Is 3043 pdf download

Such joints should be protected by a heavy contamination of the domestic water supply should coat of bitumen.

Follow Blog via Email Enter your email address to follow this blog and receive notifications of new posts by email. In any position, subject to corrosion, b Single Point Bonding — This method is as the finished joint should be protected by described in Additional protection against Additional to P1 or P2: September 16, at. These should be interlocked with operation with an incoming mains supply — When the the incoming mains supply circuit-breaker so that generating sets have direct or resistance earthing they are open during parallel operation of the set and are used as standby to the mains, earthing with the mains, but one is closed at all other contactors are needed if parallel running is a times see Fig.

NOTE 1 — These ratings correspond to those of fuse-bases. Soil Resistivity and Corrosion Range of Soil Resistivity Class of Soil ohm-metres Less than 25 Severely corrosive Moderately corrosive Mildly corrosive Above Very mildly corrosive This following methods can be adopted to safeguard Conductor against excessive corrosion: August 31, at 6: July 2, at 8: For interconnecting switchboards protected through the earth return path.

The main purpose of this The materials used for making connections have to be compatible with the earth rod and the copper earthing conductor so that galvanic corrosion is minimized. Jignesh Parmar Jignesh Parmar has completed M. Similar motors, DBs wiring conduits shall have earthing GI wires of appropriate cross section. In laying out the earth path in relation to the earth resistivity.

The readings are taken as uniform. The rated current of the outgoing or incoming circuits shall be as follows: Warning systems should be provided to give an indication of leakage to Earthing conductors installed for that purpose frame.

Skip to main content. A bayonet lamp holder complying with IS: This will not affect performance minium and copper should be of the bolted since the sections are relatively large.

The earth rod shall be placed at 1. If an imported fill exercise has been carried out, the conditions of the upper layers may be altered considerably. In these cases, deeper driving of the electrode may be necessary to reach layers of reasonable resistivity and also to reach stable ground, such that the value of the electrode resistance remains stable if the top layers of the ground dry out.

The temperature coefficient of resistivity for soil is negative, but is negligible for temperatures above freezing point. At about 20C, the resistivity change is about 9 percent per degree Celsius. Below 0C the water in the soil begins to freeze and introduces a tremendous increase in the temperature,coefficient, so that as the temperature becomes lower the resistivity rises enormously. It is, therefore, recommended that in areas where the temperature is expected to be quite low, the earth electrodes should be installed well below the frost line.

Where winter seasons are severe, this may be about 2 metres below the surface, whereas in mild climates the frost may penetrate only a few centimetres or perhaps the ground may not freeze at all. Earth electrodes which are not driven below the first depth may have a very great variation in resistance throughout the seasons of the year.

Even when driven below the frost line, there is some variation, because the upper soil, when tance -. Ecological considerations are inherent before such treatment is commenced and any deleterious effect upon electrode material has to be taken into account.

However, for some temporary electrical installations in areas of high ground resistivity, this may be the most economic method for obtaining satisfactory earth contact over a short period of working; If a greater degree of permanenct: is envisaged, earth electrodes packaged in material such as bentonite are preferable. Bentonite or similar material may be used to advantage in rocky terrain. Where holes are bored Approximately 90 percent of the resistance between a driven rod and earth lies within a radius of about two metres from the rod.

This should be kept in mind when applying the agents for artificial treatment of soil. The simplest application is by excavating a shallow basin around the top of the rod, one metre in diameter and about 30 cm deep, and applying the artificial agent in this basin. The basin should subsequently be filled several times with water, which should be allowed each time to soak into the ground, thus carrying the artificial treatment, in electrolyte form, to considerable depths and allowing the artificial agent to become diffused throughout the greater part of the effective cylinder of earth surrounding the driven rod.

This condition arises in installations involving soils of high resistivity. The alternative is to reduce the resistivity of the soil immediately surrounding the earth electrode.

To reduce the soil resistivity, it is necessary to dissolve in the moisture, normally contained in the soil, some substance which is highly conductive in its water solution. The most commonly used substances are sodium chloride NaCl , also known as common salt, calcium chloride CaCls , sodiumcarbonate NasCOs , copper sulphate CuSO, , salt, and soft coke, and salt and charcoal in suitable proportions.

