Question.1. What is earthing in electricity OR What is electrical Earthing?
Answer:- 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.
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.
Earthing means connecting earth terminals to electrodes installed solidly in the mass of the earth.
A wire coming from the ground 2.5 to 3 meters deep from an electrode plate or so) is called earthing.
Question.2. What are the main purpose of earthing?
Answer:- The main purpose of earthing is
(i) The protect the operating personnel from the danger of shock in case they come in contact with the charged frame due to defective insulation.
(ii) To maintain the line voltage constant under unbalanced load conditions.
(iii) To ensure that no part of the equipment, other than Iive parts, should assume a potential that is dangerously different from that of surroundings.
(iv) To protect all machines fed from overhead lines from lightning.
(v) T0 protects large buildings and all machines fed from overhead lines against lightning.
(vi) 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.
(vii) 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.
(viii) To maintain the line voltage constant. To serve as the return conductor for telephone and fraction work.
(ix) 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.
(x) To limit overvoltages between neutral and ground and between line and ground.
(xi) To suppress dangerous potential gradients. The earth’s potential gradients, if not suppressed, may cause incorrect operation of control and protective devices and may also cause shock or injury to personnel.
(xii) To decrease noise in signal and control circuits by minimizing voltage differential between different signal reference sub-system
Question.3. What are the different types of electrical earthing?
Answer:-Grounding (earthing) can be classified into the following two categories:
- Neutral grounding (earthing) or system grounding (earthing)
- Equipment grounding (earthing) or safety grounding (earthing)
Neutral grounding deals with the grounding of the system neutrals to ensure system security and protection. Grounding of neutrals in power stations and sub-stations belongs to this category. Neutral grounding is also called, ‘system grounding’.
Equipment grounding deals with the grounding of non-current carrying metallic parts of the equipment to ensure the safety of personnel and protection against lightning. Equipment grounding is also called safety grounding’.
Question.4. What is negative earthing?
Answer:- When the negative pole of the power supply is connected to the ground source of the circuit the system is said to be a negatively grounded electrical system.
Direct current circuits that have the negative pole of the power supply connected to the ground source are negatively grounded electrical systems. Typically the positive pole connects to one or more devices on the circuit, which have their negative terminals connected to the same ground source to complete the circuit. The other terminal is positive with respect to ground. Voltages are always looked at or measured between two points with one point (ground) being the reference. The polarity of the voltage is the polarity of the terminal, not connected to the ground.
Question.5.What do you mean by equipment grounding or What is equipment earthing?
Answer:- We are frequently in touch with electrical equipment of all kinds, ranging from domestic appliances and hand-held tools to industrial motors.
The process of connecting non-current-carrying metal parts (i.e. metallic enclosure) of the electrical equipment to earth (i.e. soil) in such a way that in case of insulation failure, the enclosure effectively remains at earth potential is called equipment grounding.
Question.6. Define ground Electrode (earth electrode).
Answer:- Any wire, rod, pipe, plate, or array of conductors, embedded in-ground (earth) horizontally or vertically is known as the ground electrode or earth electrode. For distribution systems, the ground electrode may consist of a rod, about 1m long. driven vertically into the ground. In power stations and substations, elaborate grounding systems are known as ground mats, or grounding grids are used rather than individual rods.
Question.7. Define resistance of earth electrode.
Answer:- The resistance offered by the earth electrode to the flow of current into the ground. This resistance is not the ohmic resistance of the electrode but represents the resistance of the mass of the earth surrounding the earth electrode. Numerically it Is the rati0 of the potential of the earth electrode, with respect to a remote point, to the current dissipated by it.
Question.8. Explain Neutral grounding or Neutral earthing.
Answer:- Neutral grounding or neutral earthing means electrically connecting the neutrals of the system (i.e., the neutral points of star-connected 3-phase windings of power transformers, generators, motors, grounding transformers, etc.) to low resistance ground (ground electrode/ground mat) either directly or through some circuit element (resistance or reactance).
The neutral grounding is an important aspect of the power system design since the performance of the system in terms of faults, stability, protection, etc., is greatly affected by the state of the neutral. Neutral grounding does not have any effect on the operation of a 3-phase system under balanced steady-state conditions.
