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  1. [21]
    alhakimi
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  2. [22]
    محب الشرقية
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  3. [23]
    هشام محمد حامد
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    مهندس نور اليك مقال صغير لما طلبت عن الارضي و حساباته م/هشام حامد
    لو قابلتك اي مشكلة في فهم المعادلات الرجو الرسال لي
    DESIGN AND CALCULATION OF AN EARTH ELECTRODE
    J. Gomez
    Abstract
    People using electrical installations have to be protected against electrical shock. For purposes of
    protection a distinction is made between direct and indirect contact. Direct contact is contact with
    a live conductor. Protection is provided by the insulation of cables or the screening of live parts.
    An indirect contact happens when someone touches exposed metal parts which are not intended
    to carry current but have become live as a result of a fault. In this case metallic parts raise the
    metal to a dangerous potential (contact voltage). Here protection is provided by connecting the
    exposed metal part (i.e. the case of the electrical machine) to the earthing point of the installation.
    A protective device will disconnect the circuit as soon as a fault current flows to earth. The earth
    fault value will depend on the impedance of the path taken by the fault current, which is known
    as the earth fault loop. The resistance of the earth electrode plays an important role in the final
    impedance of the earth fault loop, especially when the neutral of the transformer is earthed
    without any impedance. A good earth electrode should have the lowest possible value compared
    with the rest of the earth loop (voltage divisor). If this can be achieved, the contact voltage will
    be limited and the current that trips the protections will be of a higher value. The maximum
    duration of a contact voltage is established according to IEC 60364-4 413.1.1.1. An important
    constraint in obtaining a good resistance value for the earth electrode is the resistivity of the soil.1 SOIL RESISTIVITY
    The resistivity of a given type of soil will vary by several orders of magnitude as a result of small changes
    in the moisture *******, salt concentration, and soil temperature. A broad variation of resistivity occurs as
    a function of soil types. Knowledge of exactly what type of soil we have at a given site will help to design
    in a precise manner the earth electrode.
    Table 1: Resistivity values of the earthing medium
    Medium Resistivity (•m)
    Minimun Average Maximun
    City, industrial area 1000 10 000
    Surface soil, loam 1 50
    Clay 2 100
    Sand and gravel 50 100
    Surface limestone 100 10 000
    Limestone 5 4000
    Shale 5 100
    Sandstone 20 2000
    Granite, basalt 10 000
    Decomposed gneiss 50 500
    Freshwater lake 200 200 000
    Sea water 20 100 200
    Pastoral, low hills, rich soil 30
    Marsh 2 100
    Pastoral, medium hills 200
    Fill, ash, cinder 6 25 70
    Rocky soil, steep hills 10 500 1000
    Clay, shale, gumbo 3 40 200
    Gravel, sandstone, with little clay or
    loam, granite
    500 1000 10 000
    Sandy, dry, typical coastal country 300 500 5000
    2 PRINCIPAL TYPES OF EARTH ELECTRODE (ADVANTAGES AND DISADVANTAGES)
    Type Advantages Disadvantages
    Ring ground Easy to design and to install
    (especially around an existing
    facility). Readily available. Can be
    extended to reach water table.
    Not useful where large rock
    formations are near surface.
    Horizontal bare wires (radials) Can achieve low resistance where
    rock formations prevent use of
    vertical rods.
    Subject to resistance fluctuations
    with soil drying.
    Horizontal grid (bare wire) Minimum surface potential
    gradient. Easy to install if done
    before construction. Can achieve
    low resistance contact in areas
    where rock formations prevent the
    use of vertical rods.
    Can be combined with vertical rods
    to stabilize resistance fluctuations.
    Subject to resistance fluctuations
    with soil drying if vertical rods not
    used.
    Vertical rods Simple design. Easiest to install
    particularly around an existing
    facility. Hardware readily available.
    Can be extended to reach water
    table.
    Not useful where large rock
    formations are near surface. Step
    voltage on earth surface can be
    excessive under high fault currents.
    Plates Can achieve low resistance contact
    in limited area.
    Most difficult to install.
    Incidental electrodes (pipes,
    foundations, buried tanks)
    Can show very low resistance. Little or no control over future
    alterations. Must be employed with
    other electrodes.
    3 CALCULATION OF EARTH RESISTANCE
    For this purpose a straightforward design based on graphs is proposed.
    - General points:
    x Choose the configuration of the earth electrode according to the shape of the facility.
    x Calculate the resistance to earth for the configuration.
    x Check if the calculated resistance meets the design goal.
    x Complete the design to include all necessary interconnections.
    - Steps to calculate the earth electrode:
    x Starting data: configuration, resistivity, length and diameter of rods.
    x Determine the resistance of one of the ground rods from Fig. 1. Start by drawing a line between
    the chosen diameter ‘d’ and the length ‘l’. Then indicate the crossing point in ‘q’. From this point
    place a line between ‘q’ and the resistivity and read the resistance at the point where the line
    crosses ‘R’.2 PRINCIPAL TYPES OF EARTH ELECTRODE (ADVANTAGES AND DISADVANTAGES)
    Type Advantages Disadvantages
    Ring ground Easy to design and to install
    (especially around an existing
