How to Select the Best Earthing System
Earthing System Components – Part One
Earthing System Components – Part Two
Earthing System Components – Part Three
Introduction to Grounding System Design – Part One
Introduction to Grounding System Design – Part Two
Types of Earthing System – Part One
Types of Earthing System – Part Two
How to Select the Best Earthing System In Article " Types of Earthing System – Part One ", I listed the Different Types of Earthing Systems which can be divided according to the following factors: 1.
Function,
2.
System size,
3.
Neutral point connection to earth,
4. Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts.
And I explained the first two categories in this Article, showing that the Types of Earthing Systems according to its Function can be divided to Six types as follows: 1.
Static grounding,
2.
Equipment grounding,
3.
System grounding,
4.
Lightning grounding,
5.
Electronic (including computer) grounding,
6.
Maintenance safety grounding.
And the Types of Earthing Systems according To Its Size can be divided to Two types as follows: 1.
simple,
2.
complex.
Also, In Article " Types of Earthing System – Part Two ", I explained the Types of Earthing Systems according To Neutral point connection to earth, which were: 1.
The solidly (or directly) earthed neutral,
2.
The unearthed neutral,
3.
High impedance-earthed neutral,
4.
Resistance earthing,
5.
Reactance earthing,
6.
Petersen coil earthing.
And the Types of Earthing Systems according To Neutral point connection to earth + the connection method of the electrical installations exposed conductive parts, which were: 1.
IT (Unearthed transformer neutral, earthed frame),
2.
TT (Transformer neutral earthed, and frame earthed),
3.
TNC (If the N and PE neutral conductors are one and the same (PEN))
4.
TNS (If the N and PE neutral conductors are separate),
5.
TNC-S (Use of a TN-S downstream from a TN-C (the opposite is forbidden).
Today I will explain How to select the best earthing system for a new construction from the above types as follows.
You can preview the following Articles for more info:
Introduction to Grounding System Design – Part One
Introduction to Grounding System Design – Part Two
How to select the best earthing system for a new construction
Who decide which Type of Earthing Systems to be used? For any new construction, three parties share in the Choice of earthing system: 1. 2. 3.
Electrical power users, Network operators (electrical service), Installation’s design engineering firms.
Experience shows however that the choice is mainly made by the engineering firms designing the installation.
Criteria For Selection Of The Best Earthing System the criteria used to make the best choice will be according to: 1. 2. 3.
Country Regulations, Country development, Type of building,
4. 5. 6. 7.
Type of networks and building Loads, Availability of Electrical Service, Costs, Complexity of design.
1- Country regulations The regulations or standards applied in the country in some cases impose certain types of earthing system arrangements (SEA). The most common systems in most countries are TT and TN; a few countries, in particular Norway, use the IT system. The table in below figure#1 lists some examples for LV earting systems for public distribution (LV consumers) for different countries. This table shows that Anglo-Saxon countries mainly use the TN-C, whereas the TT is used in the rest of the world.
Fig.1: examples for LV earting systems for public distribution (LV consumers) for different countries
2- Country development The degree of development of the country should also be taken into consideration, as should be national practices, climate, etc. If we plot an axis from North to South, as regards public distribution, we find the IT earthing system in Norway, TN-C in Germany, TT in France and in most African countries. In temperate, industrial countries, all three earthing systems are used
in private installations.
3- Type of building 1- In a certain number of countries, for some buildings or parts of a building, the choice is laid down by legislations or standards, e.g. for hospitals, schools, navy, worksites, mines, etc. In other cases, certain earthing systems are strictly prohibited, for example the TN-C in premises with explosion risks. 2- The required level of dependability should determine which earthing system is chosen for a specific building type. Dependability means that electrical power should thus always be available and be completely risk-free, i.e. «out of sight, out of mind». The elements making up installation dependability:
Safety; Availability; Reliability; Maintainability, Proper operation of low current communicating systems
These elements must therefore be optimised. The table in below figure# 2 reviews the strong and weak points in each earthing system:
Fig.2: Comparison of system earthing arrangements
Note: In terms of the protection of persons, the three system earthing arrangements (SEA) are equivalent if all installation and operating rules are correctly followed. Consequently, selection does not depend on safety criteria.
