Test: Estimation & Costing- 2 - Electrical Engineering (EE) MCQ

In earthed neutral system, the neutral is earthed either directly or through resistance or reactance depends on the requirement.

The advantages of neutral earthing are,

1. The arcing grounds are prevented from occurring by employing suitable switchgears

2. As the neutral point is not shifted in this system, thus the voltages of healthy phase’s remains nearly constant

3. The faulty part of the system can be isolated from the remaining system with the help of earth fault relays

4. The magnitude of transient voltage is very small

5. This is system is more reliable, provides safety to personnel and equipment with reduced operational and maintenance cost than ungrounded system 

For proper earthing, the maximum value of earth resistance while carrying out the testing of earth continuity path should be around 1 ohm.

Aluminum is used for wiring continuous bus bar because its cost is lesser than the copper.

The unforeseen items are the items which are unpredictable. The list of vague and unforeseen items is included in contingencies.

According to Indian Electricity rules, the maximum length of the flexible conduit in motor installation is should be less than 1.25 m

Coal salt mixture is preferred for filling around the earth electrode for effective earthing.

Coal is made of carbon which is good conductor minimizing the earth resistant. Salt use as electrolyte to form conductivity between GI Plate Coal and Earth with humidity. Sand is used to form porosity to cycle water & humidity around the mixture.

Under dry conditions, the resistance offered by the human body may be as high as one mega ohms. Wet or broken skin may drop the body's resistance to 1000 ohms. Adding that, high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms.

Class C type fire extinguishers can be used for electrical fires. While the extinguishment of electrical fire,

1. If possible, remove the source of energy by unplugging or disconnecting the electrical equipment

2. Use carbon dioxide, ordinary (BC rated) dry chemical, multi-purpose dry chemical and halon fire extinguishers

3. Do not use water on any energized electrical equipment

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 valu

A guard wire or earth wire is the top most conductors in high voltage transmission lines. It is used mainly to protect lines from lightning. If in case lightning struck then it carries the excessive current inrush to ground.

Its radius is much smaller than the actual transmission wire because as the resistance is inversely proportional to area of cross section and as the cross section decreases, the resistance increases. The increased resistance of earth wire is able to withstand high inrush of current caused due to lightning and it will safely guide this inrush into ground.

Vulcanized Indian Rubber: These consists of a copper conductor covered with an insulation layer of VIR. A cotton tape covering is provided over this insulation layer to protect the wire from moisture and to provide mechanical strength to the wire. These type of wires are used in 250 V CTS wiring, casing or capping or cleat wiring.

Polyvinyl Chloride: These wires consists of a conductor over which an insulation layer made up of Polyvinyl Chloride is provided. These wires cannot resist much heat and they have relatively low melting points, so they aren’t used in hot places and also these wires are not used with heating appliances. These wires are applied in casing wiring and also used along with cleat wiring

As per IE rules the maximum allowable variation between declared and actual voltage at consumer’s premises should be ± 3%

As per IE rules Minimum distance of underground cable from the foundation of building should be 10 cm.

While using wooden poles in a services line, the span should not exceed 40 to 50 m.

As per IE rules the permissible variation of voltage at the consumer end is ±6%.

In the case of low or medium voltage, it is more than 6 percent.

In the case of high voltage, it is more than 6 per cent on the higher side or by more than 9 per cent on the lower side.

The minimum permissible size of the earth continuity conductor is 6 sq. mm.

Setting depth of poles is 1/5 part of the length of pole. The depth for most heavy poles should be at least two feet. Another rule of thumb is that the hole’s depth should be 1/5 of the length of the poles and below the frost line.

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