Test: Estimation & Costing- 1 - Electrical Engineering (EE) MCQ

According to IE rule 80,

Where a high or extra-high voltage over-head line passes above or adjacent to any building or part of a building it shall have on the basis of maximum sag a vertical clearance above the highest part of the building immediately under such line, of not less than

(a) For high voltage lines up to and including 33,000 volts - 3.7 metres

(b) For extra-high voltage lines -  3.7 metres plus 0.3 metre for every additional 33,000 volts or part thereof

According to IE rule 77,

No conductor of an overhead line, including service lines, erected across a street shall at any part thereof be at a height less than -

(a) For low and medium voltage lines - 5.8 metres

(b) For high voltage lines - 6.1 metres

According to IE rules, Overhead lines for power supply to tram cars are should be at a minimum height of 10 m

The screwed conduit circuits are used to protect the electrical wiring. It is used for the voltage levels in between 250 V to 600 V.

Electrical schedule is a list or a plan of a building providing information of number of points in each room

1. Wood is not used for mounting switchgears

2. All equipment used in power wiring shall be of iron clad

3. Electrical looping is loop/loops created between two light with 1 single wire connected to multiple fittings. Usually looping is done for neutral wire. Neutral wire is a return wire for the current in an electrical circuit, i.e. it carries electricity from the output device back to the service panel/board

4. The length of flexible conduit is less than 1.8 m

The maximum load that can be connected in a circuit connecting only lighting points is 800 watts

Factor of safety describes the load carrying capacity of a system beyond the expected or actual loads. The factor of safety used for current ratings in a power installation is 2

In the method of strip earthing, strip electrodes of cross-section not less than 25mm x 1.6mm (1in x 0.06in) is buried in a horizontal trenches of a minimum depth of 0.5m. If copper is used then the dimension will be cross-section of 25mm x 4mm (1in x 0.15in) and if galvanized iron or steel is used then dimension of 3.0 mm² is used.

Solid grounding: In this type of neutral grounding, the neutral of the system is directly connected to the ground through a conductor of negligible resistance and reactance.

Resistance grounding: In this type of neutral grounding, the neutral of the system is connected to ground through one or more resistance. Resistance grounding limits the fault currents. It protects the system from transient over voltages

Reactance grounding: In this method, a reactance is inserted between the neutral and ground to limit the fault current. This method has high transient voltages appear under fault conditions

The effect of electric shock on human body depends on current, voltage and the duration of contact.

The earthing is essential because,

1. The earthing protects the personnel from the short circuit current hence it provides safety against shock

2. The earthing provides the easiest path to the flow of short circuit current even after the failure of the insulation

3. The earthing protects the apparatus and personnel from the high voltage surges and lightning discharge

Since the resistance of the earth near each ground rod will be very high, adding a second ground rod we can slightly reduce the overall grounding resistance. Ground rods would need to be spaced 7.6 m apart to achieve the best grounding effect.

Earth is chosen as a place of zero electric potential because it has almost constant potential.

Tower footing resistance is the resistance offered by the tower footing to the dissipation of ground. The effective ground wire depends to a large extend on the tower footing resistance. The tower top potential depends on this resistance.

Low value of tower footing resistance results in less voltage stresses across line insulation. A tower footing resistance of 20 ohm for EHV lines and 10 ohm for HV lines provides sufficient lightning protection.

Tower footing resistance depends on

1) Type of electrode configuration employed

2) Soil resistivity

3) Electrode shapes (Hemisphere, vertical driven rod, and Buried horizontal wires)

Methods for tower grounding are:

Buried Conductor: One or more conductors are connected to tower legs and buried in back filled of tower foundation. It is used when soil resistivity is low

Counterpoise Wire: A wire/ Strip of length of 50 m is buried horizontally at depth of 0.5 m below ground. This wire is connected to tower legs. It is used when earth resistance is very high and soil conductivity is mostly confined to upper layer

Ground rod: Pipe/Rod of 3 to 4 m is driven into ground near the tower and top of rod is connected to tower by suitable wire/strip. It is used where ground conductivity increase with depth

Treated Earth Pits: Pipe/Rod of 3 to 4 m are buried in treated earth pits and top of rod is connected to tower by suitable wire/strip. It is used in very high resistivity near tower

For reducing tower footing resistance, it is better to use ground rod and counterpoise only.

The resistance offered by the earth electrode to the flow of current into the ground is known as the earth resistance or resistance to earth. It comprises of

1) Resistance of soil away from electrode

2) Contact resistance between electrode and soil

3) Resistance of metal electrode

The size of aluminium conductor used for sub circuit in domestic wiring should be 

Cleat wiring is most suitable for a temporary shed.

Various specifications in respect to earthing electrode as recommended by Indian Standards are given below.

1. An earthing electrode should not be situated (installed) close to the building whose installation system is being earthed at least more than 1.5m away.

2. The earth resistance should be low enough to cause the flow of current sufficient to operate the protective relays or blow fuses. Its value is not constant as it varies with weather because it depends on moisture (but should not be less than 1 Ohm).

3. The earth wire and earth electrode will be the same material.

4. The earthing electrode should always be placed in a vertical position inside the earth or pit so that it may be in contact with all the different earth layers.