Fuse MCQ Quiz - Objective Question with Answer for Fuse - Download Free PDF

Explanation:

The Maximum Fault Current and Cut-off Current

Definition: The maximum fault current reached before a fuse melts is referred to as the cut-off current. It represents the peak current value that the fuse allows to pass during a fault condition before it interrupts the circuit. This parameter is crucial in designing protective devices and ensuring the safety of electrical systems.

Working Principle: Fuses are protective devices designed to safeguard electrical circuits from overcurrent conditions. When a fault occurs (e.g., a short circuit or an overload), the current in the circuit rises rapidly. If this current exceeds the rated capacity of the fuse, the fuse element heats up and melts, breaking the circuit and preventing further damage to the system.

However, before the fuse melts, the current often reaches a peak value. This peak value is called the cut-off current. The time required for the fuse to melt depends on the current's magnitude and the fuse's characteristics, such as its material and design. The cut-off current is an essential parameter because it determines the maximum stress imposed on the electrical system during a fault.

Advantages of Considering Cut-off Current:

• Protects electrical components from thermal and mechanical damage caused by excessive current.
• Helps in designing robust and reliable protective devices for electrical systems.
• Ensures the safety of both the equipment and personnel by interrupting fault currents swiftly.

Applications: The concept of cut-off current is widely used in electrical engineering, particularly in the design and selection of protective devices like fuses and circuit breakers. It is also crucial in determining the withstand capability of electrical components during fault conditions.

Correct Option Analysis:

The correct option is:

Option 3: Cut-off current

This option accurately describes the phenomenon where the maximum fault current is reached before a fuse melts. The term "cut-off current" is used to denote this peak value, which is a critical parameter in the functioning of fuses and other protective devices. The fuse interrupts the circuit as soon as this current is reached, protecting the system from further damage.

Important Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Rupturing Current

Rupturing current refers to the maximum current that a protective device, such as a fuse or circuit breaker, can safely interrupt without causing damage to itself or the system. While related to the concept of cut-off current, rupturing current is not the same. The cut-off current pertains to the peak current value just before the fuse melts, whereas rupturing current is the maximum current the device can handle while breaking the circuit.

Option 2: Holding Current

Holding current is a term commonly associated with devices like thyristors or SCRs (Silicon Controlled Rectifiers). It represents the minimum current required to keep the device in a conducting state. If the current falls below this value, the device turns off. This term is unrelated to the operation of fuses and fault currents.

Option 4: Peak Current

Peak current refers to the highest instantaneous current value in an electrical circuit during its normal operation or under fault conditions. While the cut-off current is a type of peak current specific to fault conditions in fuses, not all peak currents are cut-off currents. Therefore, this option is not precise in the context of the given question.

Option 5: Not Provided

This option is not relevant as it does not describe any specific phenomenon related to the operation of fuses or fault currents.

Conclusion:

The concept of cut-off current is pivotal in understanding the operation of fuses and other protective devices in electrical systems. It represents the peak fault current that occurs just before a fuse melts, safeguarding the circuit from further damage. By analyzing the other options, it is evident that the term "cut-off current" provides the most accurate description of the phenomenon in question. Electrical engineers must carefully consider this parameter when designing and selecting protective devices to ensure the safety and reliability of electrical systems.

Explanation:

Purpose of a Fuse in an Electric Circuit

Definition: A fuse is a safety device used in electrical circuits to protect electrical equipment and wiring from excessive current caused by overloads or short circuits. It is designed to act as a sacrificial device, interrupting the circuit when the current exceeds a specified safe level.

Working Principle: The fuse contains a thin wire or strip made of a material with a low melting point, such as tin, lead, or a combination of alloys. When an excessive current flows through the circuit due to a fault, the heat generated by the high current causes the fuse wire to melt. This breaks the circuit, stopping the flow of current and preventing further damage to the equipment or the wiring.

Advantages:

• Provides protection against overcurrent and short circuits by interrupting the circuit before damage occurs.
• Simple, cost-effective, and easy to replace after it blows.
• Ensures the safety of both electrical devices and users by preventing potential fire hazards or equipment failures.

Disadvantages:

• Once blown, the fuse must be replaced to restore the circuit's functionality.
• May not be reusable, unlike circuit breakers, which can be reset after tripping.
• Requires careful selection of the correct fuse rating to ensure effective protection without unnecessary interruptions.

Applications: Fuses are widely used in various applications, including residential electrical systems, industrial machinery, automotive circuits, and electronic devices, to provide overcurrent protection and ensure safe operation.

Correct Option Analysis:

The correct option is:

Option 1: To safeguard the installation against heavy current.

This option accurately describes the primary purpose of a fuse in an electric circuit. The fuse is specifically designed to protect electrical installations, devices, and wiring from damage caused by heavy current or overcurrent conditions. When the current exceeds the safe limit, the fuse blows, breaking the circuit and preventing the flow of excessive current, thereby safeguarding the installation.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: To reduce the current flowing in the circuit.

