Earth Resistance

What is "IS"?

IS ( Indian Standards)

Metric or Imperial units?

_{OK, that's a joke.}

Earth resistance is based on transformer capacity rating, because from the size in KVA you can determine the full load current or amps as a basis of determining the suitable size of earthing system. We dont have IS as Indian Standard.. what we have is IS as International Standards, so then i can't give you my IS because might be different than your IS(Indian Standard) and is not applicable in your locality or else Indian concerned Authorities will throw you in jail because it is not conform with Indian Standard.

Give the international standard

Is the installation in India?

Refer IS:3043

0Ω.

Negative values are very difficult to obtain.

Quotes from IS 3043 on the value of Earth Resistance:

0.9.2 While detailed guidelines are covered in specific portions of the Code, the following shall be noted:

a) For solidly grounded systems, it shall be sufficient to check whether the characteristics of protective device for automatic disconnection, earthing arrangements and relevant impedances of the circuits are properly coordinated to ensure that voltages appearing between simultaneously accessible, exposed and extraneous conductive parts are within the magnitudes that would not cause danger;

b) For systems where the earthing is deemed to be adequate, it shall be checked whether the main over current protective device is capable of meeting the requirements in the wiring code; and

c) Where the main over current protective device did not fulfil the requirements or where the earthing is considered inadequate, then a separate residual current device would be necessary to be installed, the earth fault loop impedance and the tripping characteristics so chosen that they comply with safe touch voltage limits.

4.6 Each earth system shall be so devised that the testing of individual earth electrode is possible. It is recommended that the value of 'any earth system resistance shall be such as to conform with the degree of shock protection desired.

6.0.3 The system earth-resistance should be such that, when any fault occurs against which earthing is designed to give protection, the protective gear will operate to make the faulty main or plant harmless.

6.0.4 In the case of underground systems, there is no difficulty whatever but, for example, in the case of overhead-line systems protected by fuses or circuit-breakers fitted with over current protection only, there may be difficulty in arranging that the value of the system earth-resistance is such that a conductor falling and making good contact with the ground results in operation of the protection. A low system-earth resistance is required even in the cases where an arc suppression coil is installed, as its operation may be frustrated by too high an earth-electrode resistance.

7.0.2 Voltage Exposure - When there is unintentional contact between an energized electric conductor and the metal frame or structure that encloses it ( or is adjacent, the frame or structure tends to become energized to the same voltage level as exists on the energized conductor. To avoid this appearance of this dangerous, exposed shock hazard voltage, the equipment grounding conductor must present a low impedance path from the stricken frame to the zero potential ground junction. The impedance should also be sufficiently low enough to accept the full magnitude

of the line-to-ground fault current without creating an impedance voltage drop large enough to be dangerous.

7.0.3 Avoidance of Thermal Distress - The earthing conductor must also function to conduct the full ground fault current (both magnitude and duration ) without excessively raising the temperature of the earthing conductor or causing the expulsion of arcs and sparks that could initiate a fire or explosion. The total impedance of the fault circuit including the grounding conductor should also permit the required current amplitude to cause operation of the protective system.

7.0.4 Preservation of System Performance - The earthing conductor must return the ground fault current on a circuit without introducing enough additional impedance to an extent that would impair the operating performance of the over current protective device, that is, a higher than necessary ground-circuit impedance would be acceptable if there is no impairment of the performance characteristics of the protective system.

8. RESISTANCE TO EARTH

8.0 Nature of Earthing Resistance

8.0.1 The earthing resistance of an electrode is made up of:

a) resistance of the ( metal ) electrode,

b) contact resistance between the electrode and the soil, and

c) resistance of the soil from the electrode surface outward in the, geometry set up for the flow of current outward from the electrode to infinite earth;

The first two factors are very small fractions of an ohm and can be neglected for all practical purposes.

The factor of soil resistivity is discussed in 8.1.

8.1 Soil Resistivity

8.1.1 The resistance to earth of a given electrode depends upon the electrical resistivity of the soil in which it is installed. This factor is, therefore, important in deciding which of many protective systems to adopt. The type of soil largely determines its resistivity and examples are given in Table 3. Earth conductivity is, however, essentially electrolytic in nature and is affected, by the moisture content of the soil and by the chemical composition and concentration of salts dissolved in the contained water. Grain size and distribution, and closeness of packing are also contributory factors since they control the manner in which the moisture is held in the soil. Many of these factors vary locally and some seasonally so that the table should only be taken as a genera1 guide.

8.8 Artificial Treatment of Soil - Multiple rods, even in large number, may sometime fail to produce an adequately low resistance to earth.

9. EARTH ELECTRODES

9.1 Effect of Shape on Electrode Resistance

9.1.1 With all electrodes other than extended systems, the greater part of the fall in potential occurs in the soil within a few feet of the electrode surface, since it is here that the current density is highest. To obtain a low overall resistance the

current density should be as low as possible in the medium adjacent to the electrode, which should be so designed as to cause the current density to decrease rapidly with distance from the electrode.