    facility). Readily available. Can be
    extended to reach water table.
    Not useful where large rock
    formations are near surface.
    Horizontal bare wires (radials) Can achieve low resistance where
    rock formations prevent use of
    vertical rods.
    Subject to resistance fluctuations
    with soil drying.
    Horizontal grid (bare wire) Minimum surface potential
    gradient. Easy to install if done
    before construction. Can achieve
    low resistance contact in areas
    where rock formations prevent the
    use of vertical rods.
    Can be combined with vertical rods
    to stabilize resistance fluctuations.
    Subject to resistance fluctuations
    with soil drying if vertical rods not
    used.
    Vertical rods Simple design. Easiest to install
    particularly around an existing
    facility. Hardware readily available.
    Can be extended to reach water
    table.
    Not useful where large rock
    formations are near surface. Step
    voltage on earth surface can be
    excessive under high fault currents.
    Plates Can achieve low resistance contact
    in limited area.
    Most difficult to install.
    Incidental electrodes (pipes,
    foundations, buried tanks)
    Can show very low resistance. Little or no control over future
    alterations. Must be employed with
    other electrodes.
    3 CALCULATION OF EARTH RESISTANCE
    For this purpose a straightforward design based on graphs is proposed.
    - General points:
    x Choose the configuration of the earth electrode according to the shape of the facility.
    x Calculate the resistance to earth for the configuration.
    x Check if the calculated resistance meets the design goal.
    x Complete the design to include all necessary interconnections.
    - Steps to calculate the earth electrode:
    x Starting data: configuration, resistivity, length and diameter of rods.
    x Determine the resistance of one of the ground rods from Fig. 1. Start by drawing a line between
    the chosen diameter ‘d’ and the length ‘l’. Then indicate the crossing point in ‘q’. From this point
    place a line between ‘q’ and the resistivity and read the resistance at the point where the line
    crosses ‘R’.2 PRINCIPAL TYPES OF EARTH ELECTRODE (ADVANTAGES AND DISADVANTAGES)
    Type Advantages Disadvantages
    Ring ground Easy to design and to install
    (especially around an existing
    facility). Readily available. Can be
    extended to reach water table.
    Not useful where large rock
    formations are near surface.
    Horizontal bare wires (radials) Can achieve low resistance where
    rock formations prevent use of
    vertical rods.
    Subject to resistance fluctuations
    with soil drying.
    Horizontal grid (bare wire) Minimum surface potential
    gradient. Easy to install if done
    before construction. Can achieve
    low resistance contact in areas
    where rock formations prevent the
    use of vertical rods.
    Can be combined with vertical rods
    to stabilize resistance fluctuations.
    Subject to resistance fluctuations
    with soil drying if vertical rods not
    used.
    Vertical rods Simple design. Easiest to install
    particularly around an existing
    facility. Hardware readily available.
    Can be extended to reach water
    table.
    Not useful where large rock
    formations are near surface. Step
    voltage on earth surface can be
    excessive under high fault currents.
    Plates Can achieve low resistance contact
    in limited area.
    Most difficult to install.
    Incidental electrodes (pipes,
    foundations, buried tanks)
    Can show very low resistance. Little or no control over future
    alterations. Must be employed with
    other electrodes.
    3 CALCULATION OF EARTH RESISTANCE
    For this purpose a straightforward design based on graphs is proposed.
    - General points:
    x Choose the configuration of the earth electrode according to the shape of the facility.
    x Calculate the resistance to earth for the configuration.
    x Check if the calculated resistance meets the design goal.
    x Complete the design to include all necessary interconnections.
    - Steps to calculate the earth electrode:
    x Starting data: configuration, resistivity, length and diameter of rods.
    x Determine the resistance of one of the ground rods from Fig. 1. Start by drawing a line between
    the chosen diameter ‘d’ and the length ‘l’. Then indicate the crossing point in ‘q’. From this point
    place a line between ‘q’ and the resistivity and read the resistance at the point where the line
    crosses ‘R’.Figure 1: Nomograph for determining the resistance to earth of a single ground rod.
    x Because we are going to place more than one rod there will be a lower resistance which will be
    estimated according to Fig. 3.
    If the rods are distributed in a grid pattern, as will frequently be done for substations, use Fig. 2 to
    estimate the net resistance instead of Fig. 3.
    Figure 2: Graph of multiple rod resistance ratio.Conversion values:
    1 foot = 30.4 cm 1 inch = 2.54 cm
    Example:
    To illustrate this, let us assume that a 30 m u 48 m (100 ft u 160 ft) rectangular configuration like
    that shown in Fig. 3 is initially chosen. Assume that the soil resistivity measurements made during the site
    survey showed an average resistivity of 10 000 ohm-cm for the area. In addition, the site survey indicated
    that all rock formations are at depths greater than 10 feet; the water table (layer containing a big amount of
    flowing water) never drops more than 5 feet below grade; and the frost line extends only to 1 foot below
    ground level.
    Figure 3: Effective resistance of ground rods when arranged in a straight line or a large circle.
