1. The magnetic circuit breaker has ______ trip action.
A. delayed
B. instantaneous
C. both of the above
D. none of the above
Answer
B. instantaneous
Explanation:
- A magnetic circuit breaker is a safety device designed to cut an electrical current in the event of a power surge, thus protecting electrical equipment and circuitry from damage.
- Overloaded circuits, loose or faulty wiring and lightning can cause the circuit breaker to trip.
- The magnetic circuit breaker has instantaneous Trip Action.
- Thermal circuit breaker has delayed trip action.
2. In a circuit breaker the basic problem is to
A. maintain the arc
B. extinguish the arc
C. transmit large power
D. emit the ionizing electrons
Answer
B. extinguish the arc
Explanation:
- A circuit breaker is an electrical switch designed to protect an electrical circuit from damage caused by overcurrent/overload or short circuit. Its basic function is to interrupt current flow after protective relays detect a fault.
- In a circuit breaker, the basic problem is to extinguish the arc.
- Arcing usually occurs when a circuit becomes overloaded and overheats. The overheating causes damage not only to the circuit breaker but also to its connection to the bus.
- Once damaged, a circuit breaker can malfunction and continue to let electricity flow between its connection instead of tripping.
3. The arcing contacts in a circuit breaker are made of
A. copper tungsten alloy
B. porcelain
C. electrolytic copper
D. aluminum alloy
Answer
A. copper tungsten alloy
Explanation:
- 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.
- 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.
4. Air blast circuit breaker is used for
A. overcurrents
B. short duty
C. intermittent duty
D. repeated duty
Answer
D. repeated duty
Explanation:
- Airblast circuit breakers are used in arc furnace duty, traction systems, and used for the railway electrification because they are suitable for repeated duty.
- Air blast circuit breakers are used for indoor services in the medium high voltage field and medium rupturing capacity.
- Generally up to voltages of 15 KV and rupturing capacities of 2500 MVA. The air blast circuit breaker is now employed in high voltage circuits in the outdoors switch yard for 220 KV lines.
5. The steady-state stability of the power system can be increased by
A. Connecting lines in parallel
B. Connecting lines in series
C. Using machines of high impedance
D. Reducing the excitation of machines
Answer
A. Connecting lines in parallel
Explanation:
- Steady-state stability is defined as the capability of an electric power system to maintain its initial condition after a small interruption or to reach a condition very close to the initial one when the disturbance is still present.
Steady-state stability of a power system is
Pmax = EV/X
- We can increase the steady-state stability by decreasing the reactance X.
- In a double circuit line where two transmission lines are connected in parallel, the reactance is less than the single line circuit and hence the stability can be improved.
6. The inductive interference between power and communication line can be minimized by
A. transposition of the power line
B. transposition of the communication line
C. both (A) and (B)
D. increasing the distance between the conductors
Answer
C. both (A) and (B)
Explanation:
- The balance of AC power line is improved by using transposition.
- The transposition of lines reduces the induced voltages to a considerable extent.
- The capacitance of the lines is balanced by transposition leading to balance in electrostatically induced voltages.
- Using transposition the fluxes due to positive and negative phase sequence currents cancel out so the electromagnetically induced e.m.f.s are diminished.
- For zero sequence currents, the telephone lines are also transposed.
- Electromagnetic and electrostatic fields are produced by these lines having sufficient magnitude. Because of these fields, voltages and currents are induced in the neighboring communication lines. Thus it gives rise to the interference of power line with communication circuit.
7. A thermal protection switch can protect against
A. short-circuit
B. temperature
C. overload
D. overvoltage
Answer
C. overload
Explanation:
- A thermal protection switch is used to protect against overload conditions
- Thermal circuit protectors utilize a bimetallic strip electrically in series with the circuit
- The heat generated by the current during an overload deforms the bimetallic strip and trips the breaker
- Thermal protectors have a significant advantage over fuses in that they can be reset after tripping
8. The thermal circuit breaker has
A. delayed trip action
B. instantaneous trip action
C. both of the above
D. none of the above
Answer
A. delayed trip action
Explanation:
- A thermal circuit breaker is a resettable circuit protection device that is primarily designed to protect components and equipment from current overload, and short circuit, conditions.
- In a thermal circuit breaker, current flows through a bimetal strip that is connected to the two terminals.
- The thermal circuit breaker has delayed trip action.
9. The time of closing the cycle, in modern circuit breakers is
A. 0.003 sec
B. 0.001 sec
C. 0.01 sec
D. 0.10 sec
E. none of the above
Answer
A. 0.003 sec
Explanation:
- The closing time of a high-voltage circuit breaker refers to the time required for the circuit breaker from receiving a closing command (ie, applying voltage to the closing coil) to the time when the three-phase main contacts of the circuit breaker are in contact.
- The time of closing the cycle, in modern circuit breakers is 0.003 sec.
10. Transmission line has reflection coefficient as one.
A. Open circuit
B. Short-circuit
C. Long
D. None of the above
Answer
A. Open circuit
Explanation:
- In telecommunications and transmission line theory, the reflection coefficient is the ratio of the complex amplitude of the reflected wave to that of the incident wave.
- The shorted line will absorb more power than any other line. Thus the reflection coefficient is considered to be negative.
- An open circuit line has infinite output impedance. Any wave incident at the output will be completely reflected. Thus the reflection coefficient is unity.
- The reflection coefficient is 1 and -1 in open and shorted lines respectively. This value of the reflection coefficient will yield an infinite standing wave ratio.