some ships, however, GSEs are responsible for the
power relays in controllers that control equipment, such
as fuel pumps and turbine ventilation systems.
Construction and Operation
To test a relay, you must understand its construction
and operation. A relay acts according to the same basic
principle as a solenoid. In fact, the basic difference
between a relay and a solenoid is that the relay does not
have a movable core (plunger).
A relay consists of a magnetic core, and its
a s s o c i a t e d coil, contacts, springs, armature, and
mounting. Figure 5-1 shows the construction of a relay.
Figure 5-2.--Relay contact arrangement.
When the coil is energized, the flow of current through
the coil creates a strong magnetic field. This field pulls
the armature downward to contact Cl, completing the
circuit from the common terminal to Cl. At the same
Maintenance
time, the circuit to contact C2 is opened.
The relay is one of the most dependable electrical
Most of the relays in the propulsion control system
devices in use. Like any other mechanical or electrical
are the multiple-contact type. Figure 5-2 shows a
d e v i c e , relays occasionally wear out or become
multiple-contact relay that has four different contact
inoperative. If you detect that a relay is faulty, remove
combinations. Any number of sets of contacts may be
the relay and replace it with another of the same type.
built onto the relay. This configuration makes it possible
Make certain you get the same type of relay as a
to control many different circuits at the same time. This
replacement. Relays are rated in voltage, amperage, type
type of relay can be a source of trouble because the
of service, and number of contacts. Relay coils usually
motion of the armature does not assure movement of all
consist of a single coil. If a relay fails to operate, test the
the movable contacts.
coil for an open circuit, a short circuit, or a short to
ground. An open coil is a common cause of relay failure.
During preventive maintenance, you should check
for conditions that will cause a relay to fail. When
checking a relay, make certain you follow all general
safety precautions. The conditions that usually cause a
relay to fail consist of the following:
Charred or burned insulation on the relay
Darkened or charred terminal leads
Loose power terminal connections
Film buildup on the contact surfaces
Bent or broken contact arms
If a relay fails to function, examine the movement
of the contacts. Contact clearances or gap settings must
be maintained according to the operational
specifications of the relay. When the relay has bent
Figure 5-1.--Basic relay construction.
contact arms, you should use a point bender (shown in
5-5
