دورات هندسية

 

 

سؤال عن منظومة ارث Earthing system

صفحة 1 من 3 12 3 الأخيرةالأخيرة
النتائج 1 إلى 10 من 24
  1. [1]
    ضياء بصرة
    ضياء بصرة غير متواجد حالياً

    جديد

    تاريخ التسجيل: Oct 2007
    المشاركات: 9
    Thumbs Up
    Received: 1
    Given: 0

    سؤال عن منظومة ارث Earthing system

    سلام عليكم
    كيف اعمل منظومة ارث الى محولة و كيف اقيس مقاومة ارث ؟ ما هي حدودها ؟
    شاكرين تعاونكم

  2. [2]
    حاتم عبدالحميد
    حاتم عبدالحميد غير متواجد حالياً
    عضو


    تاريخ التسجيل: May 2006
    المشاركات: 20
    Thumbs Up
    Received: 0
    Given: 0
    لعمل الارث لاى محول يتطلب
    1- حربه نحاس بطول من 3 الى 9 متر حسب نوع التربه
    2- شيكارة ملح+ شيكارة فحم وزن 50ك
    3-الرينج الخاص بالفراءت هو من 3 الى 7 اوم اذى كان اقل من 3 اوم فهذا افضل
    4- الجهاز المستخدم هو ارث كلامبينج
    5- يتم دق الحربه النحاس حسب العمق
    6- يتم عمل حفره حول الحربه بقطر متر
    7- يتم اضافة الملح والفحم على طباقات سمك 20سم

    0 Not allowed!



  3. [3]
    ضياء بصرة
    ضياء بصرة غير متواجد حالياً
    جديد


    تاريخ التسجيل: Oct 2007
    المشاركات: 9
    Thumbs Up
    Received: 1
    Given: 0

    استفسار

    السلام عليكم
    اشكرك اخ حاتم حميد على الاجابة
    ممكن ان تعطيني مخطط كيفية ربط الجهاز الفحص وكذلك مخطط الى ارث
    اذا يوجد اي كتاب او مصدر بهذا الخصوص افضل

    مع فائق تقدديرنا واعتزازنا

    0 Not allowed!



  4. [4]
    kaz
    kaz غير متواجد حالياً
    عضو فعال


    تاريخ التسجيل: May 2007
    المشاركات: 92
    Thumbs Up
    Received: 0
    Given: 0
    منتظر الرد؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟؟

    0 Not allowed!



  5. [5]
    ديلان2020
    ديلان2020 غير متواجد حالياً
    عضو


    تاريخ التسجيل: May 2007
    المشاركات: 16
    Thumbs Up
    Received: 0
    Given: 0

    Talking I can help

    i can help you but only tell me which kind of tester you have; it have three or four inputs

    0 Not allowed!



  6. [6]
    amr sad
    amr sad غير متواجد حالياً
    جديد


    تاريخ التسجيل: May 2009
    المشاركات: 2
    Thumbs Up
    Received: 0
    Given: 0
    شكرا وياريت لو فى كتاب يعطى شرح كامل (عربى)

    0 Not allowed!



  7. [7]
    مهند مصطفي سعد
    مهند مصطفي سعد غير متواجد حالياً
    عضو فعال جداً
    الصورة الرمزية مهند مصطفي سعد


    تاريخ التسجيل: Sep 2008
    المشاركات: 203
    Thumbs Up
    Received: 14
    Given: 0
    this topic will help you

    0 Not allowed!


    الملفات المرفقة

  8. [8]
    امير محمود
    امير محمود غير متواجد حالياً
    عضو متميز
    الصورة الرمزية امير محمود


    تاريخ التسجيل: Nov 2007
    المشاركات: 458
    Thumbs Up
    Received: 4
    Given: 0
    نصيحة اخى يدعى المستخدم فى التأريض الكترود ارضى وليست حربة وشكرا

    0 Not allowed!



  9. [9]
    ayman amr
    ayman amr غير متواجد حالياً
    عضو فعال


    تاريخ التسجيل: Apr 2006
    المشاركات: 114
    Thumbs Up
    Received: 4
    Given: 0
    ارجو ان تستفيد من هذا الموضوع
    Ground Electrodes

