Thermal Overload Relay

bearings show signs of rust, disassemble the device

properly adjusted. Make a contact impression and

and carefully clean the rusted surfaces. Light oil can

check the spring pressure following the

be wiped on sparingly to prevent further rusting. Oil

manufacturer's instructions. If information on the

has a tendency to accumulate dust and grit, which may

correct contact pressure is not available, compare the

cause unsatisfactory operation of the device,

contact pressure with that of similar contacts. When

particularly if the device is delicately balanced.

the force is less than the designed value, the contacts

either require replacing because they are worn down,

Arc chutes or boxes should be cleaned by scraping

or the contact springs should be replaced. Always

them with a file if wiping with a cloth is not sufficient.

replace contacts in sets, not singly; and replace contact

Replace or provide new linings when they are broken

screws at the same time. Do not use emery paper or

or burned too deeply. Be certain that arc chutes are

emery cloth to clean contacts, and do not clean

securely fastened and that there is sufficient clearance

contacts when the equipment is energized.

to ensure that no interference occurs when the switch

or contact is opened or closed.

Clean all surfaces of the circuit breaker

mechanism, particularly the insulation surfaces, with

Shunts and flexible connectors, which are flexed

a dry cloth or air hose. Before directing the air on the

by the motion of moving parts, should be replaced

breaker, be certain that the water is blown out of the

when worn, broken, or frayed.

hose, that the air is dry, and that the pressure is not

Operating tests that consist of operating the circuit

over 30 psi. Check the pins, bearings, latches, and all

breakers in the manner in which they are intended to

contact and mechanism springs for excessive wear or

function in service should be conducted regularly. For

corrosion and evidence of overheating. Replace parts

manually operated circuit breakers, simply open and

if necessary.

close the breaker to check the mechanical operation.

Slowly open and close circuit breakers manually

To check both the mechanical operation and the

a few times to be certain that trip shafts, toggle

control wiring, electrically operated circuit breakers

linkages, latches, and all other mechanical parts

should be tested by the operating switch or control.

operate freely and without binding. Be certain that the

Exercise care not to disrupt any electrical power

arcing contacts make-before and break-after the main

supply that is vital to the operation of the ship or to

contacts. If poor alignment, sluggishness, or other

endanger personnel by inadvertently starting motors

abnormal conditions are noted, adjust the contacts

and energizing equipment under repair.

following the manufacturer's instructions.

Metal locking devices are available that can be

Before returning a circuit breaker to service,

attached to the handles of type AQB circuit breakers

inspect all mechanical and electrical connections,

to prevent accidental operation. All breaker handles

including mounting bolts and screws, drawout

are now provided with a 3/32-inch hole permitting

fastening the locking device with a standard cotter pin.

disconnect devices, and control wiring. Tighten where

NSTM, chapter 300, lists the stock numbers for three

necessary. Give the breaker a final cleaning with a

different sizes of breaker handle locking devices.

cloth or compressed air. Operate manually to be

certain that all moving parts function freely. Check the

OVERLOAD RELAYS

insulation resistance.

The sealing surfaces of circuit-breaker contactor

Overload relays are provided in motor controllers

and relay magnets should be kept clean and free from

to protect the motor from excessive currents.

rust. Rust on the sealing surface decreases the contact

Excessive motor current causes normally closed

force and may result in overheating of the contact tips.

overload relay contacts to open, which break the

Loud humming or chattering will frequently warn of

circuit to the operating coil of the main contactor, and

this condition. A light machine oil wiped sparingly on

disconnects the motor from the line. Overload relays

the sealing surfaces of the contactor magnet will aid

are of the thermal or magnetic type.

in preventing rust.

Oil should always be used sparingly on circuit

Thermal Overload Relay

breakers, contractors, motor controllers, relays, and

other control equipment. Oil should not be used at all

The thermal type of overload relay is designed to

unless called for in the manufacturer's instructions or

open a circuit when excessive current causes the heater

coils to reach the temperature at which the ratchet

unless oil holes are provided. If working surfaces or

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