Page 669 - Industrial Power Engineering and Applications Handbook

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Oil level indicator
Vent liberated gases in the breaker
Oil filling vent

Upper pole head
Contact tube
Connecting terminals

Spring supported tulip contacts
(fixed contacts)

Arc quenching device
Arc chamber
Moving contact rod
Contact roller guide
Connecting terminals
Contact rollers

Lower pole head
Figure 19.7 Minimum oil content circuit breaker (MOCB)
(Courtesy: NGEF Ltd.) Oil drain plug
Level connecting operating
breaker during the late 1960s and onwards. Their rupturing mechanism
capacity is also much higher than that of a BOCB and
they are extremely suitable for distribution systems with Figure 19.8 Cross-sectional view of a typical pole assembly of
moderate fault levels. The trend, however, has tilted in an 11 kV MOCB (Courtesy: NGEF Ltd.)
favour of more advanced technologies, now available in
the form of vacuum or SF, breakers. MOCBs are available
from 6 kV to 420 kV and have a rupturing capacity of so that the arc plasma produces a magnetic field through
250-25 000 MVA. these splitters and rises upwards, and splits into a number
of shorter arcs, to lose all its heat through convection.
19.5.3 Air Circuit Breakers (ACBs) This renders the TRV insufficient to cause a restrike.
The long arc length and subsequent cooling increases
Refer to the general arrangement of this breaker in Figure the resistance of the arc plasma and improves the p.f. of
19.9(a), and (b). the interrupting circuit. It thus helps to bring the current
The moving contacts make and break in air as shown phasor closer to the voltage, and make interruption on a
in Figure 19.10. During interruption, the arc is formed current zero less severe as a result of low TRV. (Refer to
(Figure 19.11) producing N2 (80%) and O2 (20%) and curves a and b of Figure 17.1 1 .) The gradual rise of arc
metallic vapours. The quenching and extinction of arc resistance after a current zero dampens the TRV and
plasma is achieved through the elongation of arc, which makes such breakers almost immune to switching surges.
increases the area of cooling, on the one hand and requires For higher currents, the arc splitter plates may be altered
a higher TRV to cause a restrike, on the other. To obtain to have a variety of designs, such as with offset slots,
this, arc chutes are provided on the top of the interrupting serpentine splitter plates or similar features to effectively
contacts, as illustrated in Figures 19.12(a) and (b). The arrest the arc plasma within the arc chutes, rendering it
design of the arc chutes is such that it drives the arc incapable of causing a restrike after a current zero.
plasma upwards and elongates it to provide the required Such breakers are normally produced for use on an LT
cooling effect. This is achieved by constructing the arc system only. At higher voltages, while interrupting heavy
chute housing of a non-magnetic material, such as glass, currents (such as on a fault) the arc energy may be so
asbestos, ceramic or Bakelite, suitable to withstand the high that a disproportionate size of arc chutes may be
very high temperature of the arc plasma. Metallic arc required to arrest and extinguish the arc, leading to
splitter plates (fins) are fixed inside the arc chute housing disproportionate size of ACB.