Though electrical equipment are manufactured and installed to give trouble free service, failure of various components cannot be ruled out. Failure of electrical equipment or system happens due to-
- Insulation puncture.
- Accidental contact between low & high voltage line.
- Breakdown of insulation between primary and secondary of a transformer.
- Lightning strokes.
These may cause serve damage to life and property. A need was felt to provide protection against above failure so as to content the damage to the minimum. Earthing is the measure in this direction. Earthing or grounding means to connect the equipment or electrical system to the general mass of the earth. There are two types of earthing:-
- System earthing.
- Equipment earthing.
SYSTEM EARTHING: It means connecting to the earth, the neutral point i.e. the star point of generator, transformer, rotating machines, and of grounding transformer. If neutral point of a system is earthed, the phase to ground voltage under earth fault condition do not rise to high value. Let us consider a system in which neutral is not earthed (refer figure 1). If there is earth fault on B phase, the voltage of R and Y (wealthy) phases to the earth becomes equal to the line voltage. The rise in voltage causes stress on the insulation. Earthing provides protection against such rise.
Equipment Earthing: It constitute the connecting of non-current carrying metal parts of a equipment to the earth. It provides protection to operating personnel and equipment by ensuring operation of protective control gear.
ADVANTAGES OF NEUTRAL EARTHING: Connecting the neutral of an electrical system to earth has following advantages:
- Arching grounds are eliminated.
- Voltage of healthy phases with respect to earth do not rise to 3 times to normal value.
- Insulation is prevented from stress of high surge voltage & hence provide long life to insulation & equipment.
- Stable neutral point.
- Earth fault relaying becomes simple and reliable.
- Greater safety to personnel and equipment.
Table of Contents
TYPE OF EARTHING
Earthing is of two types: Mesh earthing and Electrode earthing:
Mesh earthing: It is formed by steel rods (30 to 40 cm diameter) laid horizontally at a depth of about 0.5 metre below ground surface to give a shape of mesh. The length and breath are formed by welding of steel rods.
Electrode earthing: The material for earth electrode should be corrosion resistant. Earth electrodes, which are in use are:
ROD OR PIPE ELECTRODES: Pipe electrode shall not be smaller than 40mm. internal diameter, if of galvanized iron. The length of the pipe electrode shall be minimum 4.5 m. If one electrode fails to give the required resistance, number of such electrodes shall be installed and connected in parallel. The distance between two electrodes shall not be less than twice the length of electrodes. The GI Pipe shall be cut tapered at bottom and provided with holes of 12 mm diameter drilled not less than 7.5 cm from each other up to 2 m length from bottom.
STRIP ELECTRODES: It shall not be smaller in size than 40 mm² section if of copper and 25 × 4 mm if of galvanized iron. For round conductors it should be not less than 4 mm² if of copper and 5 mm² if of galvanized iron. The length of buried conductor shall not be less than 15 m laid in trench not less than 0.5 m deep.
PLATE ELECTRODE: In it, the plate is made of either copper or galvanized iron. This type of earthing is mostly used in our department. Size of copper plate shall not be less than 600 × 600 × 3 mm. and that of GI be 600 × 600 × 6 mm. Plate shall be put vertically under-ground so that minimum distance of ground level & top edge shall be 3 meter. Where earth resistance is not sufficient with one plate, two or more plate electrodes may be connected in parallel. This type is most suited for generating stations & sub-stations. (If fault current is more than 6 kA, plate electrode is recommended).
EARTHING CONDUCTOR: The conductor from Earth Electrode to Earth shall be of the same material as earth electrode i.e. GI or copper and be in the from of strip or wire. The size of Earthing conductor shall not be less than the following
- 4 mm dia. copper wire.
- 5 mm dia. G.I. wire
- 25 × 4 mm G.I. strip
- 20 × 3 mm copper strip.
For more details design of protective conductors as given may be seen. As a thumb rule the size (cross section in mm²) of earth strip/bus can be found by A = 0.008 × ISC, where ISC is the fault current in Amperes.
However earthing conductor shall not be more than 150 mm² in case of GI and 100 mm² in case of copper, unless otherwise specified.
EARTH BUS: Two no of Cu strips shall be provided as earth bus in HT substation or DG set. Both earth bus shall be bounded together. Each earth bus shall be connected to an independent earth electrode. Body of the transformer, switch gear, panels etc shall be connected by two leads each one to the bus.
The neutral of transformer or DG set shall not be connected to these earth bus. Each neutral of transformer or alternator of DG set shall be connected to two independent earth electrodes.
