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PROTECTION AND SWITCHGEAR Page 1
MAHALAKSHMI
ENGINEERING COLLEGE
TIRUCHIRAPALLI – 621213
QUESTION BANK
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Sub. Code : EE2402 Semester : VII
Subject : PROTECTION AND SWITCHGEAR Unit : IV
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PART A
1. What is resistance switching?
It is the method of connecting a resistance in parallel with the contact space(arc). The
resistance reduces the restriking voltage frequency and it diverts part of the arc
current. It assists the circuit breaker in interrupting the magnetizing current and
capacity current.
2. What do you mean by current chopping?
When interrupting low inductive currents such as magnetizing currents of the
transformer, shunt reactor, the rapid deionization of the contact space and blast effect
may cause the current to be interrupted before the natural current zero. This
phenomenon of interruption of the current before its natural zero is called current
chopping.
3. What are the methods of capacitive switching?
Opening of single capacitor bank
Closing of one capacitor bank against another
4. What is an arc?
Arc is a phenomenon occurring when the two contacts of a circuit breaker separate
under heavy load or fault or short circuit condition.
5. Give the two methods of arc interruption?
High resistance interruption:-the arc resistance is increased by elongating, and
splitting the arc so that the arc is fully extinguished Current zero method:-The arc is
interrupted at current zero position that occurs100 times a second in case of 50Hz
power system frequency in ac.
6. What is restriking voltage? (nov/dec 2011)
It is the transient voltage appearing across the breaker contacts at the instant of arc
being extinguished.
7. What is meant by recovery voltage? (nov/dec 2011)
The power frequency rms voltage appearing across the breaker contacts after the arc
is extinguished and transient oscillations die out is called recovery voltage.
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8. What is RRRV?
It is the rate of rise of restriking voltage, expressed in volts per microsecond. It is
closely associated with natural frequency of oscillation.
9. What is circuit breaker?
It is a piece of equipment used to break a circuit automatically under fault conditions.
It breaks a circuit either manually or by remote control under normal conditions and
under fault conditions.
10. What is the main problem of the circuit breaker?
When the contacts of the breaker are separated, an arc is struck between them. This
arc delays the current interruption process and also generates enormous heat which
may cause damage to the system or to the breaker itself. This is the main problem.
11. Mention the details circuit breaker rating
Rated voltage & rated current
Rated Frequency
Rated breaking capacity, symmetrical & asymmetrical
Rated making capacity
Rated short time current
Rated operating duty
12. What are the factors the ARC phenomenon depends upon?
The nature and pressure of the medium
The external ionizing and de-ionizing agent present
Voltage across the electrodes and its variation with time
The nature shape & separation of electrodes
The nature and shape of vessel and its position in relation to the electrodes
13. What are the factors the arc resistance depends upon?
Degree of ionization
Length of the arc
Cross section area of the arc
14. What is breaking current?
The breaking current of a circuit breaker of a circuit breaker is the current in that pole
at the instant of contact separation.
15. What is symmetrical breaking current?
R.M.S Value of the A.C component of the current in the pole at the instant of contact
separation
16. What is asymmetrical breaking current?
R.M.S Value of the A.C & D.C component of the current in the pole at the instant of
contact separation
PROTECTION AND SWITCHGEAR Page 3
17. What are breaking capacities?
The symmetrical value of breaking capacity is the value of the symmetrical breaking
current which the circuit breaker is capable of breaking at the stated recovery voltage
and restriking voltage under prescribed condition
18. What are the factors the recovery voltage depends upon? (nov/dec 2011)
Power factor, Armature reaction & Circuit condition
19. What is rated making current?
The making current a circuit breaker when closed on a short circuit is the RMS value
of the total current which is measured from the envelope of the current wave at the
time of its first major peak.
20. What is the basic requirement of DC circuit breaking?
Lengthening of the arc is basic requirements of D.C circuit breaker. Loss of energy
increases with increasing length of arc and more power will be required to maintain
the arc.
21. What are the problems associated with DC circuit breakers?
Natural current zero does not occur as in the case of A.C circuit breakers
The amount of energy to be dissipated during the short interval of breaking is very
high as compared to conventional A.C circuit breakers.
22. What is the purpose of protective spark gap?
A protective spark gap can be used across the circuit breaker to reduce the size of
commutation capacitor. The spark gap acts as an energy dissipating device for high
frequency currents
23. What is the purpose of arc chute?
By breaking the arcs into number of smaller arcs and by increasing the space between
two walls of arc chute the deionization of the arc can be increased
24. What are the factors RRRV depends upon?
Active recovery voltage & Natural frequency of oscillation
25. Differentiate between the static and dynamic characteristics of the arc?
If the current changes rapidly with time the characteristics are known as dynamic
characteristics. If the rate of change of current is small the characteristics are known
as static characteristics.
