Test: Circuit Breakers - Electrical Engineering (EE) MCQ

• In SF6 circuit breaker, Sulphur hexafluoride gas is used as the arc quenching medium.
• Due to its low gaseous viscosity and high molecular weight, the SF6 gas can efficiently transfer heat by convection.
• It has high insulating properties and highly electronegative properties.
• Pure Sulphur hexafluoride gas is inert and thermally stable.
• SF6 has a very high dielectric strength (breakdown strength). It has 100 times more effective than air and oil as interrupting mediums.
• Its dielectric strength is 2.5 times that of air and 30% less than that of the dielectric oil.

Concept:

 Circuit diagram for the resistance switching of circuit breaker is shown below.

• Resistance Switching in Circuit Breaker refers to a method adopted for dampening the over voltage transients due to current chopping, capacitive current breaking etc.
• In this method, a shunt resistance is connected across the contacts of circuit breaker.
• The main role of shunt resistance R is to limit the growth of re-strike voltage and cause it to grow exponentially up to recovery voltage.
• R is so selected that the circuit becomes critically damped then re-strike voltage rises exponentially till recovery voltage is reached.
• The value of R for critical damping is 
• Resistance Switching also reduces the oscillatory growth of Re-striking Voltage

• Voltage appearing across the electrodes of circuit breaker or switching voltage or prospective voltage 

Calculation:

Given that V = 132 kV, C = 0.05 μF, L = 50 mH

Critical resistance, 

Vacuum circuit breaker:

• A breaker that is used a vacuum as an arc quenching medium is called a vacuum circuit breaker.
• The arc is extinct as the contacts are separated in a high vacuum.
• It is mainly used for medium voltage ranging from 22 kV to 66 kV.

The voltage levels of different circuit breakers are given below:

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.

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.

Rated voltage of circuit breaker:

• For below 400 kV systems, the circuit breaker is designed to withstand 10% above the normal system voltage.
• For an above or equal to 400 kV system, the insulation of the circuit breaker should be capable of withstanding 5% above the normal system voltage.
• The voltage rating of the circuit breaker is normally from 1.05 to 1.10 times more than the normal operating voltage.
• this is because during no load or small loads condition voltage level of the power system is allowed to rise up to the highest voltage rating of the system

• A circuit breaker is a switching device that interrupts the abnormal or fault current
• It is a mechanical device that disturbs the flow of high magnitude (fault) current and in additions performs the function of a switch
• The circuit breaker is mainly designed for closing or opening of an electrical circuit, thus protects the electrical system from damage
• When the circuit breaker is closed, it acts as short switch; Thus, before interruption ideal circuit breaker should offer zero impedance
• When the circuit breaker is open, it acts as open switch; Thus, after interruption ideal circuit breaker should offer infinite impedance

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.

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 

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 = 1500 A

MVA rating = 2000 MVA

Rated symmetrical breaking current 

Concept:

• The use of high-speed circuit breakers improves the system stability.
• The basic objective is to enable high-speed fault clearing. The amount of kinetic energy gained during acceleration can be reduced by the use of a high-speed circuit breaker.
• Standard practice for high voltage systems is one-cycle relaying and two-cycle circuit breakers allowing the fault clearing after three cycles.

In a circuit breaker, the main contacts are usually made up of copper and conduct current in closed positions. Circuit breakers have low contact resistance and they are silver plated. The arcing contacts are solid, resistant to heat and are made up of copper tungsten alloy.

Important Points 

Contact resistance is the resistance to current flow, due to surface conditions and other causes, when contacts are touching one another (in the closed condition of the device). This can occur between contacts of breakers.

The contact resistance is in the order of μΩ and it is approximately 20 μΩ.

Insulation resistance

The insulation resistance is the resistance in ohms of wires, cables, and electrical equipment.

Insulation resistance is essential to prevent electric shocks and equipment damage from accidental discharges. 

The insulation resistance is high values resistances generally in Mega Ohm.

The value of insulation resistance of high-voltage circuit breakers is 100 Mega Ohm.

Classification of insulation resistance

The insulation resistances are classified as:

Additional Information 
Methods for measurement of resistors, inductors, and capacitors:

Vacuum Circuit Breaker

• A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium.
• A vacuum that is used as the arc quenching medium in a circuit breaker is known as a vacuum circuit breaker because a vacuum gives high insulating strength due to superior arc quenching properties.
• Alloys like Copper-bismuth, silver-tellurium and copper-lead are the ideal materials to make VCB contacts.
• It is mainly used for medium voltage ranging from 11 KV to 33 KV.
• When the fault occurs in the system, the contacts of the breaker are moved apart and hence the arc is developed between them.
• When the current-carrying contacts are pulled apart, the temperature of their connecting parts is very high due to which ionization occurs.
• Due to the ionization, the contact space is filled with the vapour of positive ions which is discharged from the contact material.
• The density of vapour depends on the current in the arcing. Due to the decreasing mode of the current wave their rate of release of vapour falls and after the current zero, the medium regains its dielectric strength provided vapour density around the contacts is reduced.
• Hence, the arc does not restrike again because the metal vapour is quickly removed from the contact zone.

Additional Information
Vacuum Circuit Breaker:

• A vacuum circuit breaker is a kind of circuit breaker where the arc quenching takes place in a vacuum medium.
• The operation of opening and closing of current carrying contacts and associated arc interruption take place in a vacuum chamber in the breaker which is called a vacuum interrupter.
• Vacuum circuit breakers are more commonly used at system voltage levels up to 72 kV.
• The vacuum contactor’s rated 1.2/3.6/7.2 kV is used for indoor metal enclosed control gear.
• The vacuum interrupters rated 3.6/7.2/12/36 kV is used for indoor metal-clad switch gear.