System earthing or intentional connection of a phase or neutral conductor to earth is for the purpose of controlling the voltage to earth within predictable limits. It also provides for a flow of current that will allow detection of an undesired connection between the system conductors and the earth and that may initiate the operation of automatic devices to remove the source of voltage from conductors with such undesired connections to the earth.
The control of voltage to earth limits the voltage stress on the insulation of conductors so that insulation performance can be predicted more readily. The control of voltage also allows for the reduction of shock hazards to any living body that might come in contact with the live conductors.
Question.9. What are the types of Neutral grounding or Neutral earthing?
Answer:- Neutral grounding can be broadly classified into the following two categories:
(i) Effective grounding
(ii) Non-effective grounding
In the effective grounding, the neutral is directly connected to the ground without inserting any intentional impedance (resistance or reactance). Effective grounding was previously called solid grounding. The coefficient of grounding of an effectively grounded system is less than 80%.
Non-effective grounding:- When an intentional resistance or reactance is connected between neutral and ground, it is called non-effective grounding. The coefficient of grounding for non-effective grounding is greater than 80%. Hence, in this case, the maximum line to ground voltage of healthy lines in the event of a single line-to-ground fault is more than 80% of line to line voltage.
Question.10. What is the difference between neutral and Earthing?
Neutral is a return path for an AC circuit that is supposed to carry current in normal condition. This current may be because of many reasons: primarily because of phase current imbalance, sometimes because of 3rd and 5th harmonics also. It is a conducting wire used in an AC circuit that provides a returning path for the flow of electrical current. It has to be connected through a neutral line.
- Earth is for safety concerns against leakage or residual currents on the system via the least resistance path.
- Earthing means connecting the dead part (it means the part which does not carry current under normal conditions).
- While phase and neutral are connected to main power wiring, the earth may be connected to the body of equipment or to any system which in normal condition doesn’t carry current but in case of some insulation failure, is supposed to carry some minor current.
- This current is not directly coming from live or phase wire but is from secondary links which were not in touch with the live system in normal condition. This current is usually much lesser than mainline current or phases current and mostly is in order of mA. But this leakage current is good enough to kill someone or may risk a fire.
- Such current is being provided with a low resistance path and sent to earth via wire.
- Because of the difference in the application, we never mix the grounding of neutral and earth.
- If both will be mixed then the earth wire which is not supposed to carry any current in normal condition may have some charges across and will become hazardous.
Question.11. Why it is important to monitor the ground resistance?
It is important to continuously monitor the ground resistance using automated or timed measurement mechanisms in order to determine the current flowing through the system and the performance of the grounding system. The methods of testing include induced frequency test methods and automated data reporting. Induced frequency tests are limited to small electrodes because the tests use frequencies in the range of kilohertz and utility is connected to the grounding system.
Question.12. What is the disadvantage of earthing?
Answer:- The disadvantage of earthing is mainly the cost: of having to provide protective conductors and earth electrodes, etc.
Question.13. When the system is operated without intentional earth it is called as?
Unearthed Systems:- Unearthed systems are the systems operated without intentional earth connection to the system conductors, except through potential indicating or measuring devices or other very high-impedance devices.
In an unearthed or high-impedance earthed network, the equipment damage is reduced, but it must have an insulation level compatible with the level of overvoltages able to develop in this type of network. In fact, these systems are earthed through the system capacitance to the ground. Mostly, this is extremely high impedance and the resulting system relationships to the ground are weak and easily distorted.
Question.14. How does earthing prevent us from getting an electric shock?
Answer:- Local earthing provides us safety from electric shock in the case of a short circuit. when excessive current flows through the live wire, it will pass to the earth through the earth wire. In the absence of local earthing, it may cause fire due to overheating of the live wire. In the case of an electric appliance, if its outer metallic body is not earthed and by chance, if it happens to touch a live wire, then metallic casing.
Question.15. What are the advantages of the unearthed system?
Answer:- The unearthed systems have two advantages. One is technical and the other is economical:
Technically: The earth fault on a system causes a small earth current to flow, so the system may be operated even in the presence of earth fault, thus improving the system continuity.
Economically: There are no expenditures required for earthing equipment or earthed system conductors.