PROTECTION AND SWITCHGEAR Page 4
PART B
1)Explain Double Frequency Transient .
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6) Explain Resistance switching with circuit diagram.
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7)Explain Arc Extinction and types of Extinction.
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8)Explain air break circuit breakers.
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All circuit breakers have common features in their operation, although details vary
substantially depending on the voltage class, current rating and type of the circuit breaker.
The circuit breaker must detect a fault condition; in low voltage circuit breakers this is
usually done within the breaker enclosure. Circuit breakers for large currents or high voltages
are usually arranged with protective relay pilot devices to sense a fault condition and to
operate the trip opening mechanism. The trip solenoid that releases the latch is usually
energized by a separate battery, although some high-voltage circuit breakers are self-
contained with current transformers, protective relays and an internal control power source.
Once a fault is detected, contacts within the circuit breaker must open to interrupt the circuit;
some mechanically-stored energy (using something such as springs or compressed air)
contained within the breaker is used to separate the contacts, although some of the energy
required may be obtained from the fault current itself. Small circuit breakers may be
manually operated, larger units have solenoids to trip the mechanism, and electric motors to
restore energy to the springs.
The circuit breaker contacts must carry the load current without excessive heating, and must
also withstand the heat of the arc produced when interrupting (opening) the circuit. Contacts
are made of copper or copper alloys, silver alloys and other highly conductive materials.
Service life of the contacts is limited by the erosion of contact material due to arcing while
interrupting the current. Miniature and molded-case circuit breakers are usually discarded
when the contacts have worn, but power circuit breakers and high-voltage circuit breakers
have replaceable contacts.
When a current is interrupted, an arc is generated. This arc must be contained, cooled and
extinguished in a controlled way, so that the gap between the contacts can again withstand the
voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas or oil as the
medium the arc forms in. Different techniques are used to extinguish the arc including:
Lengthening / deflection of the arc
Intensive cooling (in jet chambers)
Division into partial arcs
PROTECTION AND SWITCHGEAR Page 15
Zero point quenching (Contacts open at the zero current time crossing of the AC
waveform, effectively breaking no load current at the time of opening. The zero crossing
occurs at twice the line frequency, i.e. 100 times per second for 50 Hz and 120 times per
second for 60 Hz AC)
Connecting capacitors in parallel with contacts in DC circuits.
Finally, once the fault condition has been cleared, the contacts must again be closed to restore
power to the interrupted circuit.
Arc interruption
Low-voltage MCB uses air alone to extinguish the arc. Larger ratings have metal plates or
non-metallic arc chutes to divide and cool the arc. Magnetic blowout coils or permanent
magnets deflect the arc into the arc chute.
In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of
oil through the arc.
Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the arc using a magnetic
field, and then rely upon the dielectric strength of the sulfur hexafluoride (SF6) to quench the
stretched arc.
Vacuum circuit breakers have minimal arcing (as there is nothing to ionize other than the
contact material), so the arc quenches when it is stretched a very small amount (less than 2–3
mm (0.079–0.118 in)). Vacuum circuit breakers are frequently used in modern medium-
voltage switchgear to 38,000 volts.
Air circuit breakers may use compressed air to blow out the arc, or alternatively, the contacts
are rapidly swung into a small sealed chamber, the escaping of the displaced air thus blowing
out the arc.
Circuit breakers are usually able to terminate all current very quickly: typically the arc is
extinguished between 30 ms and 150 ms after the mechanism has been tripped, depending
upon age and construction of the device.
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Short-circuit current:
Circuit breakers are rated both by the normal current that they are expected to carry, and the
maximum short-circuit current that they can safely interrupt.
Under short-circuit conditions, a current many times greater than normal can exist (see
maximum prospective short circuit current). When electrical contacts open to interrupt a large
current, there is a tendency for an arc to form between the opened contacts, which would
allow the current to continue. This condition can create conductive ionized gases and molten
or vaporized metal, which can cause further continuation of the arc, or creation of additional
short circuits, potentially resulting in the explosion of the circuit breaker and the equipment
that it is installed in. Therefore, circuit breakers must incorporate various features to divide
and extinguish the arc.
In air-insulated and miniature breakers an arc chute structure consisting (often) of
metal plates or ceramic ridges cools the arc, and magnetic blowout coils deflect the arc into
the arc chute. Larger circuit breakers such as those used in electrical power distribution may
use vacuum, an inert gas such as sulfur hexafluoride or have contacts immersed in oil to
suppress the arc.
