Power System Earthing Interview Questions and Answers:
1. What is difference between system earthing and equipment earthing ?
Ans. System earthing deals with earthing of the system neutral to ensure system security and protection while equipment earthing deals with earthing of non-current carrying parts of the equipment to ensure safety of personnel and protection against lightning.
2. What is average resistance of a human body ?
Ans. About 1,000 Ω.
3. What is the current that is considered safe for human body ?
Ans. About 9 mA.
4. What is meant by an earth electrode ?
Ans. Any wire, rod, pipe, plate and an array of conductors embedded in ground horizontally or vertically is called the earth electrode
5. Give the maximum permissible values of ground resistance for (a) large power stations (b) small substations.
Ans. (a) 0.5 Ω; (h) 2.0 Ω.
6. What is meant by ‘let go current’ ?
Ans. The maximum current at which the subject is capable of releasing an electrical conductor by using muscles directly stimulated by the current is called ‘let go current’.
7. On what factor does the resistivity of soil depend ?
Ans. Soil resistivity depends upon moisture content, soil temperature, composition and amount of soluble salts in the soil, magnitude of current flowing through the soil and depth of the layer.
8. What is meant by earthing mat ?
Ans. A number of rods when joined together through copper conductors constitute an earthing mat. This is also called the earthing grid.
9. What is earthing lead ?
Ans. The wire which connects earth wire or any other apparatus to be earthed to the earth electrode is known as the earthing lead.
10. What is meant by system grounding ?
Ans. The process of connecting some part of the electrical power system (e.g., neutral point of a star-connected system, one conductor of the secondary of a transformer etc.) to ground (i.e., soil) is known as the system grounding.
11. What is neutral grounding ?
Ans. The process of connecting neutral point of a 3-phase system to ground (i.e., soil) either directly or through some circuit element (e.g., resistance, reactance etc.) is called the neutral grounding.
12. What is the chief argument that favours use of isolated neutral system ?
Ans. The chief argument that goes in favour of isolated neutral system is the possibility of maintaining continuity of supply in case one phase gets grounded until it is convenient to disconnect and repair the faulty line.
13. What is arcing ground ?
Ans. Arcing ground is a surge which is produced if the neutral is not connected to the earth. In a three-phase transmission line, each phase has a capacitance w.r.t. to the earth. When an earth fault occurs on any phase, a capacitive fault current IF flows into the earth. Such a current if exceeds 4-5 amperes, is sufficient to maintain an arc in the ionized path of the fault, even though the medium causing the fault has cleared itself. The persistency of the arc due to the flow of capacitance current gives rise to a condition known as “arcing ground” in which cyclic charging and discharging of the system capacity through the fault results in high frequency oscillations being superimposed on the whole system and a build-up of very high voltages can occur. This results in phase voltage to rise to 5 to 6 times of normal voltage. The build-up of high voltages may result in insulation breakdown.
14. What are the various methods of neutral grounding ?
Ans. The methods commonly used for grounding the neutral point of a 3-phase system are
- Solid or effective grounding
- Resistance grounding
- Reactance grounding
- Peterson coil or arc suppression coil grounding
- Voltage transformer grounding.
15. What is effectively grounded system ?
Ans. When the neutral point of a 3-phase system (e.g., 3-phase generator, 3-phase transformer etc.) is directly connected to ground (i.e.. soil) through a wire of negligible resistance and reactance, it is called the solid grounding or effective grounding.
According to IEEE definition, a system or a portion of a system can be said to be effectively grounded when for all points on the system or specified portion thereof, the ratio of zero sequence reactance to positive sequence reactance is not greater than three (i.e., X0/X1 > 3) and the ratio of zero sequence resistance to positive sequence resistance is not greater than one (i.e., R0/R1 > 1) for any condition of operation and for any amount of generator capacity.
16. What is arc suppression coil grounding ?
Ans. In the arc suppression coil method of grounding, the arcing-ground danger has been eliminated and the system is approximated to the isolated neutral system, in which one or two healthy phases continue to supply power and complete shutdown on the system is avoided till the fault was located and isolated.
An arc suppression coil, also known as Peterson coil or Ground fault neutralizer, is an iron core tapped reactor connected in neutral to ground connection. The reactor is provided with tappings so that it can be tuned to the system capacitance. The function of arc suppression coil is to make the arcing ground faults self extinguishing and, in case of sustained faults, to reduce ground fault current to a comparatively low value so that the system can be kept in operation with one line grounded.
This method of neutral grounding is usually confined to medium voltage overhead transmission lines which are connected to the system generating source through power transformer.
It has been found that arc suppression coil grounding reduces the line outages from ground faults to 20 or 30 per cent of those obtainable with other types of grounding. As such this method of grounding is advantageously employed on radial lines as this avoids the construction of duplicate circuits for maintaining service continuity and thus there is overall economy.
17. What is the use of earthing transformer ?
Ans. In cases where the neutral points of a three-phase system are not accessible or where the transformers or generators are delta connected, an artificial neutral point is required to be created. This is accomplished by means of a zig-zag transformer, called the earthing transformer.
As compared to the power transformer the rating of a zig-zag transformer is quite different. A grounding transformer is usually specified by the single-phase fault current that it handles. The two most common intervals specified for the fault current are 30 and 60 s. A 60 s grounding transformer will be more expensive as compared to a 30 one.
18. What is harmonic suppressor ?
Ans. Harmonic suppressor is an iron-cored reactor and is used in the neutral ground connections of a generator so that third harmonic currents are reduced to safe value.