Question.16. What are the disadvantages of unearthed system?
Answer:- The unearthed system can develop substantial overvoltages can develop. The presence of capacitance and inductance in the system leads to arcing and intermittent spluttering type faults, which cause the system voltage to escalate to a dangerously high value (up to six times the normal value has been produced in laboratory tests).
Also, difficulties of implementing selective protection upon the occurrence of the first fault are involved. Over-stressing of the insulation and failures in system components can be caused by voltages of this magnitude. Transient over-voltages, especially in the medium-voltage systems, increase the probability of failures and generally outweigh any advantages an unearthed system might offer.
Question.17. When do we require temporary earthing?
Answer:- Temporary earthing is required when doing maintenance and construction jobs on the lines and sub-stations. Properly applied temporary earthing creates an equipotential work zone guarding against possible threats:
- The voltage induced from adjacent energized lines
- Fault current on adjacent systems
- Lightning stroke
- Accidental re-energizing
It provides a parallel path around the worker with low resistance by earthing sets. Almost all the current passes through the parallel path.
Question.18. How many electrodes do we require for electrical earthing?
- One or more earth electrodes are installed to obtain the earthing system resistance stipulated in the National electrical code rules.
- The number of electrodes depends upon the soil resistivity and type of electrode used. The first step is to measure the soil resistivity. The resistance of a single electrode can be calculated based on the formulae and the number of electrodes required can be determined.
- The electrodes can be a combination of vertical electrodes, building columns, and horizontal electrodes.
- The combined resistance is a parallel combination of all the electrodes. This method is not very accurate but gives close results.
- The number of electrodes should be such that if one electrode is disconnected for testing the rest should be able to give the required earthing system resistance. Therefore a minimum of two electrodes should be provided (except for a house).
Question.19. What are the different methods of earthing?
Answer:- The different methods of earthing are
(i) Strip earthing
(ii) Earthing through water mains
(iii) Rod earthing:
(iv) Pipe earthing
(v) Plate earthing
Question.20. According to Rule 61 of Indian Electricity Rules, what is the minimum value of voltage for earthing requirements?
Answer:- According to Rule 61 of Indian Electricity Rules 1956, it is obligatory to earth by the following points and apparatus used in the power system, where the voltage is more than 125 V.
Question.21. Discuss three points of Indian Electricity Rules 1956?
- All the metal frames of motors, generators, transformers, and controlling equipment.
- The steel tower and steel tubular or rail poles carrying overhead conductors.
- The metal frames of portable electrical equipment such as heaters, table fans, electric iron, refrigerator, air conditioners, vacuum cleaners, etc.
- Other metal parts such as conduits, switchgear casings, etc.
- Earth terminal of all the three-pin outlet sockets.
- In the case of concentric cables, external conductors, that is, armoring of such cables.
- Stay wires of overhead lines if the stay insulator is not provided.
- All non-current carrying metal parts shall be earthed with two separate and distinct earth continuity conductors to an efficient earth electrode.
Question.22. Explain the strip earthing method?
Answer:- Wire or strip earthing is applied at the places (rocky soil) where it is difficult to dig pits of the desired depth. Usually, a copper wire of 5 SWG of enough length is buried in horizon trenches of depth.
Question.23. What is Rod’s earthing method?
Answer:- Rod earthing is used in sandy soil. Since excavation is not required, it is cheaper. A solid rod of galvanized iron of diameter 1.9 cm is laid vertically into the earth up to a depth of 200 to 300 cm. The conductor is tied to the rod with small clamps.
Question.24. Explain the Pipe earthing Method?
Answer:- Pipe earthing is the common method of earthing done using G.I. of diameter 3.8 cm. A pit of about 40 cm square area and 4 to 5-meter depth is dug in the soil. To increase the dampness and moisture, charcoal and salt are filled in the pit in alternate layers up to about two meters from the bottom. Earth pipe can be protected from mechanical damage by covering it with cement concrete work. A funnel with mesh can also be provided to pour water.
Question.25. Why double earthing is used in electrical systems?
Double earth is used for a 3-phase machine and equipment. This gives minimum resistance to the flow of the whole current of that apparatus in case of short circuit, leakage, etc. When double earth is used, there is an advantage of redundancy.
