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ELECTRIC PROTECTIVE SYSTEM
SYSTEMS OPERATION
ELECTRIC PROTECTIVE SYSTEM
INTRODUCTION
The electric protective system is designed to activate an
The electric protective system consists of the basic
alarm or shut the engine off if there is a problem or a
components that follow:  magnetic pickup, electronic
failure in any of four different engine systems.  The
speed switch, water temperature contactor switch, two
engine systems monitored are:
engine overspeed,
time delay relays and two slave relays.  This system
starter motor crank terminate, engine oil pressure and
monitors the engine from starting through rated speed.
engine coolant temperature.
COMPONENT DESCRIPTION
operation, two different oil pressure switches are used
Magnetic Pickup (MPU) The magnetic pickup is a
[280 kPa (40 psi), 140 kPa (20 psi)]. Once the step oil
single pole, permanent magnet generator made of wire
pressure speed setting is made, an engine that runs
coils around a permanent magnet pole piece.
above this speed setting must maintain an oil pressure
As the teeth of the flywheel ring gear go through the
that is more than 280 kPa (40 psi). An engine that runs
magnetic lines of force around the pickup, an AC voltage
at a speed below this speed setting must maintain an oil
is made. A positive voltage is made when each tooth
pressure that is more than 140 kPa (20 psi). If either
goes by the pole piece. Each time the space between
condition is not correct, a switch will close to activate an
the teeth goes by the pole piece, a negative voltage is
alarm or cause the fuel to be shut off to the engine.
made.
Engine speed is then determined by the
frequency of these signals.
Water Temperature Contactor Switch This contactor
switch is a separate unit (mounted in the regulator
Electronic Speed Switch (ESS) The 4W2218 Electronic
housing) that is wired into the shutdown circuit. It has an
Speed Switch can easily be identified by its black color
element that feels the temperature of the coolant (it must
(the earlier speed switch was yellow). This speed switch
be in contact with the coolant). When the engine coolant
has controls (in a single unit) to monitor three of the
temperature becomes too high, the switch closes to
basic functions. These three functions are:
activate an alarm or cause the fuel to be shut off to the
engine.
Engine Overspeed An adjustable engine speed setting
(normally 118% of rated speed) that gives protection to
Time Delay Relays These relays are special ON/OFF
the engine from damage if the engine runs too fast. This
switches with two controls that will either make the relay
condition will cause a switch to close that shuts off both
activate immediately, or after a 9 second delay. One of
the inlet air and the fuel to the engine.
the time delay relays is used to arm the shutdown
system, and the other time delay relay controls the oil
Crank Terminate (Starter Motors) An adjustable engine
pressure circuits for the two oil pressure switches. Both
speed setting that gives protection to the starter motor
time delay relays have a 70 second OFF delay to be sure
from damage by overspeed. This condition will cause a
of complete engine shutdown.
switch to open that stops current flow to starter motor
circuit, and the starter motor pinion gear will then
Slave Relays These are standard type relays that, when
disengage from engine flywheel ring gear.
energized, have contacts that open across one circuit
and close across another circuit. When activated, one of
Engine Step Oil Pressure An adjustable engine speed
the relays will activate an alarm or cause the fuel to be
setting that gives protection to the engine from a failure
shut off, and the other relay will cause the inlet air to be
caused by not enough oil pressure. To maintain desired
shut off.
protection through the complete speed range of engine
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