IS r'A the other two, thus a pipe, rod or strip has a much lower resistance than a plate of equal surface area. The resistance is not, however, inversely proportional to the surface area of the electrode. The salt content is expressed in percent by weight of the contained moisture. It will be noted that the curve flattens off at about 5 percent salt content and a further increase in salt gives but little decrease in the soil resistivity. The effect of salt will be different for different kinds of so11 and for various moisture contents but the curve will convey an idea of how the soil conductivity can be improved.

Decreasing the soil resistivity- causes a corresponding decrease in the resistance of a driven earth electrode. However, it is recommended that annual or biannual measurements of earth resistivity should be made to find out if additional treatment is npeded.

The possible contamination of the domestic water supply should also be considered. This requirement is met by making the dimensions in one direction large compared with those in. Where the resistance of a single plate is higher than the required value, two or more plates may be used in parallel and the total resistance is than inversely proportional to the number employed, provided that each plate is installed outside the resistance area of any other.

This normally requires a separation of about 10 m but for sizes of plate generally employed, a separation of 2 m is sufficient to ensure that the total resistance will not exceed the value obtained from the above formula by more than 20 percent. Even at the latter spacmore economical to use two ing, it is generally plates in parallel, each of a given size, than one of twice that size.

The size employed is, therefore, normally not greater than l-2 x l-2 m. Plate electrodes shall be of the size at least 60 cm x 60 cm. Plates are generally of cast iron not less than 12 mm thick and preferably ribbed. The earth connection should be joined to the plate at not less than two separate points. Plate electrodes, when made of GI or steel, shall be not less than 6. Plate electrodes of Cu shall be not less than 3. Suitable methods of ointing are a taper pin driven into a reamed ho 1e and riveted over or a copper stud screwed into a tapped hole and riveted.

Such joints should be protected by a heavy coat of bitumen. The connection between the earth plate and the disconnecting link should be set vertically and the depth of setting should be such as to ensure that the surrounding soil is always damp. The minimum cover should be mm except that where the underlying stratum is solid, for example, chalk or sandstone and near the surface, the top of the plate should be level with the top of the solid stratum.

Sufficient solid stratum should be removed and replaced with fine soil or other suitable infill to ensure as low a resistance as possible. The use of coke breeze as an infill is not recommended as it may result in rapid corrosion not only of the electrode itself but also of cable sheaths, etc, to which it may be bonded. Driven rods generally consist of round copper, steel-cored copper or galvanized steel see 9.

For conventional sizes, the resistance is approximately inversely proportional to the linear dimensions, not the surface area, that is a O-9 m x O-9 m plate would have a resistance approximately 25 percent higher than a l-2 x 1. The current. Plate electrodes shall be buried such that its top edge is at a depth not less than 1 5 m from the surface of the ground. However, the depth at which plates are set should be such as to ensure that the surrounding soil is always damp.

Where the underlying stratum is solid, for example chalk or sandstone and near the surface, the top of the plate should be approximately level with the top of the solid stratum. The curves of Fig. Chlnge of diameter has a relatively minor effect and size of pipe is generally governed by resistance to bending or splitting. It is apparent that the resistance diminishes rapidly with the first few feet of driving, but less so at depths greater than 2 to 3 m in soil of uniform resistivity.

A number of rods or pipes may be connected in parallel and the resistance is then practically proportional to the reciprocal of the number employed so long as each is situated outside the resistance area of any other.

In practice, this is satisfied by a mutual separation equal to the driven depth. Little is to be gained by separation beyond twice the driven depth. A substantial gain is effected even at 2 m separation. Pipes may be of cast iron of not less than mm diameter, to 3 m long and 13 mm thick. Such pipes cannot be driven satisfactorily and may, therefore, be more expensive to instal than plates for the same effective area. Alternatively, mild steel water-pipes of 38 to 50 mm diameter are sometimes employed.

These can be driven but are less durable than copper rods. Cruciform and star shaped sections are also available and are more rigid while being driven, but the apparent additional surface does not confer a noticeable advantage in current-carrying capacity or reduction of resistance.

