41. The under voltage relay can be used for
A. generators
B. busbars
C. transformers
D. motors
E. all of the above
Answer
E. all of the above
Explanation:
Under voltage relay:
- Under voltage relay is a relay that has contacts that operate when the voltage drops below a set voltage
- These are used for protection against voltage drops, to detect short-circuit faults, etc.
- These are used in motors, alternators and in busbars
Note:
- An induction motor runs at a speed determined by the mains frequency. If the voltage drops, the motor continues to rotate at the same speed (same output power) but because of the lower voltage at a higher current. The current heats the windings according to a square law. So it would overheat. Hence the under voltage relay is mainly used for motor protection.
42. In order that current should flow without causing excessive heating or voltage drop, the relay contacts should
A. Have low contact resistance
B. Be clean and smooth
C. Be of sufficient size and proper shape
D. Have all above properties
Answer
D. Have all above properties
Explanation:
Relay contacts should have following properties
- Have low contact resistance
- Be clean and smooth
- Be of sufficient size and proper shape
- For higher-power relays switching many amperes, such as motor circuit contactors, contacts are made with a mixtures of silver and cadmium oxide, providing low contact resistance and high resistance to the heat of arcing.
43. On which of the following effects of electric current a fuse operates?
A. Photoelectric effect
B. Electrostatic effect
C. Heating effect
D. Magnetic effect
Answer
C. Heating effect
Explanation:
This is due to the heating effect of the electric current. It means the current flowing through the resistance conductor produces the heat.
The use of the fuse is to save the other components of the circuit from the serious damage due to the high current.
Photoelectric effect, phenomenon in which electrically charged particles are released from or within a material when it absorbs electromagnetic radiation. The effect is often defined as the ejection of electrons from a metal plate when light falls on it.
An electrostatic effect is an effect that relates to an electric field, or is created by an electric charge. As electrostatic effects are dependent upon the potential and frequency, to produce the most powerful action it is desirable to increase both as far as practicable.
The heating effect of an electric current depends on three factors: The resistance, R of the conductor. A higher resistance produces more heat.The higher the current the larger the amount of heat generated.
Magnetic Effect of Electric Current – A magnetic field is a force field that is created by magnetic dipoles and moving electric charges, and it exerts a force on other nearby moving charges and magnetic dipoles. Magnetic Field is a vector quantity because it has both magnitude and direction.
44. Ionization in circuit breaker is facilitated by
A. high temperature
B. increase of mean free path
C. increasing field strength
D. all of the above
Answer
D. all of the above
Explanation
In a circuit breaker, ionisation is facilitated by
1) Increase of mean free length
2) Increase of filed strength
3) High temperature of surrounding medium
45. Arc in a circuit breaker is interrupted at
A. zero current
B. maximum current
C. minimum voltage
D. maximum voltage
Answer
A. zero current
Expalnation:
- When the current carrying contacts of the circuit breaker are moved apart, an arc is formed, which insist for a short period after the separation of contacts
- This arc is dangerous on account of the energy generated in it in the form of heat which may result in explosive force
- The circuit breaker should be capable of extinguishing the arc without causing any disturbances to the equipment
- This phenomenon takes place at zero current
- There are two methods of arc extinction in circuit breakers; They are high resistance method and low resistance or zero current interruption method
46. Fault diverters are basically
A. fuses
B. relays
C. fast switches
D. circuit breakers
Answer
C. fast switches
Explanation:
- Fault diverters, or crowbars, have proven to be very effective protection against transient-induced power arcs within accelerator oscillator tubes.
- This device short circuits the oscillator-plate power supply in the event of an over-current, thus removing the power flow from the fault within a few microseconds.
- Fault diverters are basically fast switches.
47. Relays can be designed to respond to changes in
A. resistance, reactance or impedance
B. voltage and current
C. light intensity
D. temperature
E. all above
Answer
E. all above
Explanation:
Protective Relays:
- It is a sensing device.
- It senses the fault or given situation when the current attains a certain predetermined value.
- It knows its position and finally, it gives the tripping command to the circuit breaker.
- The protective relay gives the command to the circuit breaker to disconnect the faulted element.
- Relays can be designed to respond to changes in resistance, reactance, impedance, voltage, current, light intensity and temperature.
Important Notes:
Overload relay:
- An overload relay is an electrical device used to protect an electric motor from overheating. It is mainly three types.
Thermal relay:
- Thermal overload relay is a protective device, and that is mainly designed to cut the power whenever the motor uses too much current for an extended time period.
- Once extreme current supplies throughout the motor circuit, then the relay gets open because of the improved temperature of the motor.
Magnetic relay:
- Magnetic overload relay can be operated by detecting the magnetic field strength which is generated by the flow of current towards the motor.
- This relay can be built with a variable magnetic core within a coil that holds the motor current.
Solid-state relay:
- Solid-state relays are semiconductor equivalents of the electromechanical relay and can be used to control the electrical loads without the use of moving parts.
48. On which of the following routine tests are conducted ?
A. Oil circuit breakers
B. Air blast circuit breakers
C. Minimum oil circuit breakers
D. All of the above
Answer
D. All of the above
Explanation:
Routine test conducted on
- Oil circuit breakers
- Air blast circuit breakers
- Minimum oil circuit breakers
49. Relays are used for phase faults on long line.
A. Mho
B. Reactance
C. Either of the above
D. None of the above
Answer
A. Mho
Explanation:
Important points:
- The relay which is selected for a long transmission line should be less affected due to power swings. Hence Mho relay is preferred.
- Mho relay is used for the phase faults in a long transmission line.
- Impedance relay is used for the phase faults in the medium transmission line.
- Reactance relay is suitable for the protection of a short transmission line because its operation is independent of arc resistance.
- Reactance relay is used for the phase faults in a short transmission line.
50. By which of the following methods major portion of the heat generated in a H.R.C. fuse is dissipated ?
A. Radiation
B. Convection
C. Conduction
D. All of the above
Answer
C. Conduction
Expalantion:
- Conduction is the process by which heat energy is transmitted through collisions between neighboring atoms or molecules.These vibrating molecules collide with their neighboring molecules, making them also vibrate faster. As these molecules collide, thermal energy is transferred via conduction to the rest of the pan.
- Radiation is energy that comes from a source and travels through space at the speed of light. This energy has an electric field and a magnetic field associated with it, and has wave-like properties. You could also call radiation “electromagnetic waves”.
- Convection, process by which heat is transferred by movement of a heated fluid such as air or water. Forced convection involves the transport of fluid by methods other than that resulting from variation of density with temperature. Movement of air by a fan or of water by a pump are examples of forced convection.
51. Surge diverters and Surge absorbers are the types of
A. Relays
B. Circuit breakers
C. Overvoltage protection devices
D. Reactors
Answer
C. Overvoltage protection devices
Explanation:
Surge diverter:
- A surge diverter (or arrestor) is a device attached to the main power board which gives protection against damaging power surges, either from outside or from within (such as from faulty wiring or appliances).
- The device prevents excess power from damaging or destroying sensitive electrical and electronic appliances such as computers and audio gear.
Surge absorber:
- A surge absorber is a protective device that reduces the steepness of wavefront of a surge by absorbing surge energy.
- Although both surge diverter and surge absorber remove the surge, the manner in which it is done is different in the two devices. The surge diverter diverts the surge to earth, but the surge absorber absorbs the energy of a surge.