QUE-ANS

10-30 words

[1]        What are the functions of circuit breakers fuses relays?

The circuit breakers and switches are the means of controlling the flow of electric current. The fuses and the relays protect the distribution system from large fault currents.

[2]        What is system protection discrimination?

System protection discrimination is provided so the generator breaker only trips if a feeder breaker fails to do so or a bus bar fault occurs.

[3]        Why can the fault current be higher when the alternator is cold?

The fault current can be higher when the alternator is cold because the field resistance will be low then.

[4]        In practice what is the time delay from 10% full load capacity?

In practice the time delay from 10% full load currents is 20 seconds.

[5]        What is the usual short circuit condition time delay for alternator over current protection?

The usual short circuit condition time delay for alternator over current protection is 0.1 and 3 seconds, the actual setting depending on discrimination requirements.

[6]        What is the common characteristic of all over current relays?

The common characteristic of all over current relays is the bigger the current, the faster it will operate.

[7]        Why is higher viscosity oil used in marine dashpots?

Higher viscosity oil is used in marine dashpots because of the higher ambient temperatures.

[8]        By what percentage would the rating of a circuit breaker change from its free air value due to switchboard mounting?

The percentage the rating of a circuit breaker would change from its free air value due to mounting in a switchboard would be around 80 or 90%.

[9]        What is the maximum temperature allowable with rated full load current?

The maximum temperature allowable with rated full load current is 80 degrees C (i.e… 40 degrees C rise above ambient of 40 degrees C).

[10]      What determined the size of the short circuit fault current?

The size of the short circuit current is determined by the total impedance of the generators, cables and transformers in the circuit between the generator and the fault.

[11]      What apart from overload can initiate preference tripping?

Apart from overload, preference tripping may be initiated by low generator frequency.

[12]      What kind of over current protection is found in MCCB and MCBs?

The type of over current protection found in MCCB’s and MCB’s are thermal relays.

[13]      How is preference tripping tested?

Preference tripping is tested by current injection.

[14]      What can be used instead of reverse power protection?

Instead of reverse power protection, electrical interlocks or contacts which will respond to various condition such as closing of the fuel or steam admission valve.

[15]      What is the most common form of overload protection?

The most common type of overload protection is the bimetallic thermal overload relay.

[16]      What is the advantage of the thermal relay over the magnetic relay?

The advantage of a thermal relays over a magnetic relay is that they can be approximately designed to follow the motor heating curve.

[17]      What is the disadvantage of thermal relays?

The disadvantage of thermal relays is that when circumstances require a motor can not be restarted immediately until the bimetallic strips have cooled and reset.

[18]      What is used to delay the operation of a magnetic relay?

The delay in a mechanical relay can be an oil dashpot, the time lag on all oil dashpots being dependent on the viscosity of the oil used; this is further affected by temperature.

[19]      What built in motor overload protection can be provided?

Built in motor overload protection can be achieved by using thermisters.

[20]      What does this protect against primarily?

Primarily this protects against causes of motor overheating which are not reflected in the motor current i.e., blocked ventilation.

[21]      What is the advantage of the thermostat over the thermisters?

The advantage of a thermostat over a thermisters is that they are not as bulky and as expensive and they can operate on control gear without additional equipment.

[22]      What circuit protection devices are fitted on switchboards?

Circuit safety devices fitted to switchboards are; circuit breakers, fuses, reverse relays, current sensitive relays to provide protection against faults in the distribution system.

[23]      Under fault conditions what should be the last protection device to operate?

Under fault conditions, the last protection device to operate should be the generator circuit breaker.

[24]      Give two methods of tripping non vital loads to ensure essential supplies?

Two methods of tripping non vital loads to ensure essential supplies, is automatic tripping of all non vital supplies or have a timed sequence of tripping of non vital loads until the overload is reduced to an n acceptable value.

Ø 30-60 words

[1]         Why HRC fuse is normally preferred for electrical installation onboard?

1. Time/current fusing characteristics are very consistent.
2. HRC fuses are reliable and non-deteriorating in service
3. Fuses have very high breaking capacity.
4. They also have very high speed of operation at high short circuit fault, faster than a circuit Breaker.

Ø 60-100 words

[1]        Compare the effectiveness of filled cartridge and rewire able fuses for three phase circuit protection.

Effectiveness of cartridge fuses in AC 3 phase circuit protection are:

Cartridge fuses or high rupturing fuses can be used for very high fault current levels.

Time / current fusing characteristic are very consistent.

Cartridge fuses are sized to ensure that the correct fuses are inserted.

While rewire able fuses :

Rewire able fuses tend to deteriorate with use.

They are corroded by the surrounding atmosphere.

They do not have any accurate time – current characteristic.

[2]        How will you come to know whether a fuse is blown out or not, explain two methods?

1. Live circuit: By using multi-tester in voltage mode check the voltage at either ends respectively w.r.t. to neutral line or other phase. If voltage is available on both ends fuse is OK else it is blown off.
2. Cold check; By using the multi-tester in ohm mode connect the leads on both ends of the fuse. If it showing no resistance it is OK else it is blown off.

[3]        How a fuse is selected for circuit protection ?

1. Rating of fuse for lighting or heating circuit: it is the current which a fuse will carry continuously. For a 50A circuit, a 50A fuse would be appropriate, unless it was a motor circuit.
2. Rating of fuse for motor circuit: in motor circuits, fuse should be large enough to carry starting current for the time necessary to start the motor plus necessary margin.

Ø 100-150 words

[1]        Explain why it is not good practice to replace only single fuse in a 3 phase motor and why fuse clips and connections need examination during fuse renewal.

It is not a good practice to replace only a single fuse in a three phase motor because:

Due to aging of the fuses and cause dampness of the fuse.

Material properties f the fuse will be weakened due to expose to the increased current.

Rating will be drop and could operate at a much lower operating value than it was originally designed.

Thus the loss of machinery during normal operation could occur and it could be dangerous if it were on essential service equipment. As we know, burnt of one fuse can cause single phasing resulting the current in the energized winding approx. to double. Frequent single phasing can damage the respective machinery which is unsafe while operating.

Fuse clips and connections should be examined when renewing fuses as a scored contact may cause, operation below the correct value due to being oxidized, melted or weakened by the high current.