In circumstances where it is convenient to do so, the addition of radial strips will be advantageous. Such rods may be coupled together to give longer lengths. Except in special conditions, a number of rods in parallel are to be preferred to a single long rod. Deeply driven rods are, howver, effective where the soil resistivity decreases with depth or where substrata of low resistivity occur at depths greater than those with rods, for economic reasons, are normally driven.

In such cases the decrease of resistance with depth of driving pay be very considerable as is shown by the measurements plotted in Fig. The rapid reduction in resistance, when the electrodes penetrated the latter, was very marked, The mean resistivity up to a depth of 8 m in one case was Qm; at 11 m the mean value for the whole depth was 20 R m moving to the low resistivity of the clay stratum. Similarly for curve C, the transition from gravely soil to clayey at a depth of about l-5 m was very effective.

In the case of curve B, however, no such marked effect occurred, although there was a gradual. Other factors that affect a decision whether to drive deep electrodes or to employ several rods or pipes in parallel are the steep rise in the energy required to drive them with increase in depth and the cost of couplings. The former can be offset by reducing the diameter of the rods, since a 13 mm diameter rod can be driven to considerable depths without deformation or bending if the technique of using a large number of comparatively light blows is adopted rather than a smaller number of blows with a sledge hammer.

Power-driven hammers suitable for this purpose are available. If round conductors are used as earth electrodes, their crosssectional area shall not be less than the sizes recommended for strip electrodes.

The resistance R is given by:. In cases where impenetrable atrata or highresistivity soil occur at relatively small depths, considerable advantage may result from driving rods at an angle of about 30 to the horizontal, thus increasing the length installed for a given depth. Care should be taken in positioning these electrodes, especially to avoid damage by agricultural operations.

Figure 13 shows the variation of calculated earth-resistance of strip or conductor electrodes. IS with length for a soil resistivity of a. The effect of conductor size and depth over the range normally used is very small,. If several strip electrodes are required for connection in parallel in order to reduce the resistance, they may be Snstalled in parallel lines or they may radiate from a point. In the former case, the resistance of two strips at a separation of 24 m is less than 65 percent of the individual resistance of either of them.

For existing installations in which a water pipe is used as a sole earth electrode; an independent means of earthing should be provided at the firs1 practicable opportunity. In the majority of cases, the resistance to earth of such a system is less than I Q.

A freshly installed jute or hessian served cable is insulated from earth, but the insulation resistance of the jute deteriorates according to the moisture content and nature of the soil.

However, cable sheaths are more commonly used to provide a metallic path to the fault current returning to the neutral. For this reason, it is essential to measure the resistance to earth of any structural steetwork that it is employing and at frequent intervals thereafter. NOTE - In urban districts and other areas where piped water supply is available the use of water pipes for consumers earth electrodes has been common in the past.

Though this was generally very effecttve when consumers pipes and water-mains to which they were connected were all metal-to-metaljoints, the use of public water-pipes for this purpose has not been acceptable for many yeara because of the use of nonconducting material for pipes on new installations and for replacement purposes.

Jointing techniques now being used do not ensure electrical continuity of metallic pipes. The earth strap should be bonded to a minimum of four piles and all the piles between the bonds should be bonded together. Each set of four piles should be connected to the mai nngearthi-strap of the substation. The electrolyte is generally the ground in which the structure is either wholly or partially buried and the protection system relies upon maintaining the metalwork at a slightly more negative potential than it would exhibit by half cell measurements, if no corrective action had been taken.

For new installations, therefore, a public waterpipe may not be used as a means of earthing. Metallic pipe systems of services other than water service for example, for flammable liquids or gases, heating systems, etc shall not be used as earth electrodes for protective purposes. Bonding of the water service with the exposed metalwork of the electrical installation on the consumers side of any insulating insert and any other extrametalwork to the installation earthing neous terminal is, however, permissible and indeed ne-.

The application of cathodic protection varies according to circumstances between bare metal in contact with ground and metal that has been IS : - deliberately coated or wrapped against corrosion. In the latter case, cathodic protection is used to supplement the coating and guard against localized corrosion due to coating flaws or faults. Protective system current drain is proportional to the area of bare metal in earth contact and if a normal earthing electrode is attached to a cathodically protected structure, the increased drain current taken by the electrode could be completely unacceptable.

