IS t 3043 • 1987 or the series protective devices in case where there is no ground fault detecting devices. The main criterion is that the value of the earth return resistance should not be so high as not to produce the required ground fault current for actuating the protective devices within the stipulated time. Or in other words, the optimum value of the earth resistance is closely related to setting of the earth fault protective devices used in the system. For a small installation, as a general rule, in the event of a direct earth fault the earth fault cur rent produced should not be less than five times the highest rating of the maximum protective fuses or the setting of the earth fault relay if such a device is provided.
index up to —10 is recommeded for steel/GI earthing conductors while designing an earthing scheme, situations of corrosion index of below - 10 should not be allowed. 22.5.3 In the case of systems where standard protective arrangments have been provided for isolating the ground faults instantaneously, due consideration can be given to this aspect in decid ing upon the size of the earthing conductor by giving due allowance to lower duration of the ground fault currents. 22.6 Correlation Between Earth Fault Protection
22.5 Design of Earth Bus
22.5.2 Bare copper, PVC covered aluminium or Gl subject to relevant restrictions based on the location and nature of installation may be used as earthing to conductors. The size of the earthing conductors will be calculated according to guidelines given in the code. The time dura tion of the fault current as recommended is 3 seconds. According to standards developed in this regard, the size of the earthing conductors will be based upon current densities as given in Section 2 of this code. A corrosion factor of 5 percent of unit drop in the value of corrosion
and
22.6.1 The phase fault protective device normally used in systems operating at 415 V afford reasonable protection against arcing ground faults. The ground fault current depends upon the impedance to zero sequence current flows and depends to a large extent on the grounding network and the earth resistivity. The pick up value of the ground fault relays or the value of the phase fault protective device should be co ordinated for the required protection for the system. In case the impedance of the earth return path for ground fault current cannot be regulated so as to produce adequte fault current for operat ing the phase fault protective devices like fuses, such circuits should be protected by separate ground fault protective devices. Hence, the nece ssity of separate ground fault protection depends on the grounding network and its effective impe dance and earth grid design is closely related to the effectiveness of the phase fault protective device in clearing a ground fault in place where separate ground fault protective devices are not provided.
22.4 Determination of Earth Resistivity — As has already been pointed out, the value of the earth resistivity plays an important role in the design of the earth electrodes. In the conven tional method, the earth resistivity which is to be applied in the design calculations is taken as the arithmetic mean of a number of measured values in the area under consideration. The figure so obtained seldom projects a realistic value. A more scientific approach is to measure the earth resis tivity in different radial directions from a central point which may be taken as the proposed load centre. With the values so obtained, a polar curve is drawn. The polar curve is converted to an equivalent circle (see 36.6). The radius of the circle is taken to be the average value of the earth resistivity figure which is to be applied in design calculations. Necessary allowance should, of course, be given for factors such as variations in climatic conditions, treatment of soil, etc
22.5.1 Design of earth bus is based upon the general guidelines given in Seetion 2. The size of the main earth grid will be decided on the basts of line to ground fault current assumed to be symmetrical short-circuit current in the system. This assumption is fairly reasonable in the case of a solidly earthed system where the ratio between XO/XI is limited to less than 3 and the ohmic value of the earth return path to the supply neutral is reasonably low. The minimum fault level existing at the supply point will be assumed to be 131 kA or the actual fault current,whichever is greater for premises at voltages above 1 kV.
Grounding
22.7 Grounding tection
and
Ground
Fault Pro
22.7.1 In recent, years, there has been an increasing interest in the use of ground fault protection in industrial distribution circuits. This interest has been brought about by a disturbing number of electric failures. Hence it is worthwhile to explore the need for better ground fault pro tection and to examine the grounding practices in the light of the required protection. 22*7.2 Distribution circuits which are solidly grounded or grounded through low impedances require fast clearing of ground faults. This in volves high sensitivity in detecting low groud fault currents as wall as the co-ordination between main and feeder circuit protective devices. Fault clearing must be extremely fast where arcing is present. 22.7.3 The appeal of effective ground fault protection is based on the following: 1) The majority of electric faults involve ground. Ungrounded systems are also sub-
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