Magnetic Overload Relay

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

INSTANTANEOUS TYPE.-- This type

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

ELECTRICAL CABLES

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

MIL-C-24640.

damage, accomplishment of ship alterations, and

some electrical repairs may require that changes or

CLASSIFICATIONS OF CABLES

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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