The maximum short-circuit current that a breaker can interrupt is determined by
testing. Application of a breaker in a circuit with a prospective short-circuit current higher
than the breaker's interrupting capacity rating may result in failure of the breaker to safely
interrupt a fault. In a worst-case scenario the breaker may successfully interrupt the fault,
only to explode when reset.
MCB used to protect control circuits or small appliances may not have sufficient
interrupting capacity to use at a panel board; these circuit breakers are called "supplemental
circuit protectors" to distinguish them from distribution-type circuit breakers.
PROTECTION AND SWITCHGEAR Page 17
9)Explain T-1 series circuit Breaker.
T-1 Series Circuit Breakers SftMt 25 Amp ~ 150 Amp
5000 Ampere interrupt capacity meets primary circuit protection requirements for large DC
systems.
Vapor Proof (conforming to SAE J1171)—safe for installation aboard gas-powered
boats
Weatherproof
Combines switching and circuit breaker functions into one unit
“Trip Free”—designed to trip when subjected to a fault current, even if the reset lever
is held in the ON position.
Agency Specifications
Meets SAE J553 & J1625 Circuit Breaker Standards for Surface Vehicles.
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Meets SAE J1171 Marine Circuit Breaker Standards Including External Ignition
Protection
PN Description
7120 T-1 Circuit Breaker 25 Amp
7121 T-1 Circuit Breaker 30 Amp
7122 T-1 Circuit Breaker 35 Amp
7123 T-1 Circuit Breaker 40 Amp
7124 T-1 Circuit Breaker 50 Amp
7125 T-1 Circuit Breaker 60 Amp
7126 T-1 Circuit Breaker 70 Amp
7127 T-1 Circuit Breaker 80 Amp
7128 T-1 Circuit Breaker 90 Amp
7129 T-1 Circuit Breaker 100 Amp
7130 T-1 Circuit Breaker 110 Amp
7131 T-1 Circuit Breaker 120 Amp
7132 T-1 Circuit Breaker 135 Amp
7133 T-1 Circuit Breaker 150 Amp
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Specifications
Circuit Breaker Class Type III – Switchable /
Manual Reset – Trip Free
Type Thermally Responsive Bi-Metal Blade
Body Material Thermoset Polyester
UL Rated 94V-0, 338°F (170°C)
Amperage Range 25 to 150 Amperes
Voltage Rating 48 Volt DC Maximum
Interrupt Rating DC 5,000 Amperes @ 12 Volts
3,000 Amperes @ 24 Volts
1,500 Amperes @ 42 Volts
Weight 0.50 Lb / 0.23 Kg Panel Mount
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0.58 Lb / 0.26 Kg Surface Mount
T-1 Series Circuit Breakers Panel Mount 25 Amp ~ 150 Amp
5000 Ampere interrupt capacity meets primary circuit protection requirements for large DC
systems
Vapor Proof (conforming to SAE J1171)—safe for installation aboard gas-powered boats
Weatherproof
Combines switching and circuit breaker functions into one unit
“Trip Free”—designed to trip when subjected to a fault current, even if the reset lever is
held in the ON position.
PROTECTION AND SWITCHGEAR Page 21
Agency Specifications
Meets SAE J553 & J1625 Circuit Breaker Standards for Surface Vehicles.
Meets SAE J1171 Marine Circuit Breaker Standards Including External Ignition
Protection
PN Description
7020 T-1 Circuit Breaker 25 Amp
7021 T-1 Circuit Breaker 30 Amp
7022 T-1 Circuit Breaker 35 Amp
7023 T-1 Circuit Breaker 40 Amp
7024 T-1 Circuit Breaker 50 Amp
7025 T-1 Circuit Breaker 60 Amp
7026 T-1 Circuit Breaker 70 Amp
7027 T-1 Circuit Breaker 80 Amp
7028 T-1 Circuit Breaker 90 Amp
7029 T-1 Circuit Breaker 100 Amp
7030 T 1 Circuit Breaker 110 Amp
7031 T 1 Circuit Breaker 120 Amp
7032 T 1 Circuit Breaker 135 Amp
7033 T 1 Circuit Breaker 150 Amp
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Specifications
Circuit Breaker Class Type III – Switchable /
Manual Reset – Trip Free
Type Thermally Responsive Bi-Metal Blade
Body Material Thermoset Polyester
UL Rated 94V-0, 338°F (170°C)
Amperage Range 25 to 150 Amperes
Voltage Rating 48 Volt DC Maximum
Interrupt Rating DC 5,000 Amperes @ 12 Volts
3,000 Amperes @ 24 Volts
1,500 Amperes @ 42 Volts
Weight 0.50 Lb / 0.23 Kg Panel Mount