Ratings of Circuit Breakers MCQ Quiz - Objective Question with Answer for Ratings of Circuit Breakers - Download Free PDF

Explanation:

Three-Phase Breaker Making Current

Problem Statement: A three-phase breaker is rated at 2000 MVA and 33 kV. The task is to calculate its making current.

Solution:

To calculate the making current of the three-phase breaker, we use the standard formula:

Making Current = √2 × Symmetrical Breaking Current

The symmetrical breaking current can be determined using the formula:

Symmetrical Breaking Current (I_sym) = Rated MVA / (√3 × Rated Voltage)

Given data:

• Rated MVA (S) = 2000 MVA
• Rated Voltage (V) = 33 kV = 33 × 10³ V

Step 1: Calculate the symmetrical breaking current:

I_sym = S / (√3 × V)

Substituting the values:

I_sym = 2000 × 10⁶ / (√3 × 33 × 10³)

We know that √3 ≈ 1.732, so:

I_sym = 2000 × 10⁶ / (1.732 × 33 × 10³)

I_sym = 2000 × 10⁶ / 57.156 × 10³

I_sym = 35,000 A or 35 kA

Step 2: Calculate the making current:

Making Current = √2 × I_sym

We know that √2 ≈ 1.414, so:

Making Current = 1.414 × 35 kA

Making Current = 49.49 kA

Since the making current is typically rounded off to the nearest whole number, the making current is approximately 49 kA.

Correct Answer: Option 3) 49 kA

Important Information

To analyze the other options, let us evaluate them:

Option 1: 70 kA

This value is significantly higher than the calculated making current of 49 kA. A making current of 70 kA would correspond to a much higher symmetrical breaking current or a different breaker rating.

Option 2: 89 kA

This value is even higher than option 1 and is unrealistic for the given breaker ratings. A making current of 89 kA would require a symmetrical breaking current of over 63 kA, which does not match the given MVA and voltage ratings.

Option 4: 35 kA

This value represents the symmetrical breaking current, not the making current. The making current is always higher than the symmetrical breaking current due to the factor of √2.

Option 5: Not Provided

This option is irrelevant as it does not present any numerical value for consideration.

Conclusion:

The making current of the given three-phase breaker is accurately calculated to be approximately 49 kA, as derived from the rated MVA and voltage. Understanding the relationship between the symmetrical breaking current and the making current is crucial for accurately determining the breaker specifications and ensuring proper system protection.

Braking Capacity of the Circuit Breaker

The breaking capacity of a circuit breaker is the maximum power it can safely interrupt without damage.

It is usually expressed in MVA (Mega Volt-Amperes).

Given Specifications:

• Rated Current = 1500 A
• Breaking Capacity = 1000 MVA
• Voltage Rating = 33 kV
• Time Duration = 3 seconds

From the options provided, Option 3: 1000 MVA is correct, as it matches the given breaking capacity of the circuit breaker.

Explanation:

Rated Peak Short-Circuit Current of an Isolator

Definition: The rated peak short-circuit current of an isolator refers to the maximum current that the isolator can safely handle during a short-circuit fault condition. This rating is crucial for ensuring the isolator's capability to withstand extreme electrical stress without damage, ensuring system reliability and safety.

Working Principle: During normal operations, an isolator conducts the current flowing through the circuit. However, during a short-circuit fault, the current can surge to extraordinarily high levels, potentially causing severe damage to electrical components. The rated peak short-circuit current indicates the maximum current the isolator can manage during such faults without failure. This rating ensures that the isolator can perform its function of isolating sections of the circuit without being compromised under extreme conditions.

Importance: The rated peak short-circuit current is vital for several reasons:

• Safety: Ensuring the isolator can handle high fault currents prevents damage to the electrical system and reduces the risk of fire or equipment failure.
• Reliability: Properly rated isolators contribute to the overall reliability of the power system, ensuring continuity of service even under fault conditions.
• Compliance: Electrical systems must comply with regulatory standards that specify the minimum requirements for handling short-circuit currents to ensure safety and performance.

Correct Option Analysis:

The correct option is:

Option 2: The maximum current that the isolator can handle during a short-circuit fault.

This option accurately describes the rated peak short-circuit current of an isolator. It emphasizes the isolator's capability to manage the extreme currents that occur during a short-circuit fault without sustaining damage or compromising its functionality.

Additional Information

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

Option 1: The maximum current that the isolator can handle during a power swing.

This option is incorrect because a power swing refers to oscillations in power flow in an electrical system, typically due to changes in load or generation. While these swings can cause variations in current, they are not the same as short-circuit faults, which involve much higher currents and different stress on the isolator.

Option 3: The maximum current that the isolator can handle during a lightning strike.

This option is also incorrect. Lightning strikes cause very high transient currents that can damage electrical equipment. However, the rated peak short-circuit current specifically pertains to fault conditions within the electrical system, not external events like lightning strikes. Lightning protection requires different considerations and equipment, such as surge arresters.

