21. From the point of view of safety, the resistance of the earthing electrode should be
Infinite
High
Medium
Low
Answer.4. Low
Explanation:-
From the point of view of safety, all parts of electrical equipment should be kept at earth potential, therefore these are connected to an earth electrode. The earth electrode provides a path of low resistance to the leakage currents to flow to the earth.
For safety, the resistance of the grounding electrode should be low. If the resistance is high, then due to the maximum current in the event of a fault, the equipment connected to the supply may burn. Therefore the resistance is kept low, in homes the value of electric resistance is kept as 0.5Ω to 1Ω.
(i) In the event of overvoltage which may occur due to lightning discharges and other causes it is essential to ensure that excessive overvoltages are not induced in the parts of the equipment.
ii) Also, the neutral in a three-phase circuit is connected to the earth in order to stabilize the potential of other circuits
22. The earth’s potential is taken as
Infinite
Supply voltage
1 volt
Zero
Answer.4. Zero
Explanation:-
Why the electric potential of Earth is zero?
For the measurement of level, we have a standard or a zero level, that is sea level and for the measurement of temperature, a zero of temperature, that is the temperature of melting ice, so also for the measurement of potential, we must have a standard or a zero of potential. The potential of the earth is taken to be zero of potential. For this purpose, the earth has been chosen because of its huge size, so that the small charges with which we have to deal with, if given to it or taken from it, produce no change in its potential.
Thus, a body whose potential is higher than that of the earth is said to have positive potential and one whose potential is lower than that of the earth is said to have a negative potential
23. The acceptable value of grounding resistance to a domestic application is
0.5 Ω
1 Ω
10 Ω
100 Ω
Answer.1. 0.5Ω
Application
Earth resistance should be as small as possible. It depends on the voltage level of the system to be grounded. As voltage level increases the earth resistance is required to be nearer to zero as far as possible. The NFPA and IEEE recommend a ground resistance value of 5 ohms or less.
For domestic application, the earth resistance should be between 0.2 – 0.5 ohms
24. The earth resistance of Gi wire is less than
1 Ω
0.5 Ω
3 Ω
5 Ω
Answer.3. 3Ω
Explanation:-
Earth Resistance:- The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential. The earth resistance for copper wire is 1Ω and that of GI wire is less than 3Ω. The typical value of the earth resistance at large power stations is 0.5 Ω, major sub-stations is 1Ω, small sub-stations is 2 Ω and in all other cases 5Ω.
25. Earth resistance is comprised of
Resistance of soil surrounding earth.
Contact resistance between electrode and soil.
Resistance of metal electrode
Which of the following options are true
1 only
1 and 2 only
1 and 3 only
1,2 and 3 together
Answer.D. 1,2 & 3 together
Explanation:-
The object of an earth electrode is to provide a low resistance path to fault currents that may cause injury to the operating staff or damage or the equipment. The currents will dissipate safely when properly conducted to earth via earth electrode. There are three components of the earth resistance;
(1) Ohmic Resistance of the electrode materials and connections to them,
(2) Contact resistance between the electrode and the soil surrounding it,
(3) Resistance of the earth immediately surrounding the grounding electrode or resistivity of earth, which is often the most significant factor.
The grounding electrodes are usually made of a very conductive metal (copper or copper-clad) with adequate cross-sections so that the overall resistance is negligible.
The resistance between the electrode and the surrounding earth is negligible if the electrode is free of paint, grease, or other coatings, and if the earth is firmly packed.
The only component remaining is the resistance of the surrounding earth. The electrode can be thought of as being surrounded by concentric shells of earth or soil, all of the same thickness. The closer the shell to the electrode, the smaller it’s surface; hence, the greater its resistance. The farther away the shells are from the electrode, the greater the surface of the shell; hence, the lower the resistance.
The distance at which this effect occurs is referred to as the effective resistance area and is directly dependent on the depth of the grounding electrode.
26. In house wiring, black and green wires indicate
Black electrical wire is used for power in all circuits Any circuit’s black wire should be considered hot or live. The black wire is never used for a ground or neutral wire and should be used as the power feed for a switch or an outlet.
Red electrical wire indicates the secondary live wires in a 220-volt circuit, used in some types of switch legs and in the interconnection between smoke detectors that are hard-wired into the power system.
Yellow and blue electrical wires are also used to carry power but are not for wiring the outlets for common plug-in electrical devices. These colors are used for the live wire pulled through conduit.
White and gray indicate a neutral wire.
Green indicates the grounding of an electric circuit. A green wire can connect only to another green wire and should never connect to Lanv other color wire
27. For the measurement of the earth resistance of a given earth electrode
The collecting electrode should be very near to the electrode under test
The collecting electrode should touch the electrode under test
Collecting electrode should be far way the electrode under test
None of these
Answer.3. Collecting electrode should be far way the electrode under test
Explanation:-
When performing a measurement, the aim is to position the auxiliary test electrode C far away from the earth electrode under test so that the auxiliary test electrode P will lie outside the effective resistance areas of both the earth system and the other test electrode
If the current test electrode, C, is too close, the resistance areas will overlap and there will be a steep variation in the measured resistance as the voltage test electrode is moved.
If the current test electrode is correctly positioned, there will be a ‘flat’ (or very nearly so) resistance area somewhere in between it and the earth system, and variations in the position of the voltage test electrode should only produce very minor changes in the resistance.
28. Mixture preferred for filling around the earth electrode for effective earthing is
Coal-salt mixture
Paper-salt mixture
Sawdust mixture
Lime-sand mixture
Answer.1. Coal-salt mixture
Explanation
Salt and coal are filled to maintain the moisture which reduces the resistance of the earth.
Charcoal is used to retain the moisture for a long duration also it is made of carbon which is a good conductor minimizing the earth resistance because it is an adsorbent and salt is added to increase the conductivity both these products help to pass the leakage current through the earth wire as early as possible to remove the chances of shock.
29. Resistivity of the earth increases sharply when moisture falls below
60%
40%
20%
10%
Answer.3. 20%
Explanation:-
Moisture; moist soil is best suited for earthing purpose
It is normal practice to insert an earth electrode well below the ground level, to get a stable moist soil in depth
Wet marshy land, loamy soil land, arable land, clayey land or land with loam mixed with small quantities of sand, gravel, and stones are the best choice for the purpose
Above 20 % of moisture content, the resistivity is very little affected. But below 20% the resistivity increases rapidly with the decrease in moisture content.
30. Which type of earthing is also known as fire earthing?
Plate earthing
Rod Earthing
Strip earthing
Pipe earthing
Answer.2. Rod earthing
Explanation:-
Rod earthing is also called fire earthing as it eliminates the risk of fire or explosion during an insulation failure.
In this method, a copper rod is hammered directly into the ground, and no excavation work is required. The earthing lead is joined to this rod with the help of nuts and bolts.
