Electrical Earthing MCQ | MCQ on Electrical Earthing

Answer.4. None of the above

Explanation:-

• Single-phase grounding of multi-phase circuits is avoided. All conductive objects within the workers’ reach (aerial or on-ground) are bonded to the grounding system. Their placement at points where workers may come in contact with the parts ensures adequate protection to the worksite.
• The earth is never used as a protective grounding conductor; neither should it be used as part of a circuit path between protective grounds.
• Overhead ground wires are bonded to the worksite grounding system (structural steel) with protective grounds for workers who will work nearby. Bonding of overhead ground wires to the worksite structure reduces step-and-touch exposure voltages on the ground. They divert fault currents away from the structure footings to adjacent structures in case of accidental energization.

Answer.4. All of the above

Explanation:-

According to Indian Electricity Rule No,61 the following equipment are to be connected to the earth.

1. The metal frame of motors, generators and other metallic parts of the equipment are to be earthed.
2. Earth terminals of the three-pin lighting and power plug socket should be earthed.
3. The metal casing of the portable apparatus like heater, electric iron, refrigerator, hair drier, etc., should be earthed.
4. All metal parts of the electrical installation such as metal conduits, light fittings, iron-clad main switches, iron-clad distribution, cable sheath, etc., should be earthed.

Answer.2. Very small

Explanation:-

The process of connecting the neutral point of a supply system on the non-current carrying parts of electrical apparatus to the general mass of earth in such a manner that at all times an immediate discharge of electrical energy takes place without danger is called earthing.

In an earthed neutral system, the neutral is earthed either directly or through resistance or reactance depending on the requirement. Thus the system neutral can be grounded effectively or non-effectively. In an effectively grounded system, the neutral is grounded directly and hence it is called solid grounding.

The main advantages of neutral earthing are:

1. The magnitude of transient voltage is small in this system.
2. The static charges which are induced are grounded immediately and are thus prevented from causing any disturbance.
3. As the neutral point is not shifted in this system, thus the voltages of healthy phases remain nearly constant.
4. This system is more reliable, provides safety to personnel and equipment with reduced operational and maintenance cost than the ungrounded system.

Answer.4. All of the above

Explanation:-

The advantages of neutral grounding are:

(i) Voltages of the phases are limited to phase to ground voltages.

(ii) The high voltages due to arcing grounds or transient line to ground faults are eliminated.

(iii) Sensitive protective relays against line-to-ground faults can be used.

(iv) The overvoltages due to lightning are discharged to the ground, otherwise, there will be a positive reflection at the isolated neutral of the system.

Answer.1. Distribution board

Explanation:-

Earth leakage switch:- The earth leakage switch is a protection device that turns off the supply of electricity when current flows into the earth wire. The earth leakage switch is on the distribution board. It comes after the isolator switch but before all the circuit breakers. As current flows into the earth wire when there is a faulty appliance or a short in the circuit. Then the earth leakage switch turns off the electricity supply. You can’t turn the electricity on again until you have removed the faulty appliance from the circuit.

Answer.3. Strip earthing

Explanation:-

Pipe earthing is the most common type used for residential quarters and strip earthing is used for 11 kV substation.

Answer.2. 50 to 100 milli-amp

Explanation:-

Electric Shock Hazards

Injuries related to electrical shocks are common accidents occurring every year and everywhere, especially when it comes to occupational incidents. Most workplaces nowadays are equipped with an electrical source and open electrical equipment that may expose workers to the hazard of being electrocuted.

Electricity refers to the flow of electrons through a material. The force that drives the electrons and makes electron flow possible is known as the voltage. Any material or substance through which electricity flows is called a conductor. Examples of conductors used in electrical work include copper and aluminum. These substances offer very little resistance to electron flow. Some materials offer very high resistance to electron flow and are classified as insulators. Examples are plastic, rubber, and porcelain. Electricity flows along a path or circuit. Typically, this path begins with a power source and follows through a conductor to a load. The path then flows back along another conductor to the Power source. 

A very important point to consider at this time is that the human body can, under certain conditions, readily become a conductor and a part of the electrical circuit. When this happens, the result is often fatal. Electrons flowing in the circuit have no way of detecting the difference between human beings and electrical equipment.

The resistance of the Human dry skin is about 100,000 Ohms to 500,000 ohms, during the perspiring or sweaty hand the resistance becomes 1000 ohms. While when the body is completely wet or under the water the resistance drops down to 150 ohms.

The human body reacts differently to the level of current flowing through it.  The data shows some typical reactions when a body is subjected to various amounts of current Remember that a milliampere (mA) is 1/1000 (0.001) of an ampere. It is a very small amount of current.

⇒ Less than 1mA = No Sensation
⇒ 1 mA = Possibly a tingling sensation
⇒ 5 mA = Slight shock felt; not painful, but disturbing; most people can let go; strong involuntary reactions may lead to injuries
⇒ 6 to 30 mA = Can definitely feel the shock; it may be painful and you could experience muscular contraction (which could cause you to hold on)
⇒ 50 to 150 mA = Painful shock, breathing could stop, severe muscle contractions; death is possible.
⇒ 1000 to 4300 mA = Heart convulsions (ventricular fibrillation), paralysis of breathing; usually means death
⇒ 10,000 mA = Cardiac arrest and severe burn deathly

Answer.4. All of the above

Explanation

Another name of neutral earthing is system earthing. In the neutral earthing system, the neutral of the system is connected to the ground

The advantages of neutral earthing are:

• It can make a closed path
• Elimination of arcing ground

The neutral of the power system may be connected to earth directly or through a resistor or reactor.

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 the ground through one or more resistance. Resistance grounding limits the fault currents. It protects the system from transient overvoltages

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

Answer. 3. Neutral wire: (i) and (iii);Earth wire: (ii) and (iv)

Explanation:-

Neutral:

• Neutral is the return path for an AC circuit that carries current in normal condition.
• This current could be primarily because of the phase current imbalance. The magnitude of this current is a fraction of phase current or in a few cases even double of the phase currents.
• It is connected to the neutral point of the transformer that is earthed at the substation.
• Unbalanced current flows through it

Earth:

• Earth, also known as a ground, is used for safety concerns against leakage or residual currents in the system.
• While phase and neutral are connected to main power wiring, the earth is connected to the body of equipment that doesn’t carry current in normal conditions but in case of any insulation failure, is supposed to carry some minor current.
• It is solid earth at the substations
• Only fault current flows through it

Answer.3. 1-A, 2-D. 3-B, 4-C

Explanation

Plate earthing:- It is used for large installations such as transmission towers, all substations, generating stations

Pipe earthing:-

• It found application for domestic installations such as heaters, coolers, refrigerators, geysers, electric iron, etc.
• For 11kV/400V distribution transformers For induction motors rating upto 100HP
• For conduit pipe in a wall, all wall brackets

Rod earthing: It is the cheapest method of earthing and is employed in sandy areas.

Strip earthing:- This type of earthing is used where the earth bed has rocky soil and excavation work is difficult