Marine Electrotechnology. Test 2

A 3-phase A.C. induction motor is running normally at its rated current of 150 A when a single phasing fault (open circuit) occurs in one line. The likely outcome will be:

Line currents: 150 A,150 A,150 A. Trip condition: fuses blow on short circuit.
Line currents: 0 A, 150 A, 300 A. Trip condition: trip on overload.
Line currents: 75 A, 75 A, 75 A. Trip condition: no trip but speed falls.
Line currents: 0 A, 180 A, 180 A. Trip condition: trip on overload.

A ships 3 - phase a.c. circuit has 440 V and 220 V transformer linked sections. The effect of a single earth fault on a 220 V line will cause the following earth lamp indication:

In 440 V section: All lamps equally bright. In 220 V section: One lamp dim.
In 440 V section: One lamp dim. In 220 V section: One lamp dim.
In 440 V section: Two lamps bright. In 220 V section: Two lamps bright.
In 440 section: All lamps equally bright. In 220 section: All lamps dark.

Under normal conditions, the electric power for services supplied from the emergency switchboard is supplied from:

The main diesel generator or shaft generator.
Emergency batteries
An emergency battery charger.
A compressed air driven generator.

An earth fault exists on the blue line of a 100 A bilge pump circuit. A second earth fault occurs on the yellow line of a 10 A ventilation fan circuit. The likely outcome is that:

Both motors trip out on overload
An open circuit occurs between earth fault and both motors trip out
A short circuit occurs between earth faults and the ventilation fan fuse blows
A short circuit occurs between earth fault and the bilge pump fuse blows

When a large motor load suddenly is disconnected from the switchboard and the generator is AVR controlled, the voltage will:

Remain approximately constant, due to governor action.
Initially fall, then reset to the set value.
Initially rise, then reset to the set value.
Remain approximately constant, due to AVR action.

After main power is restored, a timed sequential restart of motor-driven auxiliaries is necessary to avoid:

Overvoltage due to current surges
Generator overload due to many motors starting at simultaneously
Overloading creating earth faults
Over-frequency due to overspeed of generator

A motor is protected by a thermal overcurrent relay. After tripping on overload it will not be possible to reset the overcurrent relay because the:

Oil dash-pot has also to be reset.
Line contactor spring also has to be reset
Starter isolator automatically applies an interlock.
Bimetallic strips need time to cool down

Choose the group of electrical power supplies that best fits the identification of 'Mains'

No. 1 diesel and No. 2 diesel
Shaft generator and emergency diesel
Emergency diesel and batteries
Batteries and No. 1 diesel

Review Answers

Marine Electrotechnology. Test 2

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