WATERING ARRANGEMENT: In case of plate electrode, a watering pipe of 20 mm diameter GI shall be provided up to the plate and funnel with wire mesh be provided at the top. At the top a masonry enclosure of size 300 × 300 × 300 mm with a MS cover of 6 mm thickness shall be provided. The cover shall be equipped with locking arrangement.
EARTH RESISTANCE: The resistance of earth shall not be more than 5 Ω. In rocky areas, it may be up to 8 Ω.
FACTORS AFFECTING EARTH RESISTANCE: Resistance of earth electrode depends upon resistivity of soil in which electrode is installed. Soil resistivity depends upon:
- Nature of soil:Wet marshy soil is preferred, but clay soil/loam with sand can also be used. damp and wet sand pit will not give good value.
- Moisture content in the soil: Effect of moisture on soil resistivity is indicated in figure 2. If moisture content falls less than 20 %, resistivity of soil increases rapidly. Therefore water during dry season shall be added periodically so as to keep minimum 20 % moisture in soil.
- Presence of salt in the moisture: Water alone in absence of natural salts cannot provide adequate conductivity. Earth electrodes installed in streams of pure water will offer high resistance. However if water is added along with sodium chloride (common salt), calcium chloride etc. resistivity of soil can be reduced drastically. Effect of salt on soil resistivity is indicated in figure 3. From this figure it is seen that for salt content more than 5 %, very little advantage is obtained.
METHOD OF CHEMICAL TREATMENT: Approximately 90 % of resistance between an electrode and earth lies within a radius of 2 m. Therefore it is advisable to dig a trench about 30 cm deep around electrode as indicated in figure 4 and then pouring the chemical treated material into it. Water will cause diffusion of salt in soil.
MEASUREMENT OF EARTH RESISTANCE: Earth resistance of a electrode can be measured by earth tester (Refer to figure 5). E is the electrode whose resistance is to be measured. P and C are auxiliary electrodes of diameter 15 - 20 mm & 40 cm long bars. C is put about 25 m from E and P is put at central point between E & C. Two more readings are taken by moving P at a distance of 3 m on either side. All the three readings should be constant (approximately). However if much difference is there, C should be moved away by 6 m from E. The process is repeated till readings are constant.
SELECTION OF TYPE OF ELECTRODES: The following are general guidelines for the selection of the type of electrodes:
|TYPE OF ELECTRODE||APPLICATION|
|G.I. Pipe||Internal electrical installations, with incoming switch gear up to 200 A.|
|G.I. Plate||(a) For internal electrical installations with incoming switch gear larger than 200 A.|
|(b) Neutral earthing of transformers, EA sets up to 500 kVA.|
|(c) Lightning Conductors.|
|Copper Plate||Neutral earthing of transformers / EA sets above 500 kVA.|
|Strip / Conductor||Locations where it is not possible to use other types.|
NOS. OF EARTH ELECTRODES FOR VARIOUS USE: The number of earthing electrodes for sub-stations & generating sets shall be as under.
- For neutral earthing of transformer - 2 sets.
- For body earthing of all the transformers HT/LT panels and other electrical equipment in the substation /power house - 2 sets.
- For neutral earthing of each generating sets - 2 sets.
- For body earthing of all the generating sets, LT panels & other electrical equipment in the generator room - 2 sets
- Where the generator & sub-station equipment are located together in the same building, the body earthing can be common for all the electrical equipment in the building.
- Separate earth electrodes shall be provided for lightning arrestor / lightning conductors. Non current carrying metal ports of all apparatus utilized power supply at voltage exceeding 250 volts shall be earthed by two separate & distinct connections to the earth bus or to two separate & distinct earthing set.
- Earth electrode shall be at least 1.5 m away from the building.
This Appendix indicates details useful in the design of earthing as applicable to the installations generally encountered in the Department. For complete details, IS 3043 : 1987 shall be referred to.
- All medium voltage equipments shall be earthed by two separate and distinct connections with earth. In the case of high and extra high voltages, the neutral points shall be earthed by not less than two separate and distinct connections with earth, each having its own electrode at the generating station or substation, and may be earthed at any other point, provided no interference is caused by such earthing. If necessary, the neutral may be earthed through a suitable impedance.
- Necessary protective device shall be provided against earth leakage.
Supply system requirement:
"System Earthing" is provided to preserve the security of the supply system. This is done by limiting the potential of live conductors with reference to earth, to such values as consistent with the level of insulation applied. Earthing the neutral point of the transformer ensures reasonable potential to earth, including at the time when the HV supply is impressed on the transformer. Earthing also ensures efficient operation of protective gear in the case of earth faults. Earthing may not give protection against faults that are not essentially earth faults. For example, if a phase conductor on an overhead, spur line breaks, and the part remote from the supply falls to the ground, it is unlikely that any protective gear relying on earthing, other than current balance protection at the substation, will operate, since the earth fault current circuit includes the impedance of the load that would be high relative to the rest of the circuit.