9.2 Resistance of Common Types of Earth Electrodes

9.2.1 Plates

Sufficient solid stratum should be removed and replaced with fine soil or other suitable infill to ensure as low a resistance as possible.

9.2.5 Cable Sheaths - Where an extensive underground cable system is available, the lead sheath and armour form a most effective earth electrode. In the majority of cases, the resistance to earth of such a system is less than 1 Ω.

9.3 Selection of Metals for Earth-Electrodes

These four points lead to a compromise between the need to have a low earth value for instrumentation reference purposes, which may require a lot of buried metal, and a reasonable earth value for electrical purposes.......

11.2 In rural areas, it is by no means uncommon for the earth-path resistance to be such that faults are not cleared within a short period

18.1 Basic Purpose of Earth Fault Protection

a) the earth fault loop impedance has to be low enough to allow adequate earth fault current to flow to cause an over current protective device ( for example, a fuse or circuit breaker ) in the faulty circuit to operate in a sufficiently short time; or

b) where it is not possible to achieve a low enough earth fault loop impedance, disconnection may be initiated by fitting either a residual current device or a voltage operated earth leakage circuit breaker with the former being preferred.

18.2.2 Earthed Equipotential Bonding and Automatic Disconnection of the Supply

The second aim of this protective measure is met by limiting the upper value of the earth fault loop impedance of each circuit to a value determined by the type and current rating of the protective device concerned such that, on the

occurrence of an earth fault ( assumed to be of negligible impedance ), disconnection will occur before the prospective touch voltage reaches a harmful value.

18.3.4 Local Equipotential Bonding (18.0.10) -

The equi potential zone partially created by the bonding of extraneous conductive parts to the main earthing terminal depends for its efficacy on metal-to-metal contact of negligible impedance. Within a particular part of the zone where extraneous conductive parts are simultaneously accessible with either other extraneous conductive parts or exposed conductive parts or both, tests may show that it is necessary to carry out local equi potential bonding between the parts concerned in order to obtain satisfactory low impedance.

18.4 Cross-Sectional Areas of the Conductors of an Installation Circuit -

The cross-sectional areas of the conductors of the protective circuit are influenced by the limitation placed on earth loop impedances to ensure disconnection of the circuit in which earth fault occurs - in the prescribed time, that is, instantaneous disconnection for higher control potential and disconnection with time lag for lower voltages.

Where a protective device concerned is a fuse, miniature circuit breaker or other types of series over-current device, those disconnecting times imply that the earth fault loop impedances should be such that the earth fault current is considerably greater than the rated current of the device ( or of the same order as occurring under short circuit conditions ) Residual Current Devices ( RCDs ) shall be provided-to disconnect the circuit within the same time in case of impedance or arcing fault conditions. The device setting should be interlinked with earth fault loop impedance, safe contact potential and permissible time for disconnection.

19. SELECTION OF DEVICES FOR AUTOMATIC DISCONNECTION OF SUPPLY

19.1 General - In general, every circuit is provided with a means of over current protection. If the earth fault loop impedance is low enough to cause these devices to operate within the specified times ( that is, sufficient current can flow to earth under fault conditions ), such devices may be relied upon to give the requisite automatic disconnection of supply. If the earth fault loop impedance does not permit the over current protective devices to give automatic disconnection of the supply under earth fault conditions, the first option is to reduce that impedance. It may be permissible for this to be achieved by the use of protective multiple earthing or by additional earth electrodes. There are practical limitations to both approaches.

In case of impedance/arcing faults, series protective devices may be ineffective to clear the faults. An alternate approach is to be adopted for the complete safety of the operating personnel and equipment from the hazards that may result from earth faults. This is to use residual current devices with appropriate settings to clear the faults within the permissible time, based on the probable contact potential. This method is equally applicable where earth loop impedances cannot be improved.

19.2 Use of Over current Protective Devices for Earth Fault Protection - Where over current protective devices are used to give automatic disconnection of supply in case of earth fault in order to give shock risk protection, the basic requirement is that any voltage occurring between simultaneously accessible conductive parts during a fault should be of such magnitude and duration as not to cause danger. The duration will depend on the characteristic of the over current device and the earth fault current which, in turn, depends on the total earth fault loop impedance. The magnitude will depend on the impedance of that part of the earth fault loop path that lies between the simultaneously accessible parts.

The maximum earth fault loop impedance corresponding to specific ratings of fuse or miniature circuit breaker that will meet the criteria can be calculated on the basis of a nominal voltage to earth (U,) and the time current characteristics of the device assuming worst case conditions, that is, the slowest operating time accepted by the relevant standards. Thus, if these values are not exceeded, compliance with this code covering automatic disconnection in case of an earth fault is assured. Where it is required to know the maximum earth fault loop impedance acceptable in a circuit feeding, a fixed appliance or set of appliances and protected by an over current device, the minimum current that may be necessary to ensure operation of the over current device within the permissible time of 10 seconds for a contact potential of 65 volts is found from the characteristic curve of the device concerned. Application of the Ohm's Law then enables the corresponding earth fault loop impedance to be calculated.