4- Type of networks and building Loads
The particular characteristics of the network and the loads will affect the selection of earthing system arrangements as in below table in figure#3.
Fig.3: Influence of networks and loads on the selection of system earthing arrangements
5- Availability of Electrical Service The decision of the owner if supply is via a private MV/LV transformer (MV subscription) or the owner has a private energy source (or a separatewinding transformer). If the owner effectively has a choice, the decision on the SEA is taken following discussions with the network designer (design office, contractor), The discussions must cover:
1. First of all, the operating requirements (the required level of continuity of service) and the operating conditions (maintenance ensured by electrical personnel or not, inhouse personnel or outsourced, etc.) 2. Secondly, the particular characteristics of the network and the loads as infig.3.
6- Costs The TN-S is the least costly to install, for example if the neutral is neither protected nor switched. But be warned: the cost of curative maintenance can be high. The IT is slightly more costly to install (insulation monitoring and insulation fault tracking devices). Search for maximum availability of electrical power requires the presence of an electrical engineer, whose action will minimize curative maintenance. The TT, if enough discriminating RCDs are installed, is slightly more costly to install than the IT, but fault tracking is simple and curative maintenance less costly than in TN.
Note:
In terms of complete cost over 10 to 20 years, all three earthing systems are equivalent.
7-Complexity Of Design
Designing is simpler in TT, the same for extensions (no calculations). Designing complexity is equivalent in TN-S and IT.
Steps For Choosing The Best Earthing System Step#1: Firstly do not forget that the three system earthings can all be included in the same electrical installation: This guarantees the best possible answer to safety and availability needs (see below figure#4).
Fig.4: several earthing system included in the same LV installation. Step#2: check that the choice is not specified or stipulated by standards or legislation (decrees, ministerial decisions) Step#3: dialogue with the user to get to know his requirements and resources, and Generally (see table in figure#5 ): 1- Need for continuity of service and Whether or not there is a maintenance service:
Continuity of service and maintenance service: the IT will be chosen. Continuity of service and no maintenance service: No fully satisfactory solution: Prefer the TT whose discrimination on tripping is easier to implement and which minimises damage with respect to the TN. The installation of additionnal output is easily achieved without the necessity of further calculations. Continuity of service not essential and compent maintenance service: prefer the TN-S (rapid repairs and extensions performed according to rules), Continuity of service not essential and no maintenance service: Prefer the TT
2- Fire hazard IT if maintenance service and use of 0.5 A RCD or TT.
3- Safety The TT is the best.
4- Availability The IT is the most suitable.
5- Maintenability Fault tracking is fast in TN (thanks to the SCPD) but repair time is often long. Conversely, in IT, tracking of the first fault may be more difficult, but repairs are quicker and less costly. The TT is a good compromise.
6- Reliability The protection devices used are reliable, but reliability of the installation and loads may be affected: a- In TN-C by the fact that the PEN, not protected, may be damaged by harmonic currents; b- In TN-C and TN-S: By insufficient rigour for extensions,
By use of replacement sources with low short-circuit power, By the effects of electrodynamic forces;
c- In IT, on a double fault, the risks inherent in TN described above also exist. However if tracking and elimination of the 1st fault are rapid, installation reliability is excellent. d- in TT, by disruptive breakdown by return of the loads due to a fault in the HV/LV transformers. However the likelihood of this fault occurring is small and preventive solutions are available, e.g. use of surge arresters between one of the live conductors and the load earth connection. 7- Disturbance The TT is to be preferred to the TN-S whose high fault currents may be the source of disturbance.
Fig.5: Comparison of system earthing arrangements according to its dependability
Step#4: Finally allow for the special features of network and loads as follows: Very long network or, even more important, leakage current: Prefer the
TN-S, Use of replacement or standby power supplies: Prefer the TT, Loads sensitive to high fault currents (motors): Prefer the TT or IT, Loads with low natural insulation (furnaces) or with large HF filter (large computers): Prefer the TN-S, Supply of control and monitoring systems: Perfer the IT (continuity of service) or the TT (enhanced equipotentiality of communicating devices).
