Switchgear and Protection.pdf


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Mode of arc extinction:
1) High resistance interruption:
In this process the resistance of arc is increased by lengthening and cooling it to such an
extent that the system voltage cannot maintain the arc and thus extinguished. This is used
in Air CB and DC CB.
2) Low resistance or zero point interruption:
In this process the arc gets extinguished at the natural current zero of the alternating
current zero and is pretended from restriking again by rapid increasing the dielectric
strength of the contact space. Used in almost all ac circuit breaker.

Technical particulars of a CB:
1.
2.
3.
4.
5.

Type of medium for arc extinction.
Rated voltage.
Rated breaking current
Other rated characteristic (TRV, restriking voltage, make current etc.)
Types of construction : indoor metal clad
Outdoor type
Metal clad
gas insulated type
6. Types of operating mechanism: spring opened, spring closed mechanism
Solenoid closed, spring opened mechanism
Hydraulic mechanism
Pneumatic mechanism
7. Total break time : this is (relay+ CB time)
= (instant of fault to closer of trip circuit) +
(Closer of trip circuit to final arc extinction time)
8. Structure form : live tank type ,
Dead tank type
ȱ

Speed of CB and relay:
Before 1930: CB interrupting time: 15-30 cycles.
Relay time: 6-120 cycles
1930 --- CB time -8 cycles
1935 --- CB time-3 cycles
Present day’s relay time becomes as small as 1-3 cycles.
Switchgear and protection

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