For providing better safety, it is advisable to provide two separate earth wires, from two separate earth electrodes, connected to the same metallic body of the equipment at two different points. This is known as double earthing.
Double earthing is essential, as per Indian Electricity Rule, for metallic bodies of large rating equipment such as transformers, motors, etc. working at 400 V and above or 3-phase machines.
Advantages of double earthing are as follows:
1. Surety of safety, because if at any time, one earthing is ineffective, then another will provide earth path to fault current.
2. As the two earth wires are in parallel, the effective resistance from equipment to earth electrode is reduced.
Question.26. What is the effect of soil resistivity on the earth electrode?
The resistance to earth of earthing electrodes depends not only on the depth and surface area of the earthing electrodes but on soil resistivity as well.
Soil resistivity is the key factor that determines the resistance of an earthing electrode and the depth to which it must be driven to obtain low earth resistance. The resistivity of the soil varies widely throughout the world and changes seasonally. Soil resistivity is determined largely by its content of electrolytes, consisting of moisture, minerals, and dissolved salts.
Dry soil has higher resistivity but wet soil may also have a high resistivity if it contains no soluble salts. Because soil resistivity is directly related to moisture content and temperature, it is reasonable to assume that the resistance of any earthing system will vary during the different seasons of the year.
Question.27. What are the factors on which soil resistance depends?
Answer:- The resistivity of soil depends on the following factors:
(a) Soil type:- The type of soil governs the resistivity to a large extent. Some examples are:- Seawater 2.5 ohm-m, Tap water 20 ohm-m, clay 50 ohm-m.
(b) Moisture content:- Electrical conduction in soils is electrolytic. Soil resistivity decreases with an increase in moisture content. However, when moisture is more than 20%, the change in resistivity is negligible. The amount of water that a soil can absorb depends on soil type, grain size, and compactness.
(c) Temperature:- When the temperature is more than 0°C, its effect on soil resistivity is negligible. At 0°C, water in the soil starts freezing, and resistivity increases.
(d) Salt content:- The composition and amount of soluble salts also affect resistivity considerably. Salt is added to increase the conductivity both these products help to pass the leakage current through the earth wire as early as possible.
(e) Magnitude of current:– If the value of current being dissipated by the soil is high, it may cause significant drying of soil and an increase in its resistivity.
The soil resistivity at a particular location also changes with depth. Generally, the lower layers of soil have greater moisture content and lower resistivity. However, if the lower layer contains hard and rocky soil, resistivity may increase with depth. Since soil resistivity depends on a number of factors, it is always advisable to make proper resistivity measurements at the proposed site of earthing system.
Question.28. Discuss the type of material used in earth electrode
Answer:- The earthing wires are essentially made from the best quality copper or GI that can be used in various electrical, electronic, and automobile instruments.
Copper can be used as earthing lead but, copper strip is also used for high installation and it can handle the high fault current because of a wider area than the copper wire. A hard-drawn bare copper wire is also used as an earthing lead. In this method, all earth conductors are connected to a common (one or more) connecting points, and then, earthing lead is used to connect the earth electrode (earth plat) to the connecting point.
Earth wire and earth electrodes are made up of the same low resistance material because if the earth wire and earth electrode are of different materials then the resistivity will be different and we will not get proper grounding.
Question.29. What are the advantages of electrical earthing?
Answer:- Earthing of neutrals of all industrial power systems is always preferable. Earthing is necessary as it offers many advantages given below:
- Persistent arcing grounds are eliminated.
- Overvoltage due to restriking is minimized.
- The ground faults can be located and isolated fastly.
- Steady-state voltage stress to earth is reduced.
- Sensitive protective apparatus can be used.
- The maintenance expenditure is reduced.
- Better safety is ensured.
- Service reliability is improved.
- Earthing provides improved lightning protection
Question.30. What will be the consequences of not having proper electrical earthing?
Answer:- Disadvantages if earthing is not there
- Leakage current in machine or equipment gives shocks which may be fetal
- Damage to high buildings and domestic appliances
- The voltage will be variable if the neutral of alternative or transformer is not earthed causing damage to the equipment.
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