Earthing System Components
Earthing System Components Earthing system in an installation is normally comprised of the following components: 1. 2. 3. 4.
Earth wells and accessories, Earthing grid conductors, Marshalling earth buses (earthing distribution buses), Earthing wires and cables.
1- Earth Wells and Accessories Earth wells for a specific building or installation are actually the location, where the pure zero potential is provided and practically act as drain pits for any rush current which accidentally appears in the earthing system grid
in the event of an earth fault (connection of electrical live parts to the earthing system). There are different types of components can be used to set up an earth well Depending on the following factors: 1. Soil conductivity of the location in which the earth wells are installed, 2. The required technical specifications of the earthing system.
However, the prime components and accessories of an earth well will be:
1. 2. 3. 4. 5. 6. 7. 8. 9.
Earth Rod, Earth Plate, Earthing Clamp, Earthing Rod Coupling, Earthing Rod Tip, Earthing Rod Driving Head with driving sleeve, Earth Access Pit, Earth Pit Cover, Earth Enhancement Materials.
1.1 Earth Rods Depending on the design for a specific earth well, a number of rods are driven into the ground by means of hammering or driving to form the main earthing electrode in the earth well. The earth rod have three types:
1. 2. 3.
Notes:
Non-Sectional Ground Rods: have unthreaded top. Sectional Ground Rods: have externally threaded top. Sectional Ground Rods: have internally threaded top.
In Cases where two or more earth rods are to be driven, the individual rods are coupled to each other by means of “earth rod coupling”. Ground rods coupling process are used to help reduce ground resistance in poor soils such as sand and gravel. Doubling rod length theoretically reduces ground resistance about 40%.
1.1.1 Methods for Driving Earth Rod into Ground Two methods are used to drive the rod into the ground: 1. Hammering: by using a hammer, a drive stud and rod tip. 2. Driving: by using a Driving Hammer Tool and Ground Rod Drive Bit or using a Driving Hand Tool.
Notes: During the hammering or driving of rod into the ground, and to protect the earth rod against impact of hammering, a “driving head” is screwed onto the top of the rod. For easy and convenient driving of the earth rod into the ground an earth rod tip with sharp point is screwed to the first rod. Earth rods are used in installation of plain earthing well where, based
on design specification of the earthing system, the carbon bedding is not necessary and applicable.
1.1.2 Earth Rod Material UL467- 9.2.1 states that the solid rod electrode of copper or other suitable non-ferrous metal, or a solid rod electrode of iron or steel with a copper or other suitable non-ferrous metal or stainless steel jacket, shall have a diameter not less than ½ inch thick. Also, UL467-9.2.2 states that the stainless steel jacket shall not be less than 0.015 inches thick at any point. And UL467-9.2.6 states that the stainless steel jacket mentioned above on a stainless steel rod shall be formed of an austenitic stainless steel of the 18% chromium, 8% nickel type. And stainless steel ground rods are used in corrosive soil conditions. Usually in Europe and Middle East, Earthing rod and the associated accessories (coupling, tip and head) are made of both steel and copper. A steel core, coated with pure copper to the appropriate thickness, provides the sufficient rigidity for the earthing rod to help driving it straightly into the ground without any harm and bending. The copper coating of the earth rod provides the sufficient conductivity for the earthing system.
1.2.3 Earth Rod Dimensions a- Diameter: Depending on the design specification of the earthing system and the corresponding earthing wells, various earth rods of different dimensions would be incorporated. As per UL467- 9.2.1, the solid rod electrode shall have a diameter not less than ½ inch thick. In Europe and middle east, The range of diameter for the earth rods vary from 13 mm to 25mm (13mm,16mm, 20mm, 25mm).
b- Length:
Different lengths of earthing rods are used in design and installation of earth wells, The standard lengths are: In North America: 2, 3, 5, 8 and 10 feets. In europe and middle east: 1200mm, 2400 mm (2 × 1200 mm), 3600 mm (3 × 1200 mm) and 4800 mm (4 × 1200 mm).
1.2 Earth Plate In earth wells with carbon beddings, earthing plates are normally used instead of earthing rods. The earth plate is made of copper and shaped in the following forms: 1. 2.
