Question 1
A suspension string have 3 units. Voltage across topmost and lower most unit of string are 6.71 volts and 11 volts respectively. What is the voltage across middle string, if voltage across whole string is 25.76 volts?
A. 103.47 volts
B. 8.05 volts
C. 83.6 volts
D. 25.76 volts
View Answer
Answer: Option B
Explanation:
Voltage across whole string is equal to the sum of voltage across each unit. From this the voltage across middle unit can be calculated.
Question 2
A transmission line consists of 9 discs of suspension insulator in each string. What is the operating voltage of the transmission line?
A. 11 KV
B. 33 KV
C. 66 KV
D. 132 KV
View Answer
Answer: Option D
Explanation:
For different voltage it does not need to use different suspension insulator. Srength of insulator can easily be increased just by adding extra disc of suspension insulators. For 130 KV transmission lines 9 or 10 disks are used.
Question 3
Arcing horns are used for ______________
A. protecting insulators from birds
B. protecting insulators from cracking or breaking due to flash over
C. improving string efficiency
D. protecting insulator from deposition of dirt
View Answer
Answer: Option B
Explanation:
During flash over insulators are cracked or broken up due to the heat of Arc. Grading ring in addition to equalization of voltage distribution across the insulators unit when used in conjunction with Arcing horns protects the insulator string from flashover. It does so by keeping the arc away from the insulator string until it is interrupted by protecting devices.
Question 4
By using which of the following method hundred percent string efficiency can be achieved?
A. Using long cross arms
B. Capacitance grading
C. Static shielding
D. 100% string efficiency cannot be achieved
View Answer
Answer: Option D
Explanation:
100% string efficiency can only be achieved by making voltage distribution across each unit of string equal. In practice it is impossible to attain such uniform voltage distribution. So 100% string efficiency cannot be achieved with any of the above methods.
Question 5
If a string of suspension insulator has three units, each can withstand a maximum 11 KV and total string can withstand 25.76 KV. What is the string efficiency?
A. 234.10%
B. 46.30%
C. 68.75%
D. 78%
View Answer
Answer: Option D
Explanation:
String Efficiency = [( flashover voltage of the string) /(n × flashover voltage of 1 unit)] × 100Where ‘n’ is number of units in one stringHence, String Efficiency = [25.76/(3×11)] × 100 = 78%.
Question 6
If voltage across lowermost unit of a string is 34.8% of voltage across whole string and string efficiency is 57.46%. What is the voltage across whole string.
A. 100%of line voltage
B. 43.64% of line voltage
C. 75.2% of line voltage
D. 92.26% of line voltage
View Answer
Answer: Option A
Explanation:
Voltage across string is equal to the line voltage of conductor. So it is 100% of line voltage.
Question 7
Glass insulator cannot be used for voltage above __________
A. 25 KV
B. 11 KV
C. 33 KV
D. 50 KV
View Answer
Answer: Option D
Explanation:
Glass is cheaper than porcelain in the simplest shapes and if properly toughened and annealed gives high resistivity and dielectric strength. The main disadvantage of Glass is that moisture more readily condenses on its surface and facilitates the accumulation of dust deposit thus giving a high surface leakage. Hence, glass insulators can be used upto 25 KV under ordinary atmosphere and well upto 50 KV in dry atmosphere.
Question 8
Suspension insulator are made up of _____________
A. glass
B. porcelain
C. steatite
D. epoxy resin
View Answer
Answer: Option B
Explanation:
Suspension insulators consist of a number of porcelain disks flexibly connected in series by metal links in the form of Strings. Glass is used for making pin type insulators.
Question 9
Pin type insulator are mostly subjected to which type of mechanical stress?
A. Compressive stress
B. Tensile stress
C. Both tensile and compressive stress
D. Twisting stress
View Answer
Answer: Option D
Explanation:
The conductor is placed in the governor at the top of the insulator and is tied down. So the weight of wire acts on the top of the insulator in downward direction causing compressive stress on insulator. No tensile stress is acted on the pin type insulator.
Question 10
The unequal voltage distribution across individual unit of string of suspension insulator is __________
A. desirable and expressed by string efficiency
B. undesirable and expressed by string efficiency
C. desirable and expressed by impulse ratio
D. undesirable and expressed by impulse ratio
View Answer
Answer: Option B
Explanation:
The unequal potential distribution is undesirable and is expressed in terms of string efficiency. Impulse ratio is calculated during Impulse Frequency Flash Over Test. Impulse ratio is the ratio of impulse park over voltage to Spark over voltage at power frequency.