Installation protection requirement:
"Equipment Earthing" is provided to ensure that the exposed conductive parts in the installation do not become dangerous by attaining a high touch potential under conditions of faults. It should also carry the earth fault currents, till clearance by protective devices, without creating a fire hazard.
"Static Earthing" is provided to prevent building up of static charges, by connections to earth at appropriate locations. Example, operation theaters in hospitals.
"Clean Earth" may be needed for some of the data processing equipments. These are to be independent of any other earthing in the building.
Earthing is essentially required in protection of buildings against lightning. (For details, please refer to chapter 9 and Appendix-I of these Specifications).
TYPES OF SYSTEM EARTHING.
The various types of System Earthing in practice are indicated below, out of which TN-S and T-TN-S systems are generally applicable to installations in the Department:
TN system: It has one or more points of the source of energy directly earthed and the exposed and extraneous conductive parts of the installation are connected by means of protective conductors to the earthed points of the source i.e. metallic path for earth fault currents to flow from the installation to the earthed points of source. It is further subdivided into following:
TN-S system: Neutral is earthed at source. In addition to the phase and neutral conductors, an indepedent protective earth (PE) conductor connected to the source earth is also run with the supply line. All the exposed conductive parts of an installation are connected to this PE conductor via the main earthing terminal of the installation. In indian TN-S system, independent earth electrode is also necessary within the consumer premises at the main earthing terminal.
TN-C system: Neutral is earthed at source. No separate PE conductor is run with the supply line, nor in the internal installations, since neutral and PE are on a common conductor. All exposed conductive parts of an installation as well as the neutral line are connected to this PE&N conductor. (A CNE cable is used for wiring such installations). Additional earth electrode has to be provided for this conductor locally for 3 phase consumers.
TN-C-S system (Also called Protective Multiple Earthing PME system): Supply is as per TN-C system. The arrangement in the installation is as per TN-S system i.e. The PE and N are combined in one conductor at supply line. This is earthed at source as well as at frequent intervals. There will be independent protective conductor in the installation. Consumer also normally provides earth electrode terminating on to main earth electrode in his installation, and this is in turn "linked" to the PE&N conductor from supply line. All the exposed conductive parts in the installation are connected to the PE&N conductor, through protective conductors and this main earthing terminal
T-TN-S system (for 6, 6.6 or 11 kV bulk supply): No earth is provided with HV supply line, which is terminated in delta connected transformer primary. Neutral of the transformer (star connected) secondary is earthed. Independent earth electrodes and bus are provided for the body earthing. Protective conductors are run through out the LT distribution from the same for equipotential bonding.
TT system: Neutral is earthed only at source and no PE conductor is given with supply line. All the exposed conductive parts of the installation are connected to an earth electrode at consumer end, which is independent of the source earth, electrically.
IT system: The source has either no earth or is earthed through a high impedance. All the exposed conductive parts of the installation are connected to an earth electrode, which is independent of the source earth, electrically.
CONCEPT OF PROTECTION AGAINST INDIRECT CONTACT: The most commonly and successfully used method of protection against indirect contact is by earthed equipotential bonding and automatic disconnection of supply, details of which are elaborated in IS 732 : 1989 and IS 3043 : 1987. All the exposed conductive parts are connected through protective (loop earthing) conductors to the main earthing terminal. All the extraneous conductive parts which are simultaneously accessible with the exposed conductive parts are also bonded to the main earthing terminal through main bonding conductor so that there is no dangerous potential between the exposed and the extraneous conductive parts. The earth fault loop impedance (EFLI) and the characteristics of the tripping devices are coordinated such that the faulty circuit is automatically disconnected before there is a persistent touch voltage at the exposed conductive part over a period of time, causing a shock hazard. If the disconnecting time is not satisfactory due to large EFLI, supplementary bonding between the exposed and the extraneous conductive parts is provided. Alternatively, use of RCD's becomes very relevant in most such situations. For more details, IS 3043 : 1987 may be referred to.
Decision regarding the providing of RCD (RCCB) shall be taken in individual cases keeping in view the type, use, importance, system of earthing and nature of electrical installations to be protected by the RCD, requirements of the local Electric Supply Companies etc.
Earthing (comprising the electrode, earthing conductor, main earthing terminal etc.) and protective conductors in an installation are thus vital components in the system of protection against shock hazards. The concept is indicated diagrammatically in figure 3 (A); figure 3(B) indicates the method of ensuring the same, as envisaged in these Specifications.