Chapter 5 Earthing and Bonding n n n

Chapter 5: Earthing and Bonding

n n n The TT system has two, separate ground rods. The neutral is connected to its ground rod at the service entrance. The protective conductor is connected to its own ground rod, remote from the neutral ground rod. In some cases, the ground rod may be the steel frame of the building. In any case, there is no direct copper connection between the enclosure and the supply system.

n n n The IT system also has two, separate ground rods. The neutral is connected through an impedance to its ground rod at the service entrance. The protective conductor is connected to its own ground rod, remote from the neutral ground rod. In some cases, the ground rod may be the steel frame of the building. In any case, there is no direct copper connection between the enclosure and the supply system. One characteristic of the IT system is that the system is tolerant of a fault to ground. That is, a fault to ground does not operate the circuit breaker, so the system remains operational. (An alarm identifies the fault to ground, but the system continues to operate. ) As in the TT system, there is no direct copper connection between the enclosure and the supply system.

n n n The TN-S system has a single ground rod. At the service entrance, the neutral conductor is connected to the ground rod. The protective earth conductor is connected to the neutral at the service entrance. The “S” in the designation means that the protective earth conductor is a separate system conductor. Unlike the TT and IT systems, in the TN system the equipment and man are grounded through different paths. If the current through the different paths is different, then a potential difference will occur between the equipment and the man, and current will pass through the man. To minimize the potential difference due to the difference between the equipment and the man, it is imperative to keep the equipment ground circuit resistance as low as practicable.

n n n The TN-C system has a single ground rod. The neutral conductor is connected to the ground rod located at the service entrance. The protective earth conductor is connected to the neutral in the equipment. There is no separate protective conductor. The “C” in the designation means that the protective earth conductor is combined with the neutral conductor. The TN-C system is used for electric dryers, electric ranges, and electric water heaters in the United States.

How Does Grounding Provide Protection Against Electric Shock or a TN-S system?

n n n If the equipment resistance is 0. 1 ohm as required by the various standards, what is the voltage at accessible grounded parts for various fault currents? The analysis is for a 120 -V, 15 -A, 3% system voltage drop circuit. The current is an arbitrary 150 amperes (10 times the circuit-breaker rating). This current will clearly operate the circuit-breaker in a relatively short time. The accessible part voltage is 33 volts.

n n n If the equipment resistance is 0. 1 ohm as required by the various standards, what is the voltage at accessible grounded parts in the event of a short-circuit? Clearly, the voltage will not be less than 30 volts? How high is the voltage? The analysis is for a 120 -V, 15 -A, 3% system voltage drop circuit. The current is limited only by the source resistances and the 0. 1 ohm resistance of the equipment. (This current will clearly operate the circuit breaker in a relatively short time. ) Using similar calculations as for the 150 -ampere fault current, the accessible part voltage is 77. 7 volts.

n n n For a TN-S system, grounding does not provide an equipotential environment due to the finite resistances of the equipment grounding circuit. However, equipment grounding through its protective conductor does serve to limit the voltage for low fault currents. For higher fault currents, another scheme provides protection against electric shock: limited duration of the current through the body by means of automatic disconnection of the supply (operation of the circuitbreaker).

Grounding design n A grounding system should be installed in a manner that will limit the effect of ground potential gradients to such voltage and current level that will not endanger the safety of people or equipment under normal and fault conditions, as well as assure continuity of service Substation usually have ground grid system with ground mat as extra protection. Grid system has the form of horizontally buried grid conductor, supplemented by a number of vertical ground rods connected to the grid.

n Some of the reasons for using the combined system of vertical rods and horizontal conductors are as follow: Single rod by itself is inadequate in providing a safe grounding system. When several electrodes are connected together and to all equipment neutrals, frames and structures that are to be grounded an excellent grounding system is developed ¨ Grid is installed in a shallow depth usually 0. 3 -0. 5 m below grade, sufficient long ground rod will stabilize the performance of the combined system. Resistivity of upper layer soil vary with seasons, while the resistivity oflower soil layers remains constant. ¨ Rods penetrating the lower resistivity soil are far more effective in dissipating fault current whenever a two or multilayer soil is encountered and the upper soil layer has higher resistivity compared to lower layer. ¨