For circuits supplying socket outlets, the corresponding earth fault loop impedance can be found by a similar calculation for earthed equipment. When equipment are not earthed and connected to socket outlets without earthing facility, disconnection should be ensured for 30 mA within 10 seconds and with appropriate decrements in time for higher currents.

The earth fault loop impedance should, therefore, be low enough to cause the protective device to operate quickly enough to give that protection as well.

For a given application, the maximum permitted earth fault loop impedance would be the lower of the two values calculated for shock risk protection or thermal restraint respectively.

19.4 Selection of Earth Fault Protective Devices -The maximum permitted operating current depends on the earth fault loop impedance. The product of the net residual operating current loop impedance should not exceed 65 volts.

SECTION 4 POWER STATIONS, SUBSTATIONS AND OVERHEAD LINES

20. EARTHING IN POWER STATIONS AND SUBSTATIONS

20.1 General - The objective should, therefore, be to provide effective bonding of low impedance and adequate current-carrying capacity between parts with which anyone may be in simultaneous contact, and to arrange, as far as possible, that large fault currents do not flow between such points.

The discharge of high currents with high-frequency components requires earth connections of low resistance and reactance, that is, short connections with as few changes of direction as possible.

There is advantage in using a common earth where the earth electrode resistance, including the parallel resistance of any bonded metalwork, etc, to earth is 1 Ω or less, as is usual at power stations, large outdoor substations or substations

supplying a network of cables whose sheaths have a low impedance to earth.

The substation earth system rise of potential will not be excessive if the resistance of the earth electrode system is small compared to the total earth fault circuit impedance.

20.5 Power Station and Substation Earth Electrodes

20.5.1 General - The required characteristics of earth electrode system are:

a) a suitably low resistance, under all variations due to climatic conditions, for the fault currents envisaged;

For high voltage system earthing, the value of the resistance of the earth electrode system, with any adventitious earths due to the bonding of metalwork, etc, in contact with earth, should be such that the rise in potential of the electrode system above the potential of remote earth is as low as economically possible.

20.6 Earthing Conductors for Power Stations and Substations

20.6.1 Disposition - It is necessary to provide permanent and substantial connections between all equipment and the earth electrodes so as to afford a low resistance path for fault currents both to earth and between items 'of equipment.

a) Outdoor Equipment ( Excluding Pole Mounted Transformers: The earth connections of the diverters should be interconnected with the main earthing system since, for the effective protection of the substation equipment, a

definite connection of low impedance between the equipment and the diverters is essential.

21. EARTHING ASSOCIATED WITH OVERHEAD POWER LINES

21.7 Earth Wires and Earth Connection - Any connection between metalwork and earth has to be of low resistivity, both to provide for prompt operation of protective equipment and to minimize inductive interference with communications circuits

in the event of a flow of fault current. Electro-magnetic interference is reduced if the resistance of the earth return path is small compared with its reactance. At 50 Hz, inductive interference may be caused by the use of a high-resistivity wire ( for example. steel wire ) even if it is perfectly earthed.

21.8 Lightning Protection - The lower the impedance between aerial earth-wire and earth, the better is the protection since this reduces the possibility of a back flashover from the earthed metalwork to line conductors on the occasion of a direct strike to the earth wire.

SECTION 5 INDUSTRIAL PREMISES

22. GUIDELINES ON EARTHING OF INDUSTRIAL PREMISES

22.1 General - The neutral of the transformer should be connected to be earth electrodes by duplicate connections and adequate number of *earth electrodes should be provided with interlinking earth bus for getting an optimum value of the earth resistance depending upon the setting of the earth fault/earth leakage relays

22.1.3 As far as the value of the earth resistance is concerned, the objective from the point of safety consideration is not to attain minimum value of the earth resistance as is sometimes understood. But the consideration should be whether there is adequate co-ordination between the practically obtainable value of the earth resistance and setting of the protective relays.

22.1.3 The value of the earth resistance so obtained should be within reasonable limits,

22.2 Consideration for Earthing

22.2.3 The continuity resistance of the earth return path through the earth grid should be maintained as low as possible and in no case greater than one ohm.

22.3 The Earth Electrodes

22.3.1 The earth electrodes are provided to dissipate the fault current in case of earth faults and to maintain the earth resistance to a reasonable value so as to avoid rise of potential of the earthing grid.

22.3.2 the optimum value of the earth resistance depends upon the reasonable potential rise and setting of the earth fault isolating devices

As can be seen from the above, nowhere, I repeat, nowhere, in the IS it is said that the earth resistance shall be below a certain absolute value. Everywhere, it is mentioned only vaguely such a s"low enough" etc. Note Clause 22.1.3 in particular.

Though in Clause 22.2.3, it is mentione as 1 Ohm, please note that it is the earth grid resistance and NOT the earth resistance.

GA

Simply, it has to be low enough to operate the circuit protective devices within a specified time in the event of a fault.