Page 344 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

In three-phase circuit breakers, there are three series trip coils, three dash pots, three plungers. They can operate
the trip mechanism together or independently.
Under voltage release coils: The under-voltage release coil is used in installations where detection and isolation
of abnormally low voltage is required. The construction of the under-voltage trip coil is in Fig 5 is similar to the trip
coils discussed above except that the plunger is held away from the pole piece by a coiled spring. Under normal
operating conditions, the solenoid is energised and the plunger is held down against the force of the spring. When
the supply voltage falls, the under - voltage release coil will not be in a position to hold the plunger down against
the spring tension. Thus the plunger moves up and pushes the trip bar to trip the circuit breaker.

Fig 4 Fig 5

Repair and maintenance of CBs

Objectives: At the end of this lesson you shall be able to
• explain the procedure to carry out maintenance and repair of a OCB
• state the method to adopt checking and maintenance/repair of ACB & VCB
• explain the procedure of the condition of SF6 circuit breakers and their repair and maintenance.

Any circuit breaker has the fundamental operation is to make and break the circuit. The design and operating
procedure depends on the breaking/making load current in the circuit. Selection of quenching medium (oil, air,
vacuum or gas) and the volume is involved main factors and proper maintenance is very important to keep the
breaker accurate performance and long life.
Maintenance & repair of oil breaker

It is the first generation of circuit breaker used in the electrical protection circuit and it is still in use. High insulated oil is
the main quenching medium and the oil storage maintenance is quite difficult. Frequent purification, reconditioning,
refilling and leak proof storage etc. keeps the breaker always healthy. Due to this oil storage, recondition and
refilling problem, oil circuit breakers are replaced by modern vacuum circuit breaker. Troubleshooting chart at this
end will help to carryout smooth maintenance and repair of OCB.
Maintenance & repair of ACB, & VCB
Air circuit breaker are found in variety of applications such as very low, low, medium and high current applications.
Natural air with arc chutes in chamber found useful in very low and low to medium circuit breaker. Very high
voltage to EHT lines the VCB are used very extensively.
Natural air or forced air used for ACBs arc chutes are common in both ACB chamber, but in high voltage ACB
forced or compressed air blow is used. To produce compressed air, air chamber, air compressor is necessary in
order to operate ACB.

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116