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To perform these tasks you must first have a
Magnetic Overload Relay
working knowledge of the various types, sizes,
The magnetic type of overload relay has a coil
capacities, and uses of shipboard electrical cable. The
connected in series with the motor circuit and a
IC Electrician must also be capable of selecting,
tripping armature or plunger. When the motor current
installing, and maintaining cables in such a manner as
to ensure their adequacy.
is excessive, the armature opens the overload relay
contacts. Magnetic overload relays may be of the
For many years most of the shipboard power and
instantaneous or time-delay type.
lighting cables for fixed installation had silicone-glass
insulation, a polyvinyl chloride jacket, and aluminum
armor and were of watertight construction. It was
operates instantaneously when the motor current
determined that cables with all these features were not
becomes excessive. The relay must be set at a tripping
necessary for many applications, especially within
current higher than the motor starting current to
watertight compartments and noncritical areas above
prevent tripping when the motor is started. This type
the watertightness level. Therefore, a new family of
of overload relay is used mostly for motors that are
nonwatertight, lower-cost cable was designed. This
started on reduced voltage and then switched to full
new family of cables is electrically and dimensionally
line voltage after the motor comes up to speed.
interchangeable with silicone-glass insulated cables
TIME-DELAY TYPE.-- This type is essentially
of equivalent sizes. This cable is covered by Military
the same as the instantaneous type with the addition
Specification MIL-C-915.
of a time-delay device. The time-delay device may be
Additionally, cables jacketed with polyvinyl
an oil dashpot with a piston attached to the tripping
chloride presented the dangers of toxic fumes and
armature of the relay. This piston has a hole through
dense, impenetrable smoke when undergoing
which oil passes when the tripping armature is moved
combustion. These hazards were noticed when an
due to the excessive motor current. The size of the hole
electrical fire smoldered through the cable ways
can be adjusted to change the speed at which the piston
aboard a naval ship. Due to the overwhelming amount
moves for a given pull on the armature. For a given
of smoke and fumes, fire fighters were unable to
size hole, the larger the current, the faster the
effectively control the fire and a large amount of
operation. This allows the motor to carry a small
damage resulted. A new family of low-smoke,
overload current for a longer period of time than a
low-toxic cable was designed. This cable is
large overload current.
constructed with a polyolefin jacket vice polyvinyl
chloride jacket. This new design conforms to rigid
toxic and smoke indexes to effectively reduce the
hazards associated with the old design. This cable is
covered by Military Specification MIL-C-24643.
Shipboard electrical and electronic systems
require a large variety of electrical cables. Some
A family of lightweight cables has also been
circuits require only a few conductors having a high
introduced to aid in the elimination of excessive
current-carrying capacity; others require many
weight from the fleet. Considering the substantial
conductors having a low current-carrying capacity;
amount of cable present on a ship or submarine, a
still others may require cables with a special type of
reduction in cable weight will have a considerable
insulation, the conductors may have to be shielded, or
impact on the overall load, thus improving
in some cases the conductors may have to be of a metal
performance and increasing efficiency. This new
other than copper.
family of lightweight cables is constructed from
cross-linked polyalkene and micapolymide insulation
The proper installation and maintenance of the
and a cross-linked polyolefin jacket. The lightweight
various electrical systems aboard ship are very
c a b l e is covered by Military Specification
important to the IC Electrician. The repair of battle
damage, accomplishment of ship alterations, and
some electrical repairs may require that changes or
additions to the ship's cables be made by IC personnel.
Additionally, during shipyard and tender avail-
abilities, you may be required to inspect, test, and
Because of the varied service conditions aboard
ship, the cable must have the ability to withstand heat,
approve the new installations.

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