    The term "ground" is defined as a conducting connection by which a circuit or equipment is connected to the earth. The connection is used to establish and maintain as closely as possible the potential of the earth on the circuit or equipment connected to it. A “ground" consists of a grounding conductor, a bonding connector, its grounding electrode(s), and the soil in contact with the electrode. Grounds have several protection applications. For natural phenomena, such as lightning, grounds are used to discharge the system of current before personnel can be injured or system components damaged. For foreign potentials due to faults in electric power systems with ground returns, grounds help ensure rapid operation of the protection relays by providing low resistance fault current paths. This provides for the removal of the foreign potential as quickly as possible. The ground should drain the foreign potential before personnel are injured and the power or communications system is damaged Ideally, to maintain a reference potential for instrument safety, protect against static electricity, and limit the system to frame voltage for operator safety, a ground resistance should be zero ohms. In reality, as we describe further in the text, this value cannot be obtained. Last, but not least, low ground resistance is essential to meet NEC®, OSHA and other electrical safety standards. Figure 9 illustrates a grounding rod. The resistance of the electrode has the following components:
    (A) the resistance of the metal and that of the connection to it. (B) the contact resistance of the surrounding earth to the electrode. (C) the resistance in the surrounding earth to current flow or earth resistivity which is often the most significant factor. More specifically:
    (A) Grounding electrodes are usually made of a very conductive metal (cop- per or copper clad) with adequate cross sections so that the overall resistance is negligible. The National Institute of Standards and Technology has demonstrated the the resistance between the electrode and the surrounding earth is negligible if the electrode is free of paint, grease or other coating, and if the earth is firmly packed.
    (C) The only component remaining is the resistance of the surrounding earth. The electrode can be thought of as being surrounded by concentric shells of earth or soil, all of the same thickness. The closer the shell to the electrode, the smaller its surface; hence, the greater its resistance. The farther away the shells are from the electrode, the greater the surface of the shell; hence, the lower the resistance. Eventually, adding shells at a distance from the grounding electrode will no longer noticeably affect the overall earth resistance surrounding the electrode. The distance at which this effect occurs is referred to as the effective resistance area and is directly dependent on the depth of the grounding electrode.
    In theory, the ground resistance may be derived from the general formula:
    Length
    Resistance = Resistivity x -------
    Area
    This formula illustrates why the shells of concentric earth decrease in resistance the farther they are from the ground rod:
    R = Resistivity of Soil x Thickness of Shell
    ----------------
    Area

    In the case of ground resistance, uniform earth (or soil) resistivity through- out the volume is assumed, although this is seldom the case in nature. The equations for systems of electrodes are very complex and often expressed only as approximations. The most commonly used formula for single ground e l e c t r ode systems, developed by Professor H. R. Dwight of the Massachusetts Institute of Technology, is the following:

    {( ln 4L) -1}

    R =  -------
    r
    2  L

    R = resistance in ohms of the ground rod to the earth (or soil)
    L= grounding electrode length
    r = grounding electrode radius
    r= average resistivity in ohms-cm.

    Effect of Ground Electrode Size and Depth on Resistance
    Size :Increasing the diameter of the rod does not materially reduce its resis- tance. Doubling the diameter reduces resistance by less than 10% (Figure 10).


    Depth: As a ground rod is driven deeper into the earth, its resistance is sub- stantially reduced. In general, doubling the rod length reduces the resistance by an additional 40% (Figure 11). The NEC (1987, 250-83-3) requires a mini- mum of 8 ft (2.4 m) to be in contact with the soil. The most common is a 10 ft (3 m) cylindrical rod which meets the NEC code. A minimum diameter of 5/8 inch (1.59 cm) is required for steel rods and 1/2 inch (1.27 cm) for cop- per or copper clad steel rods (NEC 1987, 250-83-2). Minimum practical diameters for driving limitations for 10 ft (3 m) rods are:
    * 1/2 inch (1.27 cm) in average soil
    * 5/8 inch (1.59 cm) in moist soil
    * 3/4 inch (1.91 cm) in hard soil or more than 10 ft driving depths