This option is incorrect. A fuse does not reduce the current flowing in the circuit; rather, it acts as a safety device that interrupts the circuit when the current exceeds a certain threshold. The function of a fuse is to protect the circuit, not to regulate or reduce the current.

Option 3: To reduce the power consumption.

This option is also incorrect. The fuse does not play any role in reducing power consumption. Power consumption depends on the electrical load connected to the circuit and the voltage supply. The fuse is solely responsible for protecting the circuit from overcurrent or short-circuit conditions.

Option 4: To improve power factor.

This option is not relevant to the function of a fuse. Improving the power factor involves the use of capacitors or other power correction devices to reduce the phase difference between voltage and current in an AC circuit. The fuse has no impact on the power factor of the circuit.

Option 5: (Blank or not applicable)

This option does not provide any meaningful information and is not relevant to the question.

Conclusion:

Understanding the purpose of a fuse in an electrical circuit is crucial for ensuring the safety and reliability of electrical systems. The primary role of a fuse is to protect electrical installations, devices, and wiring from damage caused by overcurrent or short circuits. By interrupting the circuit when the current exceeds a safe level, the fuse prevents potential hazards such as fire, equipment failure, or electric shock. Selecting the correct fuse rating is essential for effective protection without unnecessary interruptions, ensuring the safety and longevity of the electrical system.

Concept:

Fusing current is proportional to the 3/2 power of the wire’s radius:

\( \frac{I_1}{I_2} = \left( \frac{r_1}{r_2} \right)^{3/2} \)

Substituting: \( \frac{16}{I_2} = (4)^{3/2} = 8 \Rightarrow I_2 = 2\ \text{A} \)

Fusing Factor

\(Fusing \space Factor={Minimum \space fusing \space current \over Current \space rating \space of \space fuse}\)

Minimum fusing current is the minimum value of current due to which the fuse melts.

It is always greater than 1. 

The small the value of the fusing factor, the greater is the chance of deterioration of the fusing element due to overheating and oxidation.

Explanation:

Rupturing Capacity of a Fuse

Definition: The rupturing capacity, also known as the breaking capacity or interrupting capacity, of a fuse is the maximum amount of fault current that a fuse can safely interrupt without causing an electric arc with unacceptable duration or causing damage to the electrical circuit or the fuse itself. It is a crucial parameter in ensuring the safety and reliability of electrical systems.

Working Principle: When an overcurrent or short-circuit condition occurs, the fuse element heats up due to the increased current flow, eventually melting and breaking the circuit. The rupturing capacity of the fuse ensures that this interruption occurs safely even at high current levels, preventing further damage or hazards.

Advantages:

• Provides protection against overcurrent and short-circuit conditions, ensuring the safety of electrical circuits and equipment.
• Prevents electrical fires and damage by safely interrupting excessive fault currents.
• Simple and cost-effective means of circuit protection.

Disadvantages:

• Once a fuse operates (blows), it must be replaced, unlike circuit breakers that can be reset.
• May not provide as precise protection as more sophisticated protection devices.

Applications: Fuses are widely used in various electrical and electronic systems, including residential wiring, industrial equipment, automotive applications, and consumer electronics, to protect against overcurrent conditions.

Correct Option Analysis:

The correct option is:

Option 1: MVA (Mega Volt Amperes)

This option correctly represents the rupturing capacity of a fuse. The rupturing capacity is expressed in terms of MVA, which stands for Mega Volt Amperes. MVA is a unit of apparent power, which combines both voltage and current. This unit is used because the rupturing capacity needs to account for both the voltage and the current that the fuse can interrupt safely during a fault condition. High fault currents can generate substantial energy, and the MVA rating ensures that the fuse can handle these conditions without causing damage or creating hazards.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: kVA (Kilo Volt Amperes)

This option is not correct for expressing the rupturing capacity of a fuse. While kVA is also a unit of apparent power, it is typically used for specifying the power capacity of transformers and generators rather than the rupturing capacity of fuses. The rupturing capacity usually involves higher power levels that are more suitably expressed in MVA.

Option 3: A (Amperes)

This option is incorrect as it only considers the current aspect of the fault condition. Although the current rating of a fuse is important, the rupturing capacity must also account for the voltage to ensure safe interruption. Therefore, using only Amperes (A) does not provide a complete measure of the rupturing capacity.

Option 4: kV (Kilo Volts)

Similar to Option 3, this option is also incorrect because it only considers the voltage aspect. The rupturing capacity needs to consider both the voltage and the current to ensure that the fuse can interrupt the fault condition safely. Using only Kilo Volts (kV) does not provide a comprehensive measure of the rupturing capacity.

