Power System Most Asked MCQS Part-1

1. Which of the following metals cannot be used as a fuse wire?

A. Copper
B. Iron
C. Silver
D. Lead-tin alloy

Answer: B. Iron

Explanation:
The correct answer is Iron.
The material used for fuse elements must have the following properties:

• Low melting point
• Low ohmic loss
• High conductivity (or low resistivity)
• Low cost
• Free from detraction

The materials used for fuse:

• The material mainly used as fuse elements are tin, lead, silver, copper, zinc, aluminum, and an alloy of lead and tin
• An alloy of lead and tin is used for small current rating fuses
• For current exceeding 15 A, this alloy is not used as the diameters of the wire will be larger and after fusing the metal released will be excessive.
• Beyond 15 A rating circuit, copper wire fuses are employed

Iron is not used as a fuse wire.

2. The voltage that appears across the breaker contacts after the circuit breaker is opened is called.

A. Arc voltage
B. Restriking voltage
C. Recovery voltage
D. Surge voltage

Answer: C. Recovery voltage

Explanation:
Recovery Voltage:
The RMS voltage that appears across the circuit breaker contacts after final arc interruption (when breaker opens) is called “recovery voltage”

Restriking Voltage:
It may be defined as the voltages that appears across the breaking contact at the instant of arc extinction

Active Recovery Voltage:
It may be defined as the instantaneous recovery voltage at the instant of arc extinction

Arc Voltage:

It may be defined as the voltages that appears across the contact during the arcing period, when the current flow is maintained in the form of an arc. It assumes low value except for the point at which the voltage rises rapidly to a peak value and current reaches to zero.

3. The SF6 circuit breakers are preferred for the substation with:

A. 33 kV
B. 11 kV
C. 110 kV
D. 220 kV


Answer: D. 220 kV


Explanation:

Air break

• Air at atmospheric pressure
• Low voltage up to l000 V

Tank type oil circuit breaker

• Oil
• Up to 33 kV

Minimum oil circuit breaker

• Oil
• 36 kV, 1500 MVA
• 132 kV, 3000 MVA

Air blast

• Compressed air
• 132 kV, 220 kV
• 400 kV, 760 kV

SF6

• SF6 gas
• 132 k V, 220 kV
• 400 kV, 760 kV

Vacuum C.B.

• Vacuum
• 11 kV. 33 kV

High voltage direct current C.B.

• Vacuum or SF6 gas
• ± 500 kV DC

4. Which of the following control devices is used to provide protection against accidents by rapidly interrupting dangerous contact voltages that may be present in the electrical equipments as a result of ground faults, insufficient insulation, insulation failure, or misuse and sabotage?

A. Optical switch
B. Push-button control switch
C. Float switch
D. ELCB

Answer: D. ELCB
Explanation:

• ELCB stands for Earth Leakage Circuit Breaker.
• It is a safety device used in electrical installations with high earth impedance to prevent shock.
• It detects small stray voltages on the metal enclosures of electrical equipment and interrupts the circuit if a dangerous voltage is detected.
• ELCBs are specially used to disconnect the supply underground fault condition
• An ELCB must be always connected in the circuit at the entry of supply to load after energy meter
• Earth Leakage Circuit Breaker (ELCB) works on the principle of residual current
• Current operated ELCBs are safer as they operate on the principle of the vector sum of the line currents and neutral current.
• It is used to provide protection against accident by rapidly interrupting dangerous contact voltage which may present in the faulty electrical equipment.

Note:

• The sensation of electric shock is caused by the flow of electric current through the human body to the earth
• When a person comes in contact with electrically live objects like water heaters, washing machines electric iron, etc., the extent of damages caused by this current depends on its magnitude and duration
• This kind of current is called the leakage current which comes in milli-amps
• These leakage currents being very small in magnitude, hence undetected by the fuses/MCBs are the major cause for the fires due to electricity
• Residual current operated circuit breakers provide maximum protection from electric shocks and fires caused due to earth leakage current and also prevents the waste of electrical energy
• These residual current circuit breakers (RCCB) are popularly called as earth leakage circuit breakers (ELCB)

5. The most suitable circuit breaker for having autorecloser is a/an

A. minimum oil circuit breaker
B. air-blast circuit breaker
C. vacuum circuit breaker
D. SF6 circuit breaker

Answer: B. air-blast circuit breaker

Explanation:
Concept:

• Auto-reclosers are most commonly found in overhead cable networks and they are useful because, in such networks, only around 7% of faults are permanent.
• In fact, the vast majority of faults around 80% are transient, caused by events like lightning strikes and arcing, and will disappear in less than a second.
• The remaining 13% of faults are semi-permanent and are typically caused by animals or branches bridging the power lines. Even these faults, however, usually burn away and clear in a relatively short time.
• The extra high voltage transmission lines transmit a huge amount of electric power. Hence, it is always desirable that the continuation of power flow through the lines should not be interrupted for a long time.
• There may be a temporary or permanent fault in the lines. Temporary faults get automatically cleared, and these do not require any attempt for fault rectification.
• It is normal practice by the operators that after each initial faulty tripping of the line, they close the line. If the fault is transient, the line holds after the second attempt of closing the circuit breaker.
• So, the most suitable circuit breaker for having an autorecloser is an air-blast circuit breaker.