Question 11
Voltage distribution across disc of strings of suspension insulator assembly is ______
A. same for all disks
B. maximum for unit nearest to the line
C. maximum for unit nearest to the tower
D. equal to transmission line voltage rating
View Answer
Answer: Option B
Explanation:
Thee voltage between line conductor and Earth is not distributed uniformly across individual disks. The unit nearest the conductor has the maximum value across it. The figure progressively decreases as the unit nearest the cross on is approach.
Question 12
What is the effect of rain on string efficiency?
A. It becomes very low
B. It reduces slightly
C. It does not changes
D. It is improved
View Answer
Answer: Option D
Explanation:
Due to rain insulators becomes wet and mutual capacitance increases while the shunt capacitance remains constant thereby decreasing the value of k. As k is inversely related to string efficiency so, string efficiency is improved. K is the ratio of stray capacitance to mutual capacitance.
Question 13
What is the most common cause of failure of overhead line insulators?
A. Flashover
B. Mechanical stress
C. Porosity of materials
D. Improper vitrification
View Answer
Answer: Option A
Explanation:
Failure of overhead line insulators due to mechanical stress is rare because defective pieces are separated during routine factory test. Failure due to porosity and improper vitrification is also very low. The most common cause of failure of overhead line insulator is flashover.
Question 14
What is the reason for unequal distribution of voltage among different unit of suspension insulator?
A. Unequal capacitance of different units
B. Unequal distribution of charging current caused by stray capacitance
C. Unequal resistivity of different units
D. Dirt deposited over the insulator disc
View Answer
Answer: Option B
Explanation:
The capacitance due to two metal fittings on either side of insulator is known as the mutual capacitance and the capacitance between these metal fittings and earthed pole or tower is known as shunt capacitance. Due to shunt capacitance charging current is not the same through all the discs of strings. So voltage being directly proportional to the current flowing through them will be different.
Question 15
Which of the following insulator is similar to pin type insulator?
A. Suspension insulator
B. Post insulator
C. Strain insulator
D. Shackle insulator
View Answer
Answer: Option B
Explanation:
Post insulators are very similar to pin type insulator, but has a metal base with a metal cap so that more than one unit can be mounted in series. Suspensions train and shackle insulators are completely different from pin type insulator on the basis of construction.
Question 16
Which of the following insulator is used for insulating stay wire from pole?
A. Pin type insulator
B. Shackle insulator
C. Suspension insulator
D. Stay insulator
View Answer
Answer: Option D
Explanation:
Stay insulators are also called as guy insulators and are used in guy cables. Because it is necessary to insulate the lower part of the guy cable from pole for safety of people and animals on the ground.
Question 17
Which of the following is the main field of application of pin type insulator?
A. Distribution system
B. Transmission system
C. Transmission and distribution system
D. EHV transmission system
View Answer
Answer: Option A
Explanation:
Pin type insulator become very bulky and cumbersome when designed for higher voltage. Pin insulators beyond 50,000 Volts becomes uneconomical. the modern practice is not to use pin type insulator SBI on 33kv so use of pin insulator is limited to distribution level voltage.
Question 18
Which of the following location is suitable to use shackle insulator?
A. Sharp Turn in transmission line
B. Dead end of low voltage distribution line
C. For bearing High Voltage transmission line conductor
D. Dead end of EHV transmission line
View Answer
Answer: Option B
Explanation:
Shackle insulator are almost universally used for low voltage lines. They can be used at all positions either intermediate terminal or angle where the angle exceed 60 degree deviation. They are generally used in conjunction with shackle sharps.
Question 19
Which of the following material is not used for overhead line insulators?
A. Porcelain
B. Glass
C. PVC
D. Steatite
View Answer
Answer: Option C
Explanation:
In addition to high insulation resistance and high relative permittivity, overhead line insulators must have high mechanical strength to bear the weight of line insulators, wind stress and ice loading if any. PVC have good insulation resistance but it does not have such mechanical strength so it is not suitable for overhead line insulators.
Question 20
Which type of insulator is used where there is dead end of the line or there is a corner or a sharp curve, for high voltage line?
A. Pin type insulator
B. Shackle insulator
C. Strain insulator
D. Stay insulator
View Answer
Answer: Option C
Explanation:
Pin type insulators cannot be used in such situations because they cannot take conductor mechanical load intention which often occurs in such situations for low voltage line. Shackle insulator can be used but for higher voltages transmission lines strain insulator consisting of assembly of suspension type insulator are used.