Conclusion:

Understanding the rupturing capacity of a fuse is essential for ensuring the safe and reliable operation of electrical systems. The correct representation of the rupturing capacity is in terms of MVA (Mega Volt Amperes), as it combines both the voltage and current aspects of fault conditions. This comprehensive measure ensures that the fuse can interrupt high fault currents safely without causing damage or creating hazards. Evaluating the other options, it is clear that they do not provide a complete measure of the rupturing capacity, emphasizing the importance of using MVA for this parameter.

Concept:

Fusing factor: It is the ratio of minimum fusing current and current rating of the fuse. Minimum fusing current is the minimum value of current due to which fuse melts.

The current rating of fuse is the maximum value of current due to which fuse does not get melt.

Fusing factor = (Fusing current) / (Current rating of fuse)

Explanation:

• The value of fusing factor is always greater than 1.
• The small the value of fusing factor, greater is the chance of deterioration of fusing element due to overheating and oxidation.
• For a semi enclosed or rewirable fuse which employ copper as the fuse element, the value of fuse factor is usually 2.
• Lower values of fusing factor can be employed for enclosed type cartridge fuse which uses silver or bimetallic elements.

Fuses: A fuse is a short piece of metal, inserted in the circuit, which melts when excessive current flows through it and thus breaks the circuit.

DC Fuse:

• The DC fuse opens or breaks the circuit when the excessive current flow through it.
• It is very difficult to extinct because there are no zero current flows in the circuit.
• For the same current rating, the heat generated in a particular time period in the DC fuse is more than that of AC because the equivalent value of AC is 70.7% of the DC. Therefore it has larger in size as compared to any other AC fuses.

AC Fuses:

• The AC fuses are categorized into two types they are the low voltage fuses and high voltage fuses.
• Striker type fuses,  Cartridge type fuses, the rewirable fuses are considered as low voltage fuses.
• HV fuse, Expulsion type fuse, Liquid HV HRC fuses are considered as high voltage fuse.
• Thus, the arc extinction in the AC circuit can be done easily as compared to the DC circuit.

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, 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

Iron is not used as a fuse wire.

Residual Current Circuit Breaker (RCCB):

• It is a differential current sensing device used to protect a low voltage circuit in case of a leakage fault. It is sometimes also known as Residual Current Device (RCD).
• It contains a switch device that switches off (in a fraction of a second), whenever the current leaks out and does not return back through neutral.
• The RCCB provides protection from small current leakage arising due to accidental touch by human being or insulation failure, which is not possible by MCB or fuse alone.

Ratings of RCCB:

• 30 mA is recommended for residence for safety purpose
• 100 mA is recommended for industrial establishments

The fuse rating is basically dependent on current carrying to a particular connected load.

The order of the wring done for the consumer end connection is

1. From Secondary distribution transformers, power lines are sent to each pole present in the streets.
2. From the pole, each service main wire is connected to the energy meter of each house.
3. From the main meter, it is distributed to the main circuits of the building.
4. From the main circuit, it is then divided into sub-circuit connections.
   ​

The magnitude of current-carrying by the wries in the above process is given as

i_pole > i_meter  > i_{main circuit} > i_{sub circuit}

Hence pole caries more current of different service mains of houses, pole fuse has the highest fuse rating.

Additional Information

Fusing factor:

It is the ratio of minimum fusing current and current rating of the fuse. Minimum fusing current is the minimum value of current due to which fuse melts.

The current rating of fuse is the maximum value of current due to which fuse does not get melt.

Fusing factor = (Fusing current) / (Current rating of fuse)

• The current rating is the maximum current that a fuse will carry for an indefinite period without much deterioration of the fuse element.
• Any current higher than the current rating may cause the contacts of the switch for example to ‘melt’ or ‘weld’ together.
• The current rating of the cable is defined as the maximum current carrying capacity of the power cable under normal operating conditions.
• The current rating of power cable decided the upper limit of power transfer by a cable.
• It mostly depends on the insulation temperature and electrical resistance of the conductor.
• According to the question smallest cable means the shortest cable (length is small), if the length is small then resistance is also small/low because resistance is directly proportional to the length of the cable (R ∝ l).
• If resistance is small, then the maximum permissible current will pass through the cable and thus it will be the maximum current rating of the fuse for the particular distribution system.
• Thus the current rating of a fuse in the distribution system should not exceed the current rating of the smallest cable in the circuit.
• Current rating of a fuse ≤ current rating of smallest cable in the circuit

• The electric current flowing through the fuse wire is the same as the load current which is consumed by the electrical load
• That load current only decides the rating of the fuse
• Hence, the rating of the fuse is expressed in amperes

(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

HRC fuse:

• HRC fuses are used for short circuit protection.
• It carries a heavy short circuit current for a known time period.
• HRC fuses on a transformer provided protection against external faults.

HRC fuses have various applications:

• Protection of radial lines
• Transformer protection (external fault only)
• It is also used for capacitor protection
• It also provides protection of the underground distribution system
• It is used in meter board in a residential application