- Slides: 40
The objective of a grounding system are: 1. To provide safety to personnel during normal and fault conditions by limiting step and touch potential. 2. To assure correct operation of electrical/electronic devices. 3. To prevent damage to electrical/electronic apparatus. 4. To dissipate lightning strokes. 5. To stabilize voltage during transient conditions and to minimize the probability of flashover during transients. 6. To divert stray RF energy from sensitive audio, video, control, and computer equipment.
A safe grounding design has two objectives: 1. To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service. 2. To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock.
The PRIMARY goal of the grounding system throughout any facilities is SAFETY. Why ground at all? PERSONNEL SAFETY FIRST EQUIPMENT PROTECTION SECOND
What are three main types of grounding? The three main types are: Ø EQUIPMENT GROUNDING (SAFETY) Ø SYSTEM GROUNDING Ø LIGHTNING/SURGE GROUNDING
Earth / Ground Basics Types of Grounding Systems Many different types available Ø Choice depends on local conditions and required function Ø Ø Simplest form is a single stake Ø Mostly used for: l Lightning protection l Stand alone structures l Back-up for utility ground Ground rod
Earth / Ground Basics Types of Grounding Systems Ø ground rod group Ø typically for lightning protection on larger structures or protection around potential hotspots such as substations. Ground rod group
Earth / Ground Basics Types of Grounding Systems Ø For areas where there is rock (or other poor conducting material) fairly close to the surface ground plates are preferred as they are more effective Ground plate
Earth / Ground Basics Types of Grounding Systems Ø A ground mesh consists of network of bars connected together, this system is often used at larger sites such as electrical substations. Ground mesh
Soil Characteristics Ø Soil type. Soil resistivity varies widely depending on soil type, from as low as 1 Ohmmeter for moist loamy topsoil to almost 10, 000 Ohm-meters for surface limestone. Ø Moisture content is one of the controlling factors in earth resistance because electrical conduction in soil is essentially electrolytic.
Cable(Earthing conductor) Clamp Test link Rod(Earthing electrode) Rod coupler
Recommended values of earth resistance system Recommended earth resistance(ohm) Light current 0. 5 -1 Low voltage 5 Medium voltage 2. 5 High voltage 0. 5
Substation earthing system • Step & Touch voltage • Grounding grids
Step and touch voltages
Step potential Ø “Step potential” is the voltage between the feet of a person standing near an energized grounded object. Ø It is equal to the difference in voltage, given by the voltage distribution curve, between two points at different distances from the “electrode. ” Ø A person could be at risk of injury during a fault simply by standing near the grounding point.
Touch potential “Touch potential” is the voltage between the energized object and the feet of a person in contact with the object. Ø It is equal to the difference in voltage between the energized object and a point some distance away. Ø The touch potential could be nearly the full voltage across the grounded object if that object is grounded at a point remote from the place where the person is in contact with it. Ø
Ground Testing Methods (1) Resistivity Measurement The purpose of resistivity measurements is to quantify the effectiveness of the earth where a grounding system will be installed. Differing earth materials will affect the effectiveness of the grounding system. The capability of different earth materials to conduct current can be quantified by the value E (resistivity in W. m). Resistivity measurements should be made prior to installing a grounding system, the values measured will have an effect on the design of the grounding system.
Ground Testing Methods (1) Resistivity Measurement ( Wenner method) Resistivity measurements are performed by using a four wire method Used to determine which KIND of earthing should be used, so BEFORE placing earth stakes
Ground Testing Methods (1) Resistivity Measurement From the indicated resistance value RE, the soil resistivity is calculated according to the equation : E = 2 . a. R E E RE a . . . mean value of soil resistivity (W. m). . . measured resistance (W). . . probe distance (m)
Resistance of driven rods: Ø The Ground Resistance (R) of a single rod, of diameter (d) an driven length (i) driven vertically into the soil of resistivity (ρ), can be calculated as follows: where: ρ Soil Resistivity in m l Buried Length of the electrode in m d Diameter of the electrode in m The rod is assumed as carrying current uniformly along its rod. Ø Examples (a) 20 mm rod of 3 m length and Soil resistivity 50 Ω-m. . . R=16. 1 Ω (b) 25 mm rod of 2 m length and Soil resistivity 30 Ω-m. . . R=13. 0 Ω
Ø The resistance of a single rod is not sufficiently low. Ø A number of rods are connected in parallel. Ø They should be driven far apart as possible to minimize the overlap among their areas of influence. Ø It is necessary to determine the net reduction in the total resistance by connecting rods in parallel. Ø The rod is replaced by a hemispherical electrode having the same resistance.
Rod Electrodes in Parallel Ø If the desired ground resistance cannot be achieved with one ground electrode, the overall resistance can be reduced by connecting a number of electrodes in parallel. Ø These are called “arrays of rod electrodes”. Ø The combined resistance is a function of the number and configuration of electrodes, the separation between them, their dimensions and soil resistivity. Ø Rods in parallel should be spaced at least twice their length to utilize the full benefit of the additional rods.
Ø If the separation of the electrodes is much larger than their lengths and only a few electrodes are in parallel, then the resultant ground resistance can be calculated using the ordinary equation for resistances in parallel. Ø In practice, the effective ground resistance will usually be higher than this. Ø Typically, a 4 spike array may provide an improvement of about 2. 5 to 3 times. Ø An 8 spike array will typically give an improvement of may be 5 to 6 times.
Earth clamping 1 AT-090 H Earth clamping 2 AT-087 J AT-089 J AT-093 J
METHODS OF DECREASING GROUND RESISTANCE Ø Decreasing the ground resistance of a grounding system in high resistivity soil is often a formidable task. Ø Recently, some new methods have been proposed to decrease ground resistance.
1 -Chemical Rods Chemical rods are electrodes with holes along their length, filled with mineral salts. Ø The specially formulated mineral salts are evenly distributed along the entire length of the electrode. Ø The rod absorbs moisture from both air and soil. Ø Continuous conditioning of a large area insures an ultra-low-resistance ground which is more effective than a conventional electrode. Ø
Ø If the conductive salts are running low, the rod can be recharged with a refill kit. Ø These rods are available in vertical and horizontal configurations. Ø They may be used in rocky soils, freezing climates, dry deserts, or tropical rain forests. Ø They provide stable protection for many years.
Disadvantages are: Ø Chemicals concentrated around electrodes will cause corrosion Ø Chemicals leach through the soil and dissipate Ø Scheduled replacement may be required Ø May be prohibited because they may contaminate the water table
Soil Treatment Alternatives Ø Ground enhancement material Cement-like compound l Non-corrosive l Extremely conductive l Installed around the electrode l Easy installation l Permanent
Ø Installing an EARTHLINK 101 earthling strip is simple: Dig a trench and lay in the wire.
Pour EARTHLINK 101 conductive cement, using the handy applicator bag, and shovel in a thin protective layer of soil.
Backfill the remaining soil using a front-end loader and restore the surface to grade.