Flat rectangular copper plate, Perforated rectangular copper plate (Earth Mat).
Copper ground plates are used instead or with the ground rods in the following situations:
In areas having little or no top soil, If it is required to enhance ground grid systems, In conjunction with earth enhancements materials.
For securing good electrical connections with the ground plate, ground plate use: 1. One cable connector, 2. Two dual cable connectors, 3. 18 or 24 inches (depending on the plate size) copper conductor exothermically welded to the plate cable connectors
Earth Plate Size: The common sizes of ground plates are:
In North America: 18 x 18 inches or 24 x 24 inches. In Europe: 1. The flat rectangular earth plate is normally 100 × 100 × 3 mm. 2. The standard cross section area for the copper rod or copper strips used in construction of the perforated rectangular earth plate is
normally 75 sq-mm.
Differences between Earth rods and Earth Plates Earth rods and plates or any combination thereof can be used to achieve an effective earth depending on the site conditions. The main difference between Earth rods and Earth Plates is as follows: Earth rods take advantage of lower resistivity soils at greater depths than normal excavation will allow. While, Earth plates are used to attain an effective earth in shallow soils with underlying rocks or in locations with large amounts of buried services. They can also provide protection at potentially dangerous places eg HV switching positions.
1.3 Earthing Clamp Earthing grid conductors are connected to the earth rods, already driven into the ground, by means of earthing clamps. This Connection between earth rod and Earthing grid conductors is essentially made by tightly clamping of the grid conductor to the rod by one of the two following methods:
1. 2.
Mechanical Clamps, Exothermic Welding Clamping.
1- Mechanical Clamps:
They are Used Where permanent connections are not appropriate, mechanical clamps offer the ideal solution. These are typically used on smaller scale installations where periodic disconnection for testing is required. The mechanical earhing clamp may come with one bolt or two bolts, the two bolts give more good electrical connection with the earh rod. Mechanical Earthing clamps and associated bolts nuts, washers, etc. are
made of brass, bronze or copper. The mechanical earth clamps sizes shall be selected to accommodate the diameters of earh rod and grounding conductors.
Types Of Mechanical Earthing Clamps: 1. Light duty type: this type is acceptable for electrical grounding but not for lightning protection. 2. Heavy duty type: this type is acceptable for lightning protection and requires 1-1/2 inches of surface contact between conductor and earth rod.
2- Exothermic Welding Clamping:
A simple, self-contained method of forming high quality electrical connections which requires no external power or heat source. Connections are made using the high temperature reaction of powdered copper oxide and aluminium. Exothermic Welding connections allow conductors to carry higher currents than other types of connections. They will never loosen, are highly conductive and have excellent corrosion resistance. The Exothermic Welding will be explained later in separate aricle.
1.4 Earthing Rod Coupling In some cases, Depending on the design for a specific earth well, there is need to drive two or more earth rods into the ground, the individual rods are coupled to each other by means of “earth rod coupling”. There are three types of rod couplers according to the used type of ground rods as follows: 1. Unthreaded Coupler for Non- sectional ground rods, 2. Threaded Coupler for sectional ground rods externally threaded, 3. Coupling Dowel for sectional ground rods internally threaded.
The coupling material is essentially the same as the material for the earth rod with respect to the rigidity and the required conductivity. Earth rod coupling shall have the same diameter of the earth rod.
1.5 Earth Rod Tip
The earth rod tip is used for easy and convenient driving of the earth rod into the ground because it secures a sharp head for the first rod driven into ground. The earth rod tip material is not necessarily the same as the earth rod itself, as only a rigid quality is essentially required for the tip other than conductivity. Therefore the earth rod tip is primarily made of steel with slight coating of the copper for conductivity purpose as well as protection against corrosion reasons. Earth rod tip shall have the same diameter of the earth rod.
1.6 Earth Rod Driving Head with Driving Sleeve During the hammering or driving of rod into the ground, and to protect the earth rod against impact of hammering, a “driving head” is screwed onto the top of the rod with a driving sleeve.