This is especially true where the system has been designed to protect a well wrapped or coated structure. Nevertheless, there may be a necessity to connect earth electrodes to cathodically protected structures, especially where the coating or wrapping tends to electrically insulate the structure from ground, for example:.

Prevention of power surges into the apparatus providing cathodic protection, or similar invasion of delicate low current instrumentation circuits. In addition to the guidance given in 9. The material selected should exhibit a galvanic potential with respect to ground as nearly equal to that exhibited by the structure in its natural or unprotected condition. For ferrous structures, austenitic iron austenitic cast nickel chromium alloy with spheroidal graphite present is often.

Vertically driven rods of this material are preferred in order to minimize contact area and drain, whilst thus reduce cathodic protection obtaining optimum performance frorh the electrode.

Copper should be avoided, wherever possible, not only for its increased drain but also for its ability tobecome cathodic to the protected structure. Magnesium or zinc electrodes have been used successfully, but are anodic to the protected structure and thus sacrificial in action. Tests in a wide variety of soils have shown that copper, whether Corresponding average losses for unprotected ferrous specimens for example, cast iron, wrought iron or mild steel used in the tests were as high as 2.

Considerable and apparently permanent protection appears to be given to mild steel by galvanizing, the test showing galvanized mild steel to be little inferior to copper with an average loss not greater than 0.

Only in a few cases was there any indication in all these tests that corrosion was accelerating and in these cases the indications were not very significant. The possibility on damage to cables and other underground services and structural metalwork in the vicinity of earth-electrode due to electrolytic action between dissimilar materials should not be overlooked when the material for earth-electrodes is selected. Materials compatible with other metal structures in the vicinity should be selected or other remedial action taken.

It may be essential to use materials of types other than those mentioned earlier in special circumstances, when cathodically protected structures such as pipelines are encountered. A modern high pressure gas pipeline, wrapped and cathodically protected may have a galvanic potential of - 0.

An earth electrode with a galvanic potential nearer to the protected structure has; to be used to overcome the above and be certain the pipeline is Such a material is termed an being protected. It may be necessary to earth the pipeline one or more of the following reasons: a. It may have ,instrumentation connected to it that require it to be earthed for this p,urpose and to provide a signal reference earth as well as for earthing requirement relative to electrical equipment used in hazardous areas; and.

ISr d It may require connection to earth at points to discharge unwanted induced currents and voltages from other sources sue h as overhead lines.

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 against the corrosion protection requirement to have a minimum of. NOTE - After laying the earth from the earth bus to entry conduits should be sealed with bitumin compound. Failure is fundamentally due to excessive temperature rise at the surface of the electrode and is thus a function of current density and duration as well as electrical and thermal properties of the soil.

In general, soils have a negative temperature coefficient of resistance so that sustained current loading results in an initial decrease in electrode resistance and a consequent rise in the earth fault current for a given applied voltage.

As soil moisture is driven away from the soil-electrode interface, however, the resistance increases and will ultimately become infinite if the temperature-rise is sufficient. Limitation to values below this would generally be imposed by the necessity to secure a low-resistance earth. Time to failure on short-time overload is inversely proportional to the specific loading, which is given by is, where i is the current density at the electrode surface. This results in the existence of voltages in the soil around the electrode that may be injurious to telephone and pilot cables, whose cores are substantially at earth potentional, owing to the voltage to which the sheaths of such cables are raised; the voltage gradient at the surface of the ground may also constitute a danger to life, especially where cattle are concerned.

The former risk arises mainly inconnection with large electrode systems as at power stations and substations. 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 and in such cases, animals, which frequently congregate near a pole, are liable to receive a dangerous shock.

The same trouble sometimes occurs at farms where earth electrodes are provided for individual appliances. An effective remedy is to earth the neutral conductor at some point on the system inaccessible to animals rather than earthing the neutral at the transformer itself.

Alternatively, an effective method is for pipe or rod electrodes to be buried with their tops below the surface of the soil and connection made to them by means of insulated leads. The maximum voltage gradient over a span of 2 m adjacent to a 25 mm diameter pipe electrode is reduced from 85 percent of the total electrode potential when the top of the electrode is at ground level to 20 and 5 percent when it is buried O-3 and 1. In all cases, the connections have to be mechanically strong.

