Electrical Earthing MCQ | MCQ on Electrical Earthing

Electrical Earthing MCQ

1. The earthing wire should have

  1. High resistance
  2. Medium resistance
  3. Negligible resistance
  4. Any of the above

Answer.3. Negligible resistance

Explanation:- 

Earthing is to connect any electrical equipment to earth with a very low resistance wire, making it to attain earth’s potential.

The wire is usually connected to a copper plate placed at a depth of 2.5 to 3 meters from the ground level. The potential of the earth is considered to be at zero for all practical purposes as the generator (supply) neutral is always earthed.

The body of any electrical equipment is connected to the earth by means of a wire of negligible resistance to safely discharge electric energy, which may be due to failure of the insulation, or line coming in contact with the casing, etc. Earthing brings the potential of the body of the equipment to ZERO i.e. to the earth’s potential, thus protecting the operating personnel against electrical shock.

 

2. What should be the value of earthing resistance for large power stations?

  1. 12 Ω
  2. 0.5 Ω
  3. 2 Ω
  4. 50 Ω

Answer.2. 0.5 Ω

Explanation:-

Value of Earth Resistance for Different Equipment
Equipment to be earthed Maximum Value of Resistance Under Worst Conditions
Large power stations 0.5 Ω
Major substations 1.0 Ω
Small substations 2.0 Ω
Factories substations 1.0 Ω
Lattice steel towers 3.0 Ω
. Industrial machines and equipment 0.5 Ω

According to Indian Electricity Rules, the value of earthing resistance for large power stations should be 0.5 Ω

 

3. What is the main purpose of earthing?

  1. Provide path for leakage current
  2. Provide electric shock safety
  3. Provide constant line voltage
  4. All of the above

Answer.4. All of the above

Explanation:-

Earthing means setting up an electrical bond between a given point in a network, installation, or machine and an earth electrode. This earth electrode is a conductive part that may be inserted in the ground or in a conductive medium, in electrical contact with the Earth.

Purposes of Earthing

  • It provides a surface around and under a sub-station, which would be at a uniform potential and as near to zero or absolute earth potential as possible.
  • Ensure people in the surrounding area are not exposed to electric shock.
  • Better performance of the electrical system.
  • To protect the human being from disability or death from shock in case the human body comes into the contact with the frame of any electrical machinery, appliance, or component, which is electrically charged due to leakage current or fault.
  • To maintain the line voltage constant. To serve as the return conductor for telephone and fraction work.
  • Earthing enables leakage currents to flow away safely and, if it is linked to an automatic cut-off device, can ensure that the power supply to the electrical installation is switched off.

 

4. Which of the following is/are methods of earthing?

  1. Plate earthing
  2. Strip earthing
  3. Pipe earthing
  4. All of the above

Answer.4. All of the above

Explanation:-

Method of earthing

(i) Strip earthing

(ii) Earthing through water mains

(iii) Rod earthing:

(iv) Pipe earthing

(v) Plate earthing

 

5. Earth electrode used in Plate Earthing is a

  1. Plate
  2. Pipe
  3. Rod
  4. Grid

Answer.1. Plate

Explanation:-

In the plate earthing a pit of about 3 meters depth is dug. The earth electrode used may be a copper plate having a dimension of 60 cm x 60 cm x 3.18 mm. (In the case of the G.I (Galvanized Iron) plate, the dimension is 60 cm x 60 cm x 6.36 mm).

After placing the electrode in the pit, layers of charcoal and salt are used to fill the space around it. The electrode can be connected to a G.I. pipe of 12.7 mm diameter for carrying G.I. earth wire. The pit is covered with an iron plate for a periodic check-ups. To increase moisture and dampness a pipe is provided for carrying water.

 

6. The materials used in plate earthing are

  1. Charcoal
  2. Salt
  3. GI wire
  4. All of the above

Answer.4. All of the above

Explanation:-

Following materials are required for plate earthing

(i) GI earthing plate – 60 cm × 60 cm × 6.35mm

(ii) Copper earthing plate 60cm × 60cm × 3.18mm

(iii) Earthing copper wire = 105 SWG or

(iv) Earthing GI. wire =8 SWG

(v) Earthing GI. pipe 12.7 mm dia (to take earth wire)

(vi) GL Pipe 19.5mm (to pour water around plate amp)

(vii) Salt and wooden charcoal (for moisture)

(viii) Funnel-To pour water

(ix) Nut and bolt-50 mm × 8mm (copper nut bolt for copper plate and GI nut bolt for GI plate)

(x) Cast iron cover 30 cm × 30 cm (to cover from top)

Alternate layers of charcoal and salt are used around the plate. The layers of charcoal shall be placed immediately over the plate, and thereafter, successive layers of salt and charcoal are laid to keep the surroundings sufficiently moist.

 

7. The main drawback of plate earthing is

  1. Doesn’t provide the low value of earth resistance
  2. Very expensive
  3. Difficult in maintenance
  4. All of the above

Answer.3. Difficult in maintenance

Explanation:-

The main drawback of plate earthing is as the plate is broken or any damage the repair only can be done if an earth wire is broken by re-digging of pit.

The other disadvantage is that discontinuity of earth wire and plate below the earth cannot be observed physically, hence, is misleading and sometimes result in a heavy loss in case of any fault.

 

8. The size of plate electrode (in inches) used in plate earthing is

  1. 1/4 inch
  2. 2/4 inch
  3. 3/8 inch
  4. 5/9 inch

Answer.1. 1/4 inches

Explanation:-

Plate Electrodes:- Each plate shall be at least 1/4 inch in thickness installed not less than 21/2 feet below the surface of the earth.

 

9. The minimum length of pipe electrode used for earthing should not be less than

  1. 3 m
  2. 2m
  3. 4m
  4. 10 m

Answer.2. 2m

Explanation:-

The pipe used as an earth electrode is galvanized and perforated. Its diameter is 38.1 mm and its length is 2 m. The length may be increased to 2.75 m in the case of dry soil.

Another pipe of 19.05 mm diameter and of length 2.45 m is connected at the top of the above-mentioned pipe.

 

10. What is the size of Standard Wire Gauge or (SWG) For pipe earthing is

  1. 12 SWG
  2. 16 SWG
  3. 8 SWG
  4. 20 SWG

Answer. 8 SWG

Explanation:-

The materials used in pipe earthing are as below

(i) GI pipe of 38 dia and 2-2.5 m length. The pipe is drilled many holes of 12 mm diameter.

(ii) The earth wires of 8 SWG of GI are fastened to the top section of the pipe with nut bolts.

(iii) 12.7mm diameter earthing GI pipe (earth wire is connected with this pipe.)

(iv) 1m length GI pipe of 19mm diameter which is used to fill, water around earth pipe.

(v) Salt and wood charcoal mixture (for moisture)

(vi) Washer and socket of 12.7mm GI

(vii) Funnel to fill water

(viii) Cast iron cover 30cm × 30cm sizes to cover the earthing.

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