Estimation & Costing | Electrical Engineering SSC JE (Technical) - Electrical Engineering (EE) PDF Download

Estimation And Costing

Earthing means connections of the neutral point of a supply system or the non-current carrying parts of electrical apparatus, such as metallic framework, metallic covering of cables, earth terminal of socket outlet, stay wires etc.., to the general mass of earth in such a manner that at all times an immediate discharge of electrical energy takes place without danger.

Earthing is provided 

1. To ensure that no current carrying conductor rises to a potential with respect to general mass of earth than its designed insulation.

2. To avoid electric shock to the human beings, and

3. to avoid risk of fire due to earth leakage current through unwanted path.

In an electric installation, if a metallic part of an electric appliance comes in direct contact with a bare live wire (that may be due to failure of insulation or otherwise) the metal being a good conductor of electricity is charged and static charge on it will accumulate. Now if any person comes in contact with this charged metallic part, he will get a severe shock. But if the metallic parts of the appliances are earthed, the charge will be transferred to the earth immediately, as the metallic part comes in direct contact with a bare or live wire or breakdown occurs. And as the discharge takes place to earth, the impedance of path of the current is low, a large amount of current  flows to earth, the instant, the current exceeds the limiting value, the fuse provided in the circuit will blow of and cut off the appliance from supply. Thus earthing of metallic parts of electrical equipment and appliance provides safely.

Methods of Earthing The  various methods of earthing are:

1. strip or wire earthing
2. Rod earthing
3. Pipe earthig and 
4. Plate earthing.

Strip or wiring earthing is used at places which have rocky soil earth bed because at such places excavation work of pipe/plate earthing is difficult. Rod earthing is suitable for areas which are sandy in character. This method of earthing is very cheap as no excavation work is involved. pipe earthing is the most common and best systems of earthing as compared to other systems suitable for the same earth  and moisture conditions.

INSPECTION AND TESTING OF WIRING

INSTALLATIONS

Internal wiring should be inspected once a year and the following points should be checked while carrying out inspection of the wiring installation.

1. Service Connections :- In case of overhead line, check and ensure that:

(i) The lines are terminated at a sufficient distance from the building.
(ii) The danger notice exists to warn the staff.
(iii) The fuse wire of correct rating is provided on the phase line.
(iv) The lead-in-pipe is property earthed and bonded and pipe ends are provided with insulation bushes to protect the wires from mechanical damage.

In case of underground cable check up and ensure that

(i) the cables are properly sealed adn there is no leakage of cable oil

(ii) there are earth connection to the cable armouring.

Testing of Wiring Installations The tests to be performed before a new installation or an addition to an existing installation is connected to the supply mains are as follows:

1. The insulation resistance between the wiring and earth with all fuses and lamps in and all switches ‘on’.

2. The insulation resistance between the conductors with all lamps out and all switches ‘on’.

3. Testing of polarity of nonlinked single pole switches.

4. Testing of earth continuity path

5. Testing of earth resistance.

The aim of testing of insulation resistance between the wiring and earth is to know whether the wires or cable used in the wiring system are sufficiently insulated to avoid leakage of current.

Important Consideration Regarding Motor Installation Wiring 

1. All equipment used in power wiring shall be of ironclad construction and wiring shall be of the armoured cable or conduit type (IE Rule 51)

2. Woodwork shall not be used for mounting of switchgear.

3. Looping of conductors and use of the joints shall not be done,

4. The length of  flexible conduit used for connection between the terminal boxes of motors and starters, switches and motors shall not exceed 1.25 metres.

5. Every motor, regardless, of its size, shall be provided with a switchfuse placed near it.

6. In addition to switchfuse, all motors shall be provided with suitable means for starting and starters are used to limit the starting current to a desirable value. Direct-on-starters, star-delta starters, auto-transformer-starters (or rotor resistance starters in case of slip-ring induction motors) are used for ac motors of rating up to 0.75 kW, above 0.75 kW and below 11kW and above 11kW respectively.

7. The conduit should preferably be laid in covered trenches to facilitate operator movement (safe)

8. Laying of cables must be in separate conduits for separate motors.

9. The minimum cross section of conductor that can be use for power wiring is 2.5 mm2 for aluminium conductor cables and 1.25 mm2 for copper conductor cables (ISI recommendations). Hence PVC/MLPE cable of size lower than 3/0.915 mm copper or 1/1.80 mm aluminium cannot be used for power wiring

10. The current rating of cable for supply to motor may be based on the normal full-load current of the motor but  fuse rating should be based on starting current. In no case should the rating of the fuse be greater than twice the ratin of the cable.

So the following points must be observed.

(a) For motors of capacity above 12kW having starting current lower than twice normal fullload, the current rating of the cable is to be based on normal- full load current of motors and current rating of fuses on motor starting current.

(b) For motors of capacity below 12kW, which have a very large starting current fuse should be of current rating to carry starting current safely and cable of current rating not lower than half the current rating of the fuse.

11. The conduit use in power wiring shall be electrically continuous throughout and connected to the frame of the motor. The frame of the motor shall be earthed by the owner by two separate and distinct connection so earth (IE Rule 61)

12. The wire used to earthing conductor shall be of copper or galvanised