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 ELECTRIC PROTECTIVE SYSTEM
INTRODUCTION
The electric protective system is designed to
The electric protective system consists of the
activate an alarm or shut the engine off if there is a
basic components that follow: tachometer speed sensor,
problem or a failure in any of five different engine
electronic speed switch, water temperature contactor
systems. The engine systems monitored are: engine
switch, two time delay relays and two slave relays. This
overspeed, starter motor crank terminate, engine
system monitors the engine from starting through rated
reversal, engine oil pressure and engine coolant
speed.
temperature.
COMPONENT DESCRIPTION
switches are used [280 kPa (40 psi), 140 kPa (20 psi)].
Tachometer Speed Sensor (TSS) The sensor
Once the step oil pressure speed setting is made, an
generates a signal that measures engine speed, but also
engine that runs above this speed setting must maintain
has a special characteristic that sends the signal in a
an oil pressure that is more than 280 kPa (40 psi). An
certain sequence. If the direction of rotation is changed,
engine that runs at a speed below this speed setting
the signal sequence is changed.
must maintain an oil pressure that is more than 140 kPa
(20 psi). If either condition is not correct, a switch will
Electronic Speed Switch (ESS) The speed switch has
close and cause the fuel to be shut off to the engine.
controls (in a single unit) to monitor four of the basic
functions. These four functions are:
Water Temperature Contactor Switch This contactor
switch is a separate unit (mounted in the regulator
Engine Overspeed An adjustable engine speed setting
housing) that is wired into the shutdown circuit. It has an
(normally 118% of rated speed) that gives protection to
element that feels the temperature of the coolant (it must
the engine from damage if the engine runs too fast. This
be in contact with the coolant). When the engine coolant
condition will cause a switch to close that shuts off both
temperature becomes too high, the switch closes to
the inlet air and the fuel to the engine.
cause the fuel to be shut off to the engine.
Crank Terminate (Starter Motors) An adjustable engine
Time Delay Relays These relays are special ON/OFF
speed setting that gives protection to the starter motor
switches with two controls that will either make the relay
from damage by overspeed. This condition will cause a
activate immediately, or after a 9 second delay. One of
switch to open that stops current flow to starter motor
the time delay relays is used to arm the shutdown
circuit, and the starter motor pinion gear will then
system, and the other time delay relay controls the oil
disengage from engine flywheel ring gear.
pressure circuits for the two oil pressure switches. Both
time delay relays have a 70 second OFF delay to be sure
Engine Reversal A condition where the engine starts to
of complete engine shutdown.
run backwards. This will cause a change in the signal
from the TSS, which will close a switch to cause the fuel
Slave Relays These are standard type relays that, when
to be shut off to the engine.
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 causes the fuel to be shut off, and the other
setting that gives protection to the engine from a failure
relay causes the inlet air to be shut off.
caused by not enough oil pressure. To maintain desired
protection through the complete speed range of engine
operation,
two
different
oil
pressure
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