6. Which of the following is an example of low voltage fuse?

A. Cartridge type HRC fuses
B. Liquid type HRC fuses
C. Expulsion type HRC fuses
D. Drop out type fuses

Answer: D. Drop out type fuses

Explanation:

Low voltage fuses:

Rewirable fuses:

• This type of fuses mostly used in the small current circuit or for domestic wiring.
• The fuse case and the fuse carrier are the two main parts of the rewirable fuse.
• The base of the fuse is made up of porcelain, and it holds the wires which may be made up of lead, tinned copper, aluminum, or an alloy of tin-lead.
• The fuse carrier can be easily inserted or taken out in the base without opening the main switch.

Cartridge type fuses

• The fuse element is totally enclosed in an enclosed container
• It has metal contacts on both sides.

Dropout type fuses

• In this type, the melting of the fuse causes the fuse element to drop out under gravity about its lower support.
• This type of fuse is used for the protection of outdoor transformers.

Striker type fuses

• It is a mechanical device having enough force and displacement.
• These fuses can be used for closing tripping/indicator circuits.

High Voltage fuses:

Cartridge type HRC fuses

• The fuse element of the HRC fuse is wound in the shape of the helix which avoids the corona effect at the higher voltages.
• It has two fused elements placed parallel with each other, one of low resistance and the other is of high resistance.
• The low resistance wire carries the normal current which is blown out and reducing the short circuit current during the fault condition.

Liquid type HRC fuses

• This type of fuses is filled with carbon tetrachloride (liquid) and sealed at both ends of the caps.
• When the fault occurs then the current, exceed beyond the permissible limit, and the fuse element is blown out.
• The liquid of the fuse acts as an arc extinguishing medium for the HRC fuses.
• They may be employed for the transformer protection and the backup protection to the circuit breaker.

Expulsive type HRC fuses

• The fuse elements are placed in the tubes, and the ends of the tubes are connected to suitable fittings at each end.
• The arc producing is blown off in the inner coating of the tube, and the gases thus formed extinguish the arc.
• Expulsion type fuses are widely used for the protection of feeders and transformer because of their low cost.

7. Lightning arrestors connected in the power system protect electrical equipment from:

A. Frequency fluctuation
B. Over-current due to indirect lightning stroke
C. Over-voltages due to indirect lightning stroke
D. Direct stroke of lightning

Answer: C. Over-voltages due to indirect lightning stroke

Explanation:
Lightning arresters or surge diverters:

• A lightning arrester or a surge diverter is a protective device that conducts the high voltage surges on the power system to the ground
• It consists of a spark gap in series with a non-linear resistor
• The length of the gap is so set that normal line voltage is not enough to cause an arc across the gap, but a dangerously high voltage will break down the air insulation and form an arc
• The property of the non-linear resistance is that its resistance decreases as the voltage (or current) increases and vice-versa
• One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded

Important Points:

• The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves that may reach the terminal apparatus.
• The lightning arresters or surge diverters provide protection against such surges caused by indirect lightning stroke.

8. Which is the most serious problem in vacuum circuit breaker?

A. Poor arc quenching
B. Low thermal stability
C. Current chopping
D. All of the above

Answer: C. Current chopping

Explanation:
Current chopping in circuit breaker is defined as a phenomenon in which current is forcibly interrupted before the natural current zero.



We can derive the voltage across the capacitor during current chopping




Current Chopping is mainly observed in the vacuum circuit breaker and air blast circuit breaker. There is no such phenomenon in oil circuit breaker.

9. Which metal is used in high rupturing capacity fuses and in special contacts:

A. Silver
B. Nickel
C. Tin
D. Zinc

Answer: A. Silver

Explanation:
High Rupturing Capacity fuse:

• It consists of a heat-resisting ceramic body having metal end-caps to which is a welded silver current-carrying element.
• The space within the body surrounding the element is completely packed with a filling powder.
• The filling material may be chalk, plaster of paris, quartz or marble dust and acts as an arc quenching and cooling medium
• Under normal load conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating.
• When a fault occurs, the current increases and the fuse element melts before the fault current reaches its first peak.

