Page 343 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Tripping mechanism of circuit breakers

Objectives: At the end of this lesson you shall be able to
• state the necessity of tripping mechanism
• state the types of tripping mechanism.

Tripping mechanism of circuit breakers
Trip mechanism: Trip mechanism is incorporated in the circuit breaker to switch off the circuit breaker at faulty
condition either automatically or manually at the desired time.
Fig 1 shows the arrangement. When the circuit breaker is closed, the mechanism is locked in position by a system
of linkages. This lock can be released by lifting the trip bar.
Trip bar is attached to the tripping lever which in turn can be operated manually. The tripping lever is generally
kept locked. When the trip bar is lifted the mechanism opens the breaker contacts.
Trip coils: When remote operation is desired, trip coils are used. The trip coils are small solenoids either operated
by AC or DC supplies. Fig 2 shows the general arrangement of the trip coil mechanism. A plunger moves freely
inside the solenoid. When the solenoid is energised by the trip switch the plunger moves up and release the lock
which holds the trip bar. Further the trip coils are also actuated by short circuit /overload and under -voltage relays
as described in the following paragraphs.
Shunt trip coils: The shunt trip coil requires an auxiliary supply, a C.T and a relay. The relay can be set to give
time-graded protection. The relay closes the trip coil circuit when the load current exceeds the stipulated value.
This relay is in Fig 3.

Fig 1 Fig 2

Fig 2

Fig 3

Series trip coil: The series trip coil mechanism is in Fig 4 consists of a series solenoid with a plunger controlled
by a spring. When current in the load become excessive the plunger rises and trips the mechanism.
The current necessary to trip the circuit breaker is regulated by a screw which adjusts the tension of the spring
controlling the plunger. Time-lag can be adjusted by the position of the dash pot which holds the piston of the
plunger in the oil bath.

330

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