(a)
State what protective devices are carried on, and
peculiar to AC switchboards. (3m)
(b)
State why a breaker may not open upon severe and
prolonged voltage dip. (2m)
(c)
Compare the effectiveness of filled cartridge and
rewireable fuses for three phase circuit protection. (3m)
(d)
State with reason the immediate action to be taken upon
failure of a fuse in such a system. (2m)
Answer :-
(a)
The protective devices that are fitted on AC
switchboards are :
1) overload
trip
2) under
voltage protection
3) reverse
power relay
4) preferential
trip.
For marine application, provision
of protection for each phase is very rare. The most important reason concerning
the earthing of the system neutral. Normal internal generator faults are
between one phase and earth. On a typical marine system, with an insulated
neutral this faults can be tolerated until appropriate action can be taken.
(b)
Most breakers uses magnetic contact spring forces tends
to open sufficient voltage to magnetise are for long time, contact overheated,
melts and welded together, thus it cannot be opened after a prolonged voltage
dip.
(c) Effectiveness
of cartridge fuses in AC 3 phase circuit protection are :
1) cartridge
fuses or high rupturing fuses can be used for very high fault current levels.
2) Time
/ current fusing characteristic are very consistent.
3) Cartridge
fuses are sized to ensure that the correct fuses are inserted.
While rewireable fuses :
1) rewireable
fuses tend to deteriorate with use.
2) They
are corroded by the surrounding atmosphere.
3) They
do not have any accurate time – current characteristic.
(d) Upon failure
of a fuse in such a system, it must be replaced by a fuses of same type and
same rating. The reason is because loss of one fuses out of three can cause
single phasing resulting the current in the energised winding approximately to
double.
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