120 seconds (all time delays may not be available
on all models). The P128 is the same control as the
P28 but with 1/4 inch male flare pressure connections.
Features and Benefits
♦Built-in Time Delay Relay
with Ambient Compensation
Minimizes timing fluctuations due to temperature
♦Trip-free Manual Reset Provides manual reset that cannot be overridden
by pressing and holding the reset button
♦Replaceable Time Delay
Allows easy field replacement of the time delay
relay and terminal board
♦Available with Runlight and
Allows the control to be wired for normal oil
pressure runlight signals and shutdown alarm
circuits for remote monitoring of oil pressure
Personal injury hazard.
P28 and P128 controls are
designed as lubrication
protection controls. Failure of
the P28 or P128 could allow
the refrigeration compressor
to be damaged in a way that
may not be apparent upon
visual inspection. Follow
proper procedures and the
instructions, as well as any
warning signs on or around
the equipment, when
If leakage of sensed
media (such as refrigerant or
oil) can be harmful to the
environment, or hazardous in
any way, user must provide
for proper containment.
The P28 and P128 controls measure the net oil
pressure available to circulate oil throughout a
pressure-lubricated refrigeration system. The net oil
pressure is the difference between the oil pressure at
the pump discharge and the refrigerant pressure in
the compressor crankcase.
Example: If the oil pressure pump discharge reading
is 90 psi (621 kPa) and the crankcase
pressure is 70 psi (483 kPa), the net oil
pressure is 20 psi (138 kPa).
The P28 and P128 have a built-in time delay relay.
This relay allows the oil pressure to build up for the
time delay period before the compressor trips. This
also prevents nuisance lockouts due to intermittent
loss of oil pressure. The time delay relay is a “trip free” device. The manual reset cannot be
overridden by pressing and holding the reset button.
Manual reset models are available with time delays
of 30, 45, 60, 90, or 120 seconds. Automatic reset
models are available with a 90-second time delay.
The time delay relay is compensated to minimize the
effect of ambient temperature variations. However,
the time delay relay will be affected by voltage
When the compressor starts, the timer is energized
because the net oil pressure of the system is zero.
During normal operation, the net oil pressure should
build up to the pressure switch’s cut-out setting
(scale setting) plus the switch differential (3 to 5 psi
[21 to 34 kPa]) within the required time delay,
causing the time delay relay to de-energize.
If the net oil pressure does not rise to the cut-out
pressure setting plus the switch differential within the
required time delay, the time delay relay trips and
stops the compressor.
If the net oil pressure drops below the cut-out
pressure setting during the compressor’s run cycle,
the time delay relay energizes. If the net oil pressure
returns within the time delay, the time delay relay deenergizes
and the compressor continues to operate
normally. If the net oil pressure does not return
within the time delay, the control shuts down and
locks out the compressor.
Replaces Part Numbers: P28AA-10, CARP28AA-10