This set of Basic Transformers Questions and Answers focuses on “Neutral Earthing in Transformers”.
1. Categories for a transformer earthing of a system neutral are ___
a) 2
b) 3
c) 4
d) Many
View Answer
Explanation: Fortunately for transformer designers, earthing of a system neutral can only fall into one of three categories. These are: Neutral solidly earthed, Neutral earthed via an impedance, Neutral isolated.
2. Which of the following neutral earthing method is disadvantageous?
a) Neutral solidly earthed
b) Neutral earthed via an impedance
c) Neutral isolated
d) Neutrally
View Answer
Explanation: Due to the problems and disadvantages of the third alternative that is neutral isolated, it is unlikely that it will be encountered in practice so that it is only necessary to be able to design for the first two.
3. Minimum voltage above which electrical system needs to be earthed is _____
a) 50 V
b) 5 V
c) 10 V
d) 20 V
View Answer
Explanation: According to regulations and acts made in earlier years Every electrical system rated at greater than 50 V shall be connected to earth. Here minimum voltage is defined at the level of 50 V. Various other rules are defined according to the voltage ranges.
4. What is the low voltage range according to the earthing procedures?
a) 5 V – 100 V
b) 0 V – 100 V
c) 50 V – 1000 V
d) 0 V – 1000 V
View Answer
Explanation: Low voltage is defined as exceeding 50 V but not exceeding 1000 V and is mainly referring to 415 V distribution networks. For low voltage systems the regulations say that ‘no impedance shall be inserted in any connection with earth.
5. Low voltage systems should be solidly earthed.
a) True
b) False
View Answer
Explanation: Low-voltage systems must be solidly earthed. The system of protective multiple earthing, which can be advantageous on 415 V distribution networks in some situations, is permitted on low-voltage systems subject to certain other conditions but this still requires that the neutral should be solidly earthed ‘at or as near as is reasonably practicable to the source of voltage.
6. Earthed neutral allows rapid operation of protection to ___________
a) Line to line faults
b) Earthed faults
c) Other faults
d) For all faults
View Answer
Explanation: An earthed neutral allows rapid operation of protection immediately an earth fault occurs on the system. The earthed neutral in conjunction with sensitive earth fault protection results in the faulty section being isolated at an early stage of the fault.
7. Which method will reduce the cost of insulation between earth and cables?
a) Neutral solidly earthed
b) Neutral earthed via an impedance
c) Neutral isolated
d) Neutrally
View Answer
Explanation: If the neutral is solidly earthed, the voltage of any live conductor cannot exceed the voltage from line to neutral. As under these conditions the neutral point will be at zero potential, it is possible to effect appreciable reductions in the insulation to earth of cables and overhead lines, which produces a corresponding saving in cost.
8. For an un-earthed line voltage of line to earth can be ______________
a) More than line to neutral of earthed system
b) Less than line to neutral of earthed system
c) Equal to line to neutral of earthed system
d) Can’t specify
View Answer
Explanation: On an unearthed system the voltage to earth of any line conductor may have any value up to the breakdown value of the insulation to earth, even though the normal voltage between lines and from line to neutral is maintained.
9. High voltage line is only disadvantageous in ______________
a) Small overhead lines
b) Small underground lines
c) Long overhead lines
d) Long underground lines
View Answer
Explanation: The only disadvantage of connecting a high-voltage system to earth is that this introduces the first earth from the outset and it thus increases the susceptibility to earth faults. This can be inconvenient in the case of a long overhead line, particularly in areas of high lightning incidence.
10. Earth contact material resistance should be ____________
a) Less than 2 ohms
b) More than 2 ohms
c) Depends on the circuit or system
d) Very high
View Answer
Explanation: It is not always appreciated that it is very difficult to obtain resistance values of less than about 2 ohms from a single earth plate, and often it is still more difficult to maintain the value after the earthing system has been installed for some time.
11. If parallel arrangement is done the minimum distance between two earthing conductors should be _______
a) 10 meters
b) 5 meters
c) More than or equal to 10 meters
d) 2 meters
View Answer
Explanation: Where a parallel arrangement is employed, each plate, rod, etc., all such things should be installed outside the resistance area of any other. Strictly, this requires a separation of the order of 10 meters.
Sanfoundry Global Education & Learning Series – Transformers.
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