This is Simpler disconnecting arrangements or none at all may be acceptable for small earthing installations. According to Means shall be provided in an accessible position for disconnecting the earthing conductor. Such means may conveniently he combined with the earthing terminal or bar to permit measurement of the resistance of the earthing arrangements This joint shall be disconnectable only by means of a tool, mechanically strong and ensure the maintenance of electrical continuity.

Other conduits for electrical purposes shall not be used as a protective conductor. Where a clamp is used, it shall not damage the electrode for example, a pipe or the earthing conductor.

NOTE I- Where the metal sheaths ofcables are used, as earth continuity conductors, every joint in such sheaths shall be so made that its current carrying capacity is not less than that of the sheath itself.

Where necessary, they shall be protected against corrosion. Where non-metallic joint-boxes are used, means shall be provided to maintain the continuity such as a metal strip having a resistance not greater than that of the sheath of-the largest cable entering the box. NOTE 2 - Metal conduit pipe should generally not be used as an earth-continuity conductor but where used, a very high standard of workmanship in installation is essential.

Slackness in joints may result in deterioration and even complete loss of continuity. Plain slip or pin-grip lockets are insufcient to ensure satisfactory continuity of joints. In the case of screwed conduit, lock nuts should aIso be used. Gas pipes be used as protective conductors. The k factors for protective conductors of copper, steel and aluminium are shown in Fig. If application of the formula produces nonstandard sizes, conductors of the nearest higher standard cross-sectional area shall be used.

NOTE 1 -It is necessary that the cross-sectional area so calculated be compatible with the conditions imposed by fault loop impedance. NOTE2 - Maximum permissible temperatures for.

NOTE- Account should be taken of the current-limiting effect of the circuit impe dances and the limiting capability joule integral of the protective device. Values of k. In checking of compliance with If the application of this table produces nonstandard sizes, conductors having the nearest higher standard cross-sectianal area are to be used.

IS I - The values in Table 7 are valid only if the protective conductor is made ofthe same metal as the phase conductors. If this is not so, the crosssectional area of the protective conductor is to be determined in a manner which produces a conductance equivalent to that which results from the application of Table 7 see also NOTE - This requirement is necessary to prevent inadvertently the voltage-sensitive element being bridged.

However, the minimum cross-sectional area of a PEN conductor may be 4 mma, provided that the cable is of a concentric type conforming to Indian Standards and that duplicate continuity connections exist at all joints and terminations in the run of the concentric conductors.

This requirement is considered to be fulfilled if the. IS I - At the point of separation, separate terminals or bars shall be provided for the protective and neutral conductors. The PEN conductor shall be connected to the terminal or bare intended for the protective conductor. This protective measure necessitates coordination of the types of system earthing and the characteristics of the protective devices.

This Section discusses the basic criteria for achieving this protection. One of such measures is protection by automatic disconnection of supply. The assuming the total fault current to be passing thermal rating of the earth electrodes are then the earth electrodes. It includes ensure that operators and attendants shall be at specific guidelines on earthing system design to earth potential at all times.

Skip to main content. N O T E 2 — Maximum permissible temperatures for joints should be taken into account. Rating of Individual Circuits: The former risk arises mainly in connection with large elect rode systems as at power stations and substation. T h e presence of stray currents in the soil is indicated by a wandering of the tivity thus obtained may be used for the design of instrument pointer, but an increase or decrease of the earthing grid and other computations and the generator handle speed will cause this to disappear.

Thus, if these values are not exceeded, compliance with this code covering In TT systems, there is an earhing option automatic disconnection in case of an earth fault of the use of fault voltage operated protective is assured. A difference of a few percent moisture will vel, as in general no advantage results from an therefore, make a earthiing marked difference in the increase in moisture content above about 15 to 20 effectiveness of earth connection if the moisture percent.

The generally accepted correlation between the electrical resistivity of soil and its corrosivity is as indicated in the table below: According to standards developed in this However, in the to the general mass of earth from the low voltage case of a protective multiple earthing P M E energy source has to be consistent with the earth supply, three- or four-pole switching may be used.