    Grounding Monograph
    1. Select required resistance on R scale.
    2. Select apparent resistivity on P scale.
    3. Lay straight edge on R and P scale, and allow to intersect with K scale 4 Mark K scale point.
    5. Lay straightedge on K scale point & DIA scale, and allow to intersect with D scale 6 Point on D scale will be rod depth required for resistance on R scale.
    Ground Resistance Values
    NEC®250-84 (1987): Resistance of man-made electrodes:
    "A single electrode consisting of a rod, pipe, or plate which does not have a resistance to ground of 25 ohms or less shall be augmented by one addition- al of any of the types specified in section 250-81 or 250-83. Where multiple rod, pipe or plate electrodes are installed to meet the requirements of this section, they shall be not less than 6 ft (1.83 m) apart."
    The National Electrical Code®(NEC) states that the resistance to ground shall not exceed 25 ohms. This is an upper limit and guideline, since much lower resistance is required in many instances.
    "How low in resistance should a ground be?" An arbitrary answer to this in ohms is difficult. The lower the ground resistance, the safer, and for positive protection of personnel and equipment, it is worth the effort to aim for less than one ohm. It is generally impractical to reach such a low resistance along a distribution system or a transmission line or in small substations. In some regions, resistances of 5 ohms or less may be obtained without much trouble. In other regions, it may be difficult to bring resistance of driven grounds below 100 ohms. Accepted industry standards stipulate that transmission substations should be designed not to exceed one ohm resistance. In distribution substations, the maximum recommended resistance is for 5 ohms or even 1 ohm. In most cases, the buried grid system of any substation will provide the desired resistance.
    In light industrial or in telecommunication central offices, 5 is often the accepted value. For lightning protection, the arrestors should be coupled with a maximum ground resistance of 1.
    These parameters can usually be met with the proper application of basic grounding theory. There will always exist circumstances which will make it difficult to obtain the ground resistance required by the NEC® or other safety standards. When these situations develop, several methods of low ering the ground resistance can be employed. These include parallel rod systems, deep driven rod systems utilizing sectional rods and chemical treatment of the soil. Additional methods, discussed in other published data, are buried plates, buried conductors (counterpoise), electrically connected building steel, and electrically connected concrete reinforced steel.
    Electrically connecting to existing water and gas distribution systems was often considered to yield low ground resistance; however, recent design changes utilizing non-metallic pipes and insulating joints have made this method of obtaining a low resistance ground questionable and in many instances unreliable.
    The measurement of ground resistances may only be accomplished with specially designed test equipment. Most instruments use the fall-of-potential principle of alternating current(AC)circulating between an auxiliary electrode and the ground electrode under test ; the reading will be given in ohms, and represents the resistance of the ground electrode to the surrounding earth. AEMC has also recently introduced clamp on ground resistance testers.
    Ground Resistance Testing Principle
    (Fall of Potential -- 3-Point Measurement)
    The potential difference between rods X and Y is measured by a volt meter, and the current flow between rods X and Z is measured by an ammeter. (Note: X, Yand Z may be referred to as X, P and C in a 3-point tester or C1, P2 and C2 in a 4-point tester.) (See Figure 13.)
    By Ohm's Law E = RI or R = E/I, we may obtain the ground electrode resistance R. If E = 20 V and I = 1 A, then
    R = E = 20 = 20
    I 1
    It is not necessary to carry out all the measurements when using a ground tester. The ground tester will measure directly by generating its own current and displaying the resistance of the ground electrode.


    Position of the Auxiliary Electrodes on Measurements
    The goal in precisely measuring the resistance to ground is to place the auxiliary current electrode Z far enough from the ground electrode under test so that the auxiliary potential electrode Y will be outside of the effective resistance areas of both the ground electrode and the auxiliary current elec- trode. The best way to find out if the auxiliary potential rod Y is outside the effective resistance areas is to move it between X and Z and to take a reading at each location. If the auxiliary potential rod Y is in an effective resistance area (or in both if they overlap, as in Figure 14), by displacing it, the readings taken will vary noticeably in value. Under these conditions, no exact value for the resistance to ground may be determined. On the other hand, if the auxiliary potential rod Y is located outside of the effective resistance areas (Figure 15), as Y is moved back and forth the reading variation is minimal. The readings taken should be relatively close to each other, and are the best values for the resistance to ground of the ground X. The readings should be plotted to ensure that they lie in a "plateau" region as shown in Figure 15. The region is often referred to as the "62% area."


    Measuring Resistance of Ground Electrodes (62% Method)
    The 62% method has been adopted after graphical consideration and after actual test. It is the most accurate method but is limited by the fact that the ground tested is a single unit.
    This method applies only when all three electrodes are in a straight line and the ground is a single electrode, pipe, or plate, etc., as in Figure 16.


    Now consider Figure 18, where the X and Z electrodes are sufficiently spaced so that the areas of effective resistance do not overlap. If we plot the resistance measured we find that the measurements level off when Y is placed at 62% of the distance from X to Z, and that the readings on either side of the initial Y setting are most likely to be within the established tolerance band. This tolerance band is defined by the user and expressed as a percent of the initial reading: ± 2%, ± 5%, ± 10%, etc.

    Auxiliary Electrode Spacing
    No definite distance between X and Z can be given, since this distance is rel- ative to the diameter of the electrode tested, its length, the homogeneity of the soil tested, and particularly, the effective resistance areas. However, an approximate distance may be determined from the following chart which is given for a homogeneous soil and an electrode of 1" in diameter. (For a diameter of 1/2", reduce the distance by 10%; for a diameter of 2" increase the distance by 10%.)

    0 Not allowed!



  10. [10]
    boryek
    boryek غير متواجد حالياً
    جديد


    تاريخ التسجيل: Apr 2009
    المشاركات: 2
    Thumbs Up
    Received: 0
    Given: 0
    شكرا جدا
    وجزاك الله كل خير

    0 Not allowed!



  
صفحة 1 من 3 12 3 الأخيرةالأخيرة
الكلمات الدلالية لهذا الموضوع

عرض سحابة الكلمة الدلالية

RSS RSS 2.0 XML MAP HTML