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Friday, November 6, 2015

AC switch board

(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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