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In the former case, the practicable opportunity. A freshly installed jute or hessian served cable is insulated from earth, but the insulation resistance of the jute deteriorates according to the moisture content and nature of the soil. However, cable sheaths arc more commonly used to provide a metallic path to the fault current returning to the neutral.

The earth strap should be bonded to a minimum of four piles and N O T E — In urban districts a n d other areas where all the piles between the bonds should be bonded piped water supply is available the use of water pipes together.

J o i n t i n g techniques now being at a metal to electrolyte interface. The electrolyte is generally the ground in For new installations, therefore, a public water- which the structure is either wholly or partially pipe may not be used as a means of earthing.

Bonding been taken. This is especially true where the system appears to be given to mild steel by galvanizing, has been designed to protect a well wrapped or the test showing galvanized mild steel to be little coated structure. Materials compatible with other metal are substantially parallel to the route of a structures in the vicinity should be selected or high voltage overhead line; other remedial action taken. A modern high pressure gas pipeline, wrapped and cathodically protected may have a galvanic In addition to the guidance given in 9.

The material selected should exhibit a galvanic potential with respect to ground as nearly equal An earth electrode with a galvanic potential to that exhibited by the structure in its natural nearer to the protected structure has to be used to or unprotected condition.

For ferrous structures, overcome the above and be certain the pipeline is austenitic iron austenitic cast nickel chromium being protected. Such a material is termed an alloy with spheroidal graphite present is often austenitic iron and is an austenitic cast nickel- used. Vertically driven rods of this material are chromium alloy, with spheroidal graphite present.

NOTE — After laying the earth from the earth bus to the electrode through the PVG conduits at the pit entry conduits should be sealed with bitumin compound. This results in the existence of under any condition of operation on the system. Alternatively, an effective suppression coils.

Limitation to values stalled in proximity to a metal fence, to avoid the below this would generally be imposed by possibility of the fence becoming live and thus the necessity to secure a low-resistance dangerous at points remote from the substation or earth. Simpler disconnecting arrangements such sheaths shall be so made that its current o r none at all may be acceptable for small carrying capacity is not less than that of the sheath itself.

Where necessary, they shall be protected earthing installations. Where non-metallic joint-boxes are used, means shall be provided to maintain the In the case of screwed conduit, lock nuts against with a minimum of 16 mm Fe should also be used. Other conduits for every installation, a main earthing terminal or bar electrical purposes shall not be used as a protective shall be provided and the following conductors conductor.

Gas pipes enclosure with live conductors; shall not be used as protective conductors. N O T E 2 — Maximum permissible temperatures for joints should be taken into account. N O T E 3 — Values for mineral-insulated cables are where under consideration.

These material constants are given in Table 5. Indian Standards and that duplicate continuity The PEN conductor shall be Table 8 shows the values of disconnecting times t for given touch voltages for two most The disconnecting times relies on the association of two conditions given specified for different circuits in this code follows below: basically the summary in Table 8, in addition a The existence of a conducting path falt taking into account the likelihood of faults and loop to provide for circulation of fault likelihood of contact.

The protective conductors shall be earthed near each The determination of this time depends on power transformer or generator of the installation. Earthing at additional points as evenly as possible Limits of touch voltage are based on studies is desirable.

NOTE 1 — Zs may be calculated or measured. Firstly, those parts. Where several protective devices are voltages appear between exposed conductive parts used in series, this requirement applies separately and extraneous conductive parts, and if these to all the exposed conductive parts protected by parts are simultaneously accessible, these voltages each device.

Protective current to low to cause an overcurrent measures must, however, prevent danger on the protective device for example, a fuse or occurrence of two simultaneous faults involving circuit breaker in the faulty circuit to different live conductors.

The voltages referred to e Electrical separation; and earlier see The most important requirement, current-using equipment in that building are however, is that live parts and exposed conductive parts connected. Item b can also be achieved by the application of suitable supplementary or reinforced They all include b other service pipes and ducting; a high degree of isolation from earth.

If the earthing conductor is should also be earthed at one point of the of tape or strip, the thickness should be adequate winding, unless the transformer is a safety to withstand mechanical damage and corrosion.