Option 4: The maximum current that the isolator can handle during normal operation.

This option is incorrect because the rated peak short-circuit current refers to fault conditions, not normal operation. During normal operation, the current levels are much lower than those experienced during a short-circuit fault. The isolator's ability to handle normal operating currents is defined by its continuous current rating, not its short-circuit current rating.

Conclusion:

Understanding the rated peak short-circuit current is essential for selecting the appropriate isolator for an electrical system. This rating ensures that the isolator can withstand the extreme currents associated with short-circuit faults, maintaining system integrity and safety. Evaluating and distinguishing between different operational conditions, such as power swings, lightning strikes, and normal operation, is crucial for correctly interpreting the isolator's ratings and ensuring proper application in electrical systems.

Concept

The rated breaking capacity is the maximum fault current a circuit breaker can successfully interrupt without damage.

It is always expressed in KVA and the current corresponding to breaking capacity is known as the breaking current.

The breaking current is given by:

\(I_b={MVA\over \sqrt{3}\times V_{L}}\)

where, Ib = Breaking current

MVA = Breaking capacity

VL = Line-to-line voltage

The relationship between making and breaking current is:

\(I_m=2.55\times I_b\)

where, Im = Making current

Ib = Breaking current

Calculation

Given, Breaking capacity = 2000 MVA

VL = 33 kV

\(I_b={2000\space \times 10^6\over \sqrt{3}\times 33\times 10^3}\)

Ib = 34.9 kA = 35 kA

Concept

The rated breaking capacity is the maximum fault current a circuit breaker can successfully interrupt without damage.

It is always expressed in KVA and the current corresponding to breaking capacity is known as the breaking current.

The breaking current is given by:

\(I_b={MVA\over \sqrt{3}\times V_{L}}\)

where, Ib = Breaking current

MVA = Breaking capacity

VL = Line-to-line voltage

The relationship between making and breaking current is:

\(I_m=2.55\times I_b\)

where, Im = Making current

Ib = Breaking current

Calculation

Given, Breaking capacity = 2000 MVA

VL = 33 kV

\(I_b={2000\space \times 10^6\over \sqrt{3}\times 33\times 10^3}\)

Ib = 34.9 kA = 35 kA

A triple pole with a Neutral (TPN) Miniature Circuit Breaker:

• It is used to protect the circuits from short circuit faults. It is used in the place of the fuse.
• When the amount of electricity is increasing in wire, the MCB turns off and the result is it breaks the circuit.
• It prevents from burning of home appliances.
• The current rating of single-pole Triple pole with Neutral (TPN)  MCB main switches available in the local market is 63 A.

Additional Information

A miniature circuit breaker (MCB):

• It automatically switches off the power supply during overload and faults.
• This function of automatic switching is accomplished by a bimetallic strip.
• Whenever continuous overcurrent flows through MCB, the bimetallic strip is heated and deflects by bending.
• This deflection of the bimetallic strip releases a mechanical latch.
• As this mechanical latch is attached to the operating mechanism, it causes to open the miniature circuit breaker contacts, and the MCB turns off thereby stopping the current to flow in the circuit.

Concept:

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Rated symmetrical breaking current \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

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

Symmetrical making current = 2.55 × symmetrical breaking current

Calculation:

Rated current = 1200 A

MVA rating = 2000 MVA

Rated symmetrical breaking current \(= \frac{{2000 \times {{10}^6}}}{{\sqrt 3 \times 33 \times {{10}^3}}} = 35\;kA\)

Concept: 

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Rated symmetrical breaking current, \(({I_B} = \frac{S}{{\sqrt 3 {V_L}}} )\)IB=S3VL" tabindex="0">IB=S3√VLIB=S3VL

Where, S = Rating of the circuit breaker in MVA

VL = Line voltage in kV

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

Symmetrical making current = 2.55 × symmetrical breaking current

Calculation:

Given

VL = 11kV

MVA rating, S = 500 MVA

∴ Rated symmetrical breaking current,

\((I_B= \frac{{500 \times {{10}^6}}}{{\sqrt 3 \times 11 \times {{10}^3}}} = 26.24\;kA)\)

Concept: 

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Rated symmetrical breaking current, \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

Where, S = Rating of the circuit breaker in MVA

V_L = Line voltage in kV

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

Symmetrical making current = 2.55 × symmetrical breaking current

Calculation:

Given

V_L = 33 kV

Rated current = 1500 A

MVA rating, S = 2000 MVA

∴ Rated symmetrical breaking current, \(I_B= \frac{{2000 \times {{10}^6}}}{{\sqrt 3 \times 33 \times {{10}^3}}} = 35\;kA\)

So that making current can be calculated as

Symmetrical making current = 2.55 x I_B = 2.55 x 35 kA

Symmetrical making current = 89.25 kA ≈ 89 kA

Concept:

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Rated symmetrical breaking current \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

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

Symmetrical making current = 2.55 × symmetrical breaking current

Calculations:

Rated current = 1200 A

MVA rating = 2000 MVA

Breaking current: It expresses the highest number of short-circuit currents that the breakers can break under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Symmetrical breaking capacity \(= \sqrt 3 {V_{L}}{I_{B}}\)

Rated symmetrical breaking current \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

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

Symmetrical making current = 2.55 × symmetrical breaking current

Concept:

Breaking current: It expresses the highest number of short-circuit currents that the breakers are capable of breaking under specified conditions of transient recovery voltage and power frequency voltage. It is expressed in kA (RMS) at contact separation. The breaking capacities are divided into two types.