    Therefore, 3 m (10 ft) ground rods are initially selected for evaluation. (Minimum rod diameter
    14.8 mm or 3/4 inch.)
    First in Fig. 2 we place a straight edge between the point marked 3/4 on line ‘d’ and the point
    marked 10 ft on line ‘l’. Indicate on line ‘q’ where the straight edge crosses. Next, place the straight edge
    between the point just marked on ‘q’ and the 10 000 ohm-cm point on the vertical line marked as
    resistivity. Then read the resistance as 32 ohm-cm at the point where the straight edge crosses the vertical
    line tagged resistance.
    In order to prevent interactions between rods (placing rods too close increases the expected
    resistance) we assume an initial spacing of 7 m (20 m) or twice the rod length. Figure 1 shows that 26 rods
    are needed to encircle the building. Then we use Fig. 3 to determine the relative lowering of resistance of
    one rod that is produced by 26 rods in parallel. The answer is 5.5%.
    The resistance of the 26 rods in a 10 000 ohm-cm soil is:
    R = 32 x 0.055 = 1.76 ohms.Figure 3 applies to ground rods placed in a straight line or around the perimeter of a site whose
    dimensions are large with respect to the rod spacing. If the rods are distributed in a grid use Fig. 2.
    For this example, we have an area of 16 000 square feet (1450 m2) and 26 rods distributed in it, so
    the graph shows a multiplier of 0.056.
    R = 32 x 0.056 = 1.9 ohms.
    This calculation method does not consider the interconnecting cable between rods because the
    effect produced by the rod on the earth resistance is more important than the one produced by 7 m of bare
    cable. If the distance between rods is bigger then 7 m should be considered.
    Wire resistance should be considered in parallel with the rods.
    For this purpose we can use two formulas:
    - Buried straight rod or wire
    h = depth of burial (cm)
    L = total length (cm)
    r = ohm-cm
    R = 0.366 r/L (log (L/d) + log (L/4h) + 0.34) h < 0.4L
    - Buried circle of wire
    R = 0.366 r/L (log (L/d) + log (L/4h) + 0.81) h > 0L
    - Vertical rod
    R = 0.366 r/L (log (3L/d)
    When a vertical rod is driven through a high-resistivity superficial layer into a lower resistivity
    subsoil, an adjustment can be made to the resistance to earth expression by substituting a reduced
    ‘effective length’ of the ground rod. Lc will be the effective length.
    Lc = L – h (1 – (r2/r1)) where L is the length of the rod, r1 is the resistivity of the upper layer, r2 is
    the resistivity of the subsoil, and h is the depth of the upper layer.
    4 DESIGN GUIDELINES
    a) Where bedrock (meaning an underground layer formed by rocks) or other obstacles prevent the
    effective use of vertical rods, horizontal wires, grids, or radials should be used.
    b) When other alternatives are not possible or are not cost-effective, chemical enhancement (salting) is
    frequently the only choice left.
    c) The nominal spacing between rods should be between one and two times the length of the rod;
    however, it is necessary for a ground rod to be placed near each lightning down conductor, so
    spacings should be limited to not more than 15.2 m.
    d) The rods and interconnecting cable comprising the earth electrode subsystem should be positioned
    between 0.6 m and 1.8 m outside the dripline of the building or structure to insure that rain, snow, and
    other precipitation wets the earth around the rods.
    e) Where two or more structures or facilities are located in the same general area (less than 60 m away)
    and are electrically interconnected with signal, control, and monitor circuits, either provide a common
    earth electrode subsystem or interconnect the separate earth electrode subsystems with two buried
    bare cables.f) To minimize voltage differentials between the two structures, the facilities should effectively share a
    common earth electrode subsystem. Separate structures spaced closer than 6 m should have a common
    earth electrode subsystem installed that encircles both facilities.
    g) Structures or facilities having no interconnecting cables and separated by a distance greater than 60 m
    generally do not require their earth electrode subsystems to be interconnected.
    h) Metallic structures near the earth electrode subsystem should be connected to reduce the danger of
    potential differences during lightning or fault protection; their connection will also reduce the
    resistance to earth of the electrode subsystem.
    i) To minimize resistance variations caused by surface drying of the soil and by the freezing of the soil
    during winter, connections, interconnecting cables, and tops of ground rods should be buried at least
    0.3 m below grade level and the interconnecting cable at least 0.45 m below grade level.
    j) Access to the earth electrode subsystem should be provided through the installation of one or more
    grounding wells at each site. Removable access covers must be provided.
    k) More than one grounding well may be necessary depending upon the size of the facility, the extent of
    the electrode subsystem, and the degree of accessibility to the electrodes.
    l) Locate at least one of the ground wells in an area with access to open soil so that resistance checks of
    the earth electrode subsystem can be made once the building is in use.
    BIBLIOGRAPHY
    [1] Grounding, bonding, and shielding for electronic equipments and facilities – US Department of
    Defense handbook.
    [2] Grounding principle and practices II – Establishing grounds, Jensen Claude.
    [3] Guide de protections des reseaux industriels, Christophe Preve.
    [4] Puesta a tierra de equipos eléctricos, Francisco Ruiz Vasallo.
    باقي الاشكال التوضحية لم يمكنني ارسالها اسف لذلك
    ارجو الرد سريع