Typical examples of such parts are minimum cross-sectional area is 16 mm2 if the screws and nameplate, cable clips and lamp caps. However, if a clamp is used for The equipotential zone partially ereated by t h e this connection the clamp should be so designed bonding of extraneous conductive parts to the and installed as to provide reliable connection main earthing terminal depends for its efficacy on without damage to the cable.

A separately run circuit Copper and required to be insulated. Where the alloys. Particular exposed metal pipes, sinks taps, tanks, radiators, care should be taken to avoid problems with non- and where practicable and accessible, structural conducting finishes. Joints in all conduit systems cuit within the same time in case of impedance or should be painted overall after assembly.

The device setting should These precautions should be adequate, but be interlinked with earth fault loop impedance, periodical tests should be made to verify that safe contact potential and permissible time for electrical continuity is satisfactorily maintained.

It should also be remembered that every attained by the conductors do not exceed their socket outlet circuit that do not have earthing prescribed limiting values. The different systems are supplied by means of a socket outlet not having described in Fig. It should be emphsized that an installation By requirement is that any voltage occurring between and large, the types of system encountered fall in simultaneously accessible conductive parts during one or other categories shown in Fig.

If tude will depend on the impedance of that part of the earth fault loop impedance is low enough to the earth fault loop path that lies between the cause these devices to operate within the specified simultaneously accessible parts. Hence protective relay or deviecs to give automatic disconnection of the device characteristic should be such that supply under earth fault conditions, the first option this 65 volts contact potential should be is to reduce that impedance.

There are practical limitations to both a human body for about milli seconds, approaches. This is to use residual current ture circuit breaker that will meet the criteria can devices with appropriate settings to clear the faults be calculated on the basis of a nominal voltage to within the permissible time, based on the probable earth U o and the time current characteristics contact potential.

Thus, if these values are not exceeded, compliance with this code covering In TT systems, there is an additional option automatic disconnection in case of an earth fault of the use of fault voltage operated protective is assured. This denotes the dual components and is, therefore, quick and maximum fault current for which a breaker will direct in application.

Its simplicity does exclude give thermal protection but it will generally be some circuit arrangements that could give the found in practice that this value is higher than the required protection. 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. Details of the maximum permissible earth loop 21A Fuses impedance for the thermal protection of cables by fuses can also be computed.

However, the time current characteristics of a miniature circuit breaker are such that if the loop impedance is low enough to give automatic disconnection within safe disconnecting time so providing shock risk protection, it will also give the necessary thermal protection to the earth conductor likely to be used with a breaker of that specific rating.

Figure 21 shows the relationship between the adiabatic line and the characteristic of fuses and miniature circuit breaker. In order that the devices will give thermal protection to the protective conductor, operation has to be restricted to the area to the right of point A where these curves cross.

For lower part of the earthing circuit. Devices for load b Fault Voltage Operated Earth Leakage Circuit currents below A usually include the Breakers E L C B — A voltage operated transformer and contact system within the earth leakage circuit breaker comprises a same single unit, which is then described as contact switching system together with a a residual current operated circuit breaker voltage sensitive trip coil.

On installations, RCB. Any voltage rise above earth on that metalwork A wide choice of operating currents is exceeding the setting of the coil will cause avilable typical values are between 10 mA the breaker to trip so giving indirect shock and 20 A RCB's are normally non- risk protection.

Devices suitable for time conductor from the circuit breaker to the grading are more likely to be of the solid earth electrode. It can not prove other state form as are those having higher features of the installation. Whilst the voltage operated E L C B will A test device is incorporated to allow the operate when subjected to a fault voltage operation of the RCD to be checked.

It is not age principle are of little use, since milli- always possible to introduce time grading to give amperes of earth leakage current for an discrimination whereas a limited amount of current extensive industrial system is a normal discrimination can be obtained by grading the operating situation. Tripping based on sensitivities along the distribution chain.