So, the main function of driving sleeve is to prevent Mushrooming top of ground rod while driving into ground. And, the main function of a driving head stud is to prevent damage to the coupler or ground rod threads when driving the ground rods. The driving head material is not necessarily the same as the earth rod itself, as only a rigid and robust quality is essentially required for the driving head to withstand the impact of hammerings. Driving head is practically discarded when the earth rods are all driven and installed in the ground.
Earthing System Components – Part Two
1.7 Earth Access Pit
To provide access to the earth rod and its corresponding connection to the earthing grid at the top section of the rod, a small pit-like space is fabricated over the earth well, which is referred to as “earth pit”. For periodically measuring the electrical resistance of a buried ground system, inspection pits are used as a means of access to the ground conductor. To make electrical resistance measurements, remove the cover and attach a lead from a resistance measuring instrument to the ground conductor.
1- Earth Pit Sizes: Inspection pits are available in various sizes and materials.
2- Earth Pits Main Types: A- According to location:
a- Light Weight Duty Light-duty units are generally used. For example Plastic Light weight duty earth pits are schedule 40 PVC. b- Heavy Duty For areas of high vehicular traffic, you should use heavy-duty inspection wells. For example PlasticHeavy duty earth pits are schedule 80 PVC.
B- According to Material Earth pit’s side walls are constructed of insulating material to appropriately isolate the earth rod’s top connection from the surrounding soil and protect it
for furture reference test and maintenance practices. The materials used for fabrication of Earth Pits are: 1. Polymer Concrete: Polymer Concrete reinforced with heavy weave fiberglass resulting in high strength and minimal weight. Enclosures and covers rated for 10,000 lbs. maximum load. 2. Light weight Polymer Concrete, 3. HDPE (High Density Polyethylene).
Note:
Earth pits are essentially, constructed flush with respect to the surrounding finished ground.
1.8 Earth Pit Cover To protect the earth pits against ingression of foreign material, an appropriate concrete cover is provided to be placed atop the earth pit. The cover is equipped with a rigid handle for convenient removing and replacement practices. 1- Earth Pit Cover Sizes: Inspection pits covers are available in various sizes to accommodate the size of their earthing pits. 2- Earth Pit Cover Materials:
The materials used for fabrication of Earth Pits Covers are:
Cast iron grated cover, Flat steel cover, Plastic covers, Concrete covers.
the ASTM specifications for Earth Pit Covers are as in the below image.
1.9 Earth/ Ground Enhancement Materials (GEM) Only rarely do grounding system designers and contractors get to work on a site with good grounding conditions. Even under ideal circumstances, soil structure can vary and make it difficult to achieve uniform, low levels of resistivity across a wide area. Under almost all soil conditions, the use of a ground enhancement material will improve grounding effectiveness. Some are permanent and require no maintenance.
1- Where and when Earth Enhancement Materials should be used? Earth Enhancement Materials improves grounding effectiveness regardless of soil conditions and provides excellent permanent conductivity: For areas with high resistance, such as rocky ground, mountain tops, and sandy soil, As a backfill when you have to drill because the ground is too hard to drive, or where ground rods cannot be driven, when used as a backfill for earth electrodes, soil conditioning agents effectively act to increase the electrodes surface area thus lowering its resistance to earth. Where limited space makes adequate grounding difficult by conventional methods.
2- Types of Earth Enhancement Materials Many types of Earth Enhancement Materials are used improves grounding effectiveness regardless of soil conditions and provides excellent permanent conductivity, Like: A- Carbon and Salt Bedding Depending on the technical design specification of the earthing system and primarily for soil conductivity reasons of the area where the earth wells are to be installed, the earth rods are embedded in carbon bedding. To install the carbon bedded earth wells, pre-excavation of the ground, to sufficient size and dimension, would be carried out to provide room for the carbon bedding and the earthing components (rods, plates, etc.). To achieve the maximum conductivity for the earth well, an appropriate amount of salt is added to the carbon and mixed before charging into the earth well. B- Marconite Compound Marconite forms a permanent solution when mixed with cement and is used when certain ground conditions make it difficult to obtain a reliable earth resistance or installation might require a low resistance. C- Bentonite Compound: Bentonite can be supplied in powder or granular form and is a moisture retaining clay which is used to reduce soil resistivity. It has two types: 1- Granular: It’s easier to handle in granular form, the powder can cause dust in windy conditions and below away, granular is the preferred option for filling trenches where the conductor is covered with bentonite and the water poured over and mixed in the trench. 2- Powder Powder is the preferred method for pouring into bore holes to ensure the mixture is a thin enough to reach the bottom of the bore hole, if diamond drilling is required for deep holes possibly 40 meters and deeper and the bentonite is to be pumped through the core into the hole, powder will be the preferred option.