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  4. [24]
    سحر فولت
    سحر فولت غير متواجد حالياً
    عضو


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    انا عندى تقرير عن حساب مقاومة التأريض وسوف اوفيكم به

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  5. [25]
    المهندس أبو إياد
    المهندس أبو إياد غير متواجد حالياً
    جديد


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    جزاك الله خيرا ماجد محمد فرج

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  6. [26]
    شعبان ابراهيم
    شعبان ابراهيم غير متواجد حالياً
    جديد


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    اشكرك اخى العزيز وارجو موافتنا بكل ماهو جديد

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  7. [27]
    فرش التراب
    فرش التراب غير متواجد حالياً
    جديد


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    معش انا جديد هنا ومش فاهم حاجة من التاريض دة وعايز كلام مبسط
    مع العلم انا طالب فى سنة تالتة كربة قسم اتصالات جامعة اسيوط:confused:

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  8. [28]
    عبدالواحد علي
    عبدالواحد علي غير متواجد حالياً
    عضو


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    مشكور أخي على شرحك الجميل
    ـــــــــ
    لدي سؤال وهو
    عند تأريض برج إتصالات مرتفع وتركيب في قمته مانعة صواعق
    هل نجعل لمانعة الصواعق تلك سلك يأرضها لوحدها أم أننا نأرض الجسم الكامل للبرج من قواعده الأربعة ويكفي .؟؟؟؟؟؟؟؟:confused:

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  9. [29]
    هشام محمد حامد
    هشام محمد حامد غير متواجد حالياً
    عضو


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    راي ان المناخ الموجود فيه هذا البرج و الي يحدد , بمعني اذا كانت هناك صواعق بنسة كبيرة يجب ان تأرض مانعة الصواعق بسلك منفصل و الا اعمل تارضها مع البرج عادي و ذلك لتخفيف التكلفة
    وده لازم للمهندس انك تفي بالمواصفات الفنية و باقل تكلفة و الله الموفق
    my email : hisham1118222***********
    اذا اردت الاتصال بي او مراسلتي يا اخي عبد الواحد و شكرا

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  10. [30]
    هشام محمد حامد
    هشام محمد حامد غير متواجد حالياً
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    اخي فرش التراب التاريض هي وسيلة لحماية المعدات و الاشخاص من تاثير الكهرباء الزائدة و ذلك عن طريق ايجاد مسار للتيار الزائد بشرط ان يكون اقل من مقاومة الافرد حتي يسحب التيار او يمر فيه التيار بدلا من الاشخاص او المعدة وذلك عن طريق وضع قطب من مادة موصلة للكهرباء و زرعها في الارض علي عمق مناسب حتي تكون التربة حولة مؤينة ( تحتوي علي الاملاح)ثم توصيل هذا السلك للمعدات المطلوب حمايتها عن طرق ما يسمي بشبكة الارضي. علي ******
    my email : hisham1118222

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