Important Point:

Advantages of HRC fuse:

• They can clear high as well as low fault currents
• They do not deteriorate with age
• They have a high speed of operation
• They provide reliable discrimination
• They require no maintenance
• They are cheaper than other circuit interrupting devices of equal breaking capacity
• They permit consistent performance

Disadvantages of HRC fuse:

• They must be replaced after each operation
• The heat produced by the arc may affect the associated switches

10. HRC fuses are used for:

A. overvoltage protection
B. distance protection
C. overcurrent protection
D. differential protection

Answer: C. overcurrent protection

Explanation:

(HRC) High Rupturing Capacity fuse:

• It consists of a heat-resisting ceramic body having metal end-caps to which is welded silver current-carrying element
• The space within the body surrounding the element is completely packed with a filling powder
• The filling material may be chalk, plaster of paris, quartz or marble dust and acts as an arc quenching and cooling medium
• Under normal load conditions, the fuse element is at a temperature below its melting point. Therefore, it carries the normal current without overheating.
• When a fault occurs, the current increases and the fuse element melts before the fault current reaches its first peak. Hence it is used for overcurrent protection.

Important Point:

Advantages of HRC fuse:

• They can clear high as well as low fault currents
• They do not deteriorate with age
• They have a high speed of operation
• They provide reliable discrimination
• They require no maintenance
• They are cheaper than other circuit interrupting devices of equal breaking capacity
• They permit consistent performance

Disadvantages of HRC fuse:

• They must be replaced after each operation
• The heat produced by the arc may affect the associated switches

11. A thyrite lightning arrester has

A. Inverse resistance characteristics
B. a gap
C. efficient earthing
D. a combination of inverse resistance characteristics and gap

Answer: D. a combination of inverse resistance characteristics and gap

Explnation:

• A thyrite lightning arrester has a combination of inverse resistance characteristics and gap
• Thyrite arrester is most common and is mostly used for the protection against high dangerous voltages
• The manufacturing processes have been perfected so that the electrical and mechanical characteristics can be duplicated or varied as desired within practical limits
• It does not follow Ohm's law, for each time the voltage is doubled the current increases 12.6 times
• Thus, it possesses nonlinear characteristics

12. Which of the following properties must be used for the fuse material used in the wire?

A. Low melting point
B. Low conductivity
C. High melting point
D. High resistivity


Answer: A. Low melting point

Explanation:
The material used for fuse elements must have the following properties:

• Low melting point
• Low ohmic loss
• High conductivity (or low resistivity)
• Low cost
• Free from detraction

Important Points:

• The material mainly used for fuse element are tin, lead, silver, copper, zinc, aluminium, and an alloy of lead and tin
• An alloy of lead and tin is used for small current rating fuses
• For current exceeding 15 A, this alloy is not used as the diameters of the wire will be larger and after fusing the metal released will be excessive
• Beyond 15 A rating circuit, copper wire fuses are employed

13. Rewireable fuse has:

A. Inverse time current characteristic
B. Linear time current characteristic
C. Square law time current characteristic
D. None of the above

Answer: A. Inverse time current characteristic

Explanation:

• Rewireable fuse works on inverse time-current characteristics
• Rewireable fuse is also known as the kit-kat fuse
• Whenever the current exceeds the predefined value, the flow of current heats the coil and melts it and the fuse is blown out
• Once the fuse is blown out, it can be reused by replacing the fuse element with a new one
• The normally used fuse wires are lead and aluminium
• Tinned copper and an alloy of tin-lead can also be used

14. The breaking current of a circuit breaker is

A. Less than it’s making current
B. More than it’s making current
C. Equal to its making current
D. None of the other options


Answer: A. Less than it’s making current
Explanation:


Making current of a circuit breaker is the peak value of maximum current loop during sub transient condition including DC component when breaker closes.

Breaking current of a circuit breaker depends on the instant on the current wave where the contacts of breaker start separating.

Making current is more than breaking current and the relation between them is,Symmetrical making current = 2.55 × symmetrical breaking current

15. At what level of current flow during electric shock death is possible.

A. 1 to 8 milli amp
B. 50 to 100 milli amp
C. 10 to 20 milli amp
D. 20 to 50 milli amp


Answer: B. 50 to 100 milli amp

Explanation:
Current: 1 milliamps
Reaction: Just a faint tingle

Current: 5 milliamps
Reaction: Slight shock, disturbing, but not painful. Most people can “let go.” However, strong involuntary movements can cause injuries.

Current: 6-30 milliamps
Reaction: Painful shock. Muscular control is lost. This is the range where “freezing currents” start. It may not be possible to “let go.”

Current: 50-150 milliamps
Reaction: Extreme pain, respiratory arrest, severe muscular contractions, individual cannot let go. Death is possible.

Current: 1,000 – 4,300 milliamps
(1-4.3 amps)
Reaction: Ventricular fibrillation (uneven uncoordinated pumping of the heart.) Muscular contraction and nerve damage begins to occur. Death is likely.

Current: 10,000 milliamps
(10 amps)
Reaction: Cardiac arrest and severe burns occur. Death is probable




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