The above image clearly shows, GEM has a resistivity factor more than 20 times lower than bentonite clay.
3- How to Specify GEM? 1. Ground enhancement material must be permanent and maintenance-free (no recharging with salts or chemicals which may be corrosive) and maintain its earth resistance with time. 2. It must set up firmly and not dissolve or decompose or otherwise pollute the soil or the local water table. 3. The ground enhancement material shall be suitable for installation in dry form, or in slurry form. 4. The ground enhancement material shall not depend on the continuous presence of water to maintain its conductivity. 5. Ground enhancement material in its set form shall have a resistivity of not more than 20 ohm-cm.
Note:
When selecting a ground enhancement material be sure it is compatible
with the ground rod, conductor and connection material.
4- GEM Installation GEM is supplied in easy-to-handle bags for one-man installation. GEM can be installed dry or wet (recommended). GEM quickly absorbs moisture from the soil when used dry, to reach its maximum conductivity in days. To accelerate curing time, water can be added after GEM is installed, or it can be pre-mixed with water. A- Trench Installation
1. Dig a trench at least 4 inches wide x 30 inches deep or below the frost line, whichever is deeper. Spread out enough GEM to uniformly
cover bottom of trench-about 1 inch deep. 2. Place conductor on top of GEM. 3. Spread more GEM on top of conductor to completely cover conductor about 1 inch deep. 4. Carefully cover the GEM with soil to a depth of about 4 inches, making sure not to expose the conductor. Tamp down the soil, and then fill in the trench. For various trench widths and GEM thicknesses, see the below table.
B- Ground Rod Backfill Installation
1. Auger a 3 inch or larger diameter hole to a depth of 6 inches less than the length of the ground rod. 2. Place ground rod into augered hole and drive one foot (if possible) into bottom of the hole. The top of the ground rod will be approximately 6 inches below grade. At this time, make any connections to ground rod using CADWELD connections. 3. Pour the appropriate amount of GEM around the ground rod. To ensure the GEM material completely fills the hole, tamp around the ground rod with a pole. 4. Fill remainder of augered hole with soil removed during augering. For various augered-hole diameters and depths, see the below table.
Notes: If premixing GEM into a slurry form, use a standard cement mixer or hard-mix in a mixing box, wheelbarrow, etc. Use 1-1/2 to 2 gallons of clean water per bag of GEM. Excess standing water must be removed from the hole.
Enhanced Ground Rod An Enhanced Ground Rod is a conductive hollow tube ground rod, usually manufactured from 300 stainless steel or copper. They contain special hygroscopic, electrolytic salts. These salts form a saline solution by absorbing
moisture out of the atmosphere. This saline solution leaches out of the bottom of the rod, which gradually lowers resistivity of the surrounding soil, forming "electrolytic roots" over time. To increase the efficacy of the Enhanced Ground Rod, a very low resistance ground enhancement material is placed around the rod. A conductor is exothermically attached to the enhanced ground rod. This conductor is called the tail. The tail direction is very important. Enhanced Ground Rod design allows the current, either lightning or electrical fault, to maintain a downward sloping path to ground.
There are two basic styles, vertical and horizontal (L-shaped) as in above image. Enhanced Ground Rod includes a variety of lengths, sectionals and different kits to meet many specific requirements.
2- Earthing Grid Conductors All electrical earth wells in a specific residential, commercial and industrial installation should essentially be interconnected to plant earthing systems with cables, wires and tapes that form the main earthing grid. First: Different Types of ground Grid Conductors:
Interconnecting conductors used for the grid are in the following forms: 1- Cable and wire system The available conductors may be soft-drawn (copper wire that has been heat treated) or hard-drawn (copper wire that has not been annealed after drawing), the available grounding conductors are in the following forms: 1.1 Stranded Copper Conductors:
Concentric Lay Soft-Drawn Bare Copper, Green Insulated Conductors (Concentric Lay Soft-Drawn Bare Copper with PVC sheath or THW insulation), Tinned stranded copper conductor.