• Symmetrical breaking capacity of a circuit breaker
• Asymmetrical breaking capacity of a circuit breaker

Symmetrical breaking capacity \(= \sqrt 3 {V_{L}}{I_{B}}\)

Rated symmetrical breaking current \({I_B} = \frac{S}{{\sqrt 3 {V_L}}} \)

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

Symmetrical making current = 2.55 × symmetrical breaking current

Calculation:

Symmetrical breaking capacity = 3000 MVA

\(\begin{array}{l} {V_{L}} = 66kV\\ \Rightarrow {I_{{B}}} = \frac{{3000 \times {{10}^6}}}{{\sqrt 3 \times 66 \times {{10}^3}}} \end{array}\)

IB = 26.24 kA  

Making current = 2.55 × Symmetrical breaking current

= 2.55 × I_B

= 2.55 × 26.24 kA

= 66.92 kA

The correct answer is 'option 2'

Concept:

Breaking current: 

It expresses the short circuit current that the circuit breaker is capable of breaking under specified transient recovery voltage and power frequency voltage.

It is expressed in kA(RMS) at contact separation. The breaking capacities are of two types:

1. Symmetrical breaking capacity
2. Asymmetrical breaking capacity

Rated symmetrical breaking current, 

\(I_b=\frac{S}{\sqrt3V_L}\)

Here, S is the rating of the circuit breaker

V_L is the line voltage of the circuit breaker.

Making current:

It is the peak value of the maximum current loop during sub-transient conditions including the DC component when the breaker closes.

Symmetrical making current= 2.55 × symmetrical breaking current

Solution:

Rating of the machine = 2000 MVA

V_L = 33 kV

Rated symmetrical current 

\(I_b=\frac{S}{\sqrt 3V_L}=\frac{2000\;MVA}{\sqrt 3\;×\;33 k }= 34.99\;kA\)

∴ Making current = 2.55 × 34.99 kA = 89.226 kA

Methods of Arc Extinction:

There are two methods of extinguishing the arc in circuit breakers:

Additional Information

1. High resistance method:

• In this method, arc resistance is made to increase with time so that current is reduced to a value insufficient to maintain the arc.
• Consequently, the current is interrupted or the arc is extinguished.
• The principal disadvantage of this method is that enormous energy is dissipated in the arc. Therefore, it is employed only in DC circuit breakers and low-capacity AC circuit breakers.
• The resistance of the arc may be increased by:
  • Lengthening the arc
  • Cooling the arc
  • Reducing X-section of the arc
  • Splitting the arc
     

2. Low resistance or Current zero Method

• This method is employed for arc extinction in AC circuits only.
• In this method, arc resistance is kept low until the current is zero where the arc extinguishes naturally and is prevented from restriking in spite of the rising voltage across the contacts.
• All modern high-power AC circuit breakers employ this method for arc extinction.
• In an AC system, the current drops to zero after every half-cycle.
• At every current zero, the arc extinguishes for a brief moment.
• Now the medium between the contacts contains ions and electrons so that it has small dielectric strength and can be easily broken down by the rising contact voltage known as restriking voltage. If such a breakdown does occur, the arc will persist for another half cycle.
• If immediately after current zero, the dielectric strength of the medium between contacts is built up more rapidly than the voltage across the contacts, the arc fails to restrike, and the current will be interrupted.

The correct answer is option 3):(Circuit breaker)

Concept:

• Circuit breaker, the most common protection device that can make or break the circuit either manually or through remote control under normal operating conditions
• A circuit breaker is a switching device that interrupts the abnormal or faulty current
• It is a mechanical device that disturbs the flow of high magnitude (fault) current and in addition, performs the function of a switch
• A circuit breaker is mainly designed for closing or opening an electrical circuit, thus protecting the electrical system from damage When the circuit breaker is closed, it acts as a short switch;
• Thus, before interruption ideal circuit breaker should offer zero impedance When the circuit breaker is open, it acts as an open switch;
• Thus, after the interruption, the ideal circuit breaker should offer infinite impedance

Additional Information

•  A protective relay is a relay device designed to trip a circuit breaker when a fault is detected
•  A fuse is an electrical safety device that operates to provide overcurrent protection of an electrical circuit
• A lightning arrester is a device, essentially an air gap between an electric wire and ground, used on electric power transmission and telecommunication systems to protect the insulation and conductors of the system from the damaging effects of lightning.