Milliamperes of current The maximum permitted operating current in a system, where exposed conductive depends on the earth fault loop impedance. This will in no way contribute to It is often acceptable on commercial grounds shock or fire hazard. Here objectionable to have several final circuits protected by the same fault currents will be a few or a few tenths residual current devices. This, however, does of amperes. In such cases, residual current result in several circuits being affected if a fault operated devices sensitive to these currents occurs on one of the circuits so protected and the must be made use of for earth fault current financial advantages have to be weighed against and stable operation of the plant without the effects of loosing more than one circuit.

Such an earth connection together with the Consequently, a portion of groups according to their final current operating the neutral load current will be shunted away characteristics. In general, therefore, care should be metal. It should be on the circuit they are protecting. However, this isolation is not always give fire risk protection, the degree of protection practicable and the presence of a second parallel being related to the sensitivity of the device.

Table 9 shows the maximum earth electrode impedance with switch Voltage Operated 30 mA 1 different types of breaker may be used. Insulation to this level is rarely practicable. There is advantage in using a the substation, for example, plant with connections common earth where the earth electrode resistance, to pilot or telephone cables or cable sheaths. Where surge protection is provided, stations supplying a network of cables whose the connection of the protective devices to earth sheaths have a low impedance to earth.

The discharge of high currents with high-frequency components The substation earth system rise of potential requires earth connections of low resistance and will not be excessive if the resistance of the earth reactance, that is, short connections with as few electrode system is small compared to the total changes of direction as possible.

A typical earthing supplies within the substation; arrangement for connecting the reinforcement of b substations that provide an external low foundations of substation building and switchyard voltage supply; and RCC masts is shown in Fig.

NOTE — The number of electrodes and the size of the grid conductor is to be worked out as per NOTE 2 — Two extreme columns should be earthed like this in each substation. NOTE 4 — Inserts other than earthing pads may or may not be welded to reinforcement. In audition, separately from the main station earth electrode an earth mat should be provided, near the ground system see 2 0. T h e mat should be should be connected to the transformer tank by a electrically separated from the main electrode; this connection of sufficient cross-sectional area to carry is considered to be achieved by spacing the the primary short-circuit current.

T h e In the case of pole mounted transformers on tops of the main electrodes should be at least overhead line systems, difficulties may arise in mm and preferably mm below the ground, areas of high soil resistivity. This cable, trode systems may be required. That for the neutral between the bottom of the pole and the electrode of the low voltage system is usually provided not should be laid in a mm diameter earthenware nearer than one pole span away on the low voltage duct filled solid with bitumen.

Method b — can be used to achieve the same A tion than that of the equipment used in method common earth is used for the neutral earthing of a. If desired, the generator could remain in Electrodes circuit while operational arrangements were made to permit its withdrawal.

However, this imposed a The a a suitably low resistance, under all variations output from the secondary winding of the voltage due to climatic conditions, for the fault transformer could be arranged to activate an currents envisaged; alarm or trip the generator circuit as desired. Installations of neutral point ble and of such material and design to switchboards with switching of neutral points and avoid corrosions. The most efficient disposition of earthing between dissimilar metals s e e 2 3.

Except for the smallest installations, to provide an effective electrode system, without there should be a connection to the earth necessity to provide further buried electrodes. These Connections may, least four points.

When cables are so connected, the The main recommended. As an alternative to plates, cast iron devices provided with the switchgear. The frame bar although this is not always achievable. Closer interconnecting the framework of the switch spacing reduces their effectiveness. T h e true earth stratas but they may not be suitable if the site is bar should be run separately from the frame stony or has a rock sub-strata. Where it is mounted on the switch be relied upon to form an efficient earth units, it should be insulated therefrom by bond between equipment and the main insulation capable of withstanding a test earth grid, and loops bonding across voltage of 4 kV rms alternating current structural joints are required.

Wherever possible, this should be earth system and the base of the support. For larger stations, the ring should be kept well clear of the latter. The aim should be to provide a mesh earthing system since, for the effective system wherever this can be contrived with protection of the substation equipment, a reasonable economy.

That part of the grid which ture. While sites Table 6A. The generally accepted duration for connection and there is no need to fix an design purposes are one second for voltages above earth conductor along this section. It is recommended that the duration of earth fault current should be taken as one second for and kV substations, and 3 seconds while designing All lengths in metres earth grids for all other voltage levels.