1.2 Solid Copper Conductors:
Solid Single Soft-Drawn Bare Copper, Solid Single Soft-Drawn Bare Tinned Copper.
1.3 Flexible Copper Conductors: used as flexible earth bonding leads
Bare copper round braids, Tinned copper round braids.
2- Flat tape system: it give Low impedance than equivalent sized round conductor and it will be in the following forms:
Note:
Bare copper tapes, Tinned copper tapes, Bare Copper Flat Braid Conductors, Tinned Copper Flat Braid Conductors.
There are other types of grid conductors used especially for lightning protection system and will be explained later in lightning protection system Articles. Second: Selection criteria for the best grounding conductor for certain case: There are two basic criteria for grounding conductor selection: 1- Physical Characteristics The physical characteristics of the conductor must be of a robust nature, sufficient for the environment as follows: The most common ground conductor is a soft drawn, stranded copper conductor which used for direct buried grid in dry and noncorrosive grounds. Flat copper strip / tape is also popular because it offers a large surface area and usually used for direct buried grid in dry and non-corrosive grounds (soils). PVC-Covered copper strip conductor: for direct buried grid in wet or corrosive ground. Single core stranded copper conductor with PVC sheath: for direct buried grid in wet or corrosive grounds.
Note: When site conditions are corrosive towards copper (eg. sulphurous soil), a tinned copper conductor is often the first choice. 2- Maximum Fault Current The cross sectional area of the conductor must be of sufficient size, so that it shall successfully conduct the maximum fault (surge) current for a period, which allows the operation of protection equipment (or the dissipation of this energy). In some circumstances, the maximum fault current for the installation is small. While a conductor of correspondingly small size could be used, a minimum cross section often set by the governing authority or applicable Standards body (to minimize potential damage likely to occur from any future excavation on the site), is applied. Where higher fault conditions exist, the conductor size is determined by
considering the circumstances required to avoid fusing (melting) the conductor. The accepted industry Standard is IEEE 80, Guide for Safety in Substation Grounding.
Third: Ground Grid Conductors sizing Ground Grid Conductors sizing will be explained later in the Articles for grounding system design calculations.
3- Marshaling Earth Bus
To provide easy access to the earthing grid, particularly to make proper and convenient connections of the equipment to the grid, several common connection points in the form of a flat bar of copper material are established and erected through out the grid and referred to as “earthing marshalling points” or “earthing marshalling bus”, or simply as “earth bus”. The main incoming earthing cable connected to the earth bus is branched off from the main earthing grid. The outgoing earthing cables, connected to the earth bus in one end, shall be connected to the corresponding equipment on the other end. All the connections of the main incoming and outgoing earth cables shall
be made to the earth bus by means of appropriate:
1. Cable lugs the compression type and zink coating, using bolts, nuts, flat washers and spring washers for well-tight connections, 2. Exothermic Welding process.
4- Earthing Wires (Cables) Connections between the marshalling earth buses and the equipments are carried out by means of single wires or cables of appropriate size, which are referred to as “earthing wire”, or “earthing link”. The connection between the earthing buses and the earthing grid is also made by means of earthing cables. Connections of earthing wires (cables) on both ends is made by appropriate:
1. Cable lugs the compression type and zink coating, using bolts, nuts, flat washers and spring washers for well-tight connections, 2. Exothermic Welding process.
1- earthing wires (cables) Types Earthing wires and cables are used either bare or PVC-covered (preferably bare) and are normally single core of the different cross section area, depending on the design specification.
2- Earthing wires (cables) sizing The common range of the cable size used is 16mm2, 25mm2, 35,50mm2 and 70mm2. Earthing wires (cables) of smaller and higher size could be used depending on the design specification and requirements. Earthing wires (cables) sizing will be explained later in the Articles for grounding system design calculations.