n Design in difficult condition ¨ In area soil resistivity is high or the substation space is at premium, it is not possible to use grid system. Some solution to design grounding in this area is: 1. 2. 3. 4. Connection of remote ground grid and adjacent grounding facilities Use of deep driven ground rods and drilled ground wells in combination with a chemical treatment of soil or use of bentonite clays for backfilling. Use of counterpoise wire mat. Copper clad steel wires of AWG No 6 size, arrange in 0. 6 mx 0. 6 m grid pattern, installed 0. 050. 15 m depth, then the main grounding grid 0. 3 m-0. 5 m. A nearly low resistivity material can be used as extra grid. Example: clay deposit, part of large structure such as concrete mass of hydroelectric dam

Earth electrode & earth conductor

Earth chamber

Earth clip

Earth clip bar and test point

Selection of Conductors and Joints n Basic requirement ¨ 1. 2. 3. Each element in grounding system including grid conductors, joints, connecting leads and all primary grounding electrodes should be design for the expected design life of installation, the element will be: Have sufficient conductivity Resist fusing and mechanical deterioration under most adverse combination of a fault current magnitude and duration Be mechanically reliable and rugged to a high degree, especially on location exposed to corrosion and physical abuse.

n Choice of material related to corrosion problems ¨ Copper n n n ¨ Aluminum n n n ¨ High conductivity Resist to underground corrosion – cathodic with respect to other metals Disadvantage - Form a galvanic cell with buried steel structure, pipes, and any of lead-based alloy that may present in cable shealth Used less frequently Corrode in certain soils – layer of corrode material is nonconductive for all practical grounding purpose Gradual corrosion due to alternating currents Steel n n Eliminate most of the adverse effects of copper Use application of galvanized or corrosion-resistant steel in combination of cathodic protection to extend life

n Minimum size ¨ Selection of minimum size of material used for grounding depend on ambient temperature, maximum allowable temperature and magnitude of fault current. ¨ To determine the minimum size of conductor cable can be determine from the following formula

n Example: Calculate the minimum size of copper-clad steel core ground conductor required to withstand 5 k. A rms short circuit current in 1 second at ambient temperature 40 C ?

Selection of joints ¨ Connection must able to withstand mechanical stresses without any significant deterioration due to corrosion metal fatigue and electromagnetic forces for many years ¨ Another factor must be considered are connectivity, thermal capacity, mechanical strength and reliability ¨ Type of connection: exothermic welds, brazed joints and pressure type

Soil characteristics n n n Most soil behave as both as conductor or resistance and as dielectric Effect of current magnitude- soil resistivity effected by current flowing from electrode to soil. Thermal characteristic of soil and moisture content of soil will determine is a current of given magnitude and duration will cause significant drying and thus increase the effective soil resistivity. Effect of moisture temperature and chemical content – resistivity of soil rise abruptly whenever the moisture contents accounts for less then 15% of soil weight. Amount of soil depend on the grain size, compactness. Effect of soil resistivity negligible for temperatures above temperatures the freezing point. At 0 ºC the water start to freeze and the resistivity increase rapidly. The composition and amount of soluble salts, acids or alkali may consider affecting soil resistivity.

n Use of crushed-stone layer ¨ Gravel or crushed rock covering usually about 0. 08 – 0. 15 m in depth are very useful retarding evaporation of moisture and thus limiting the drying of topsoil layers during dry weather periods. ¨ Crushed rock with high resistance value reduce shock current

What is a good earthing 1. 2. 3. 4. Low electrical resistance measured in ohms Good corrosion resistance Ability to carry high currents repeatedly Ability to perform above function for 30 years or more

Method of providing good earthing 1. 2. 3. 4. 5. 6. Deep driven earth rods Parallel driven earth rods Buried conductor (wire or tape etc) Buried earth plate or mats Underground metal pipe system Steel reinforcing rods and/ or wires for concrete

Soil resistivity measurement n Why measure? Data obtained from measurement used to make sub-surface geophysical surveys as an aid identifying ore locations, depth to bedrock and other geological phenomena ¨ Resistivity has direct impact on the degree of corrosion in underground pipelines. Decrease in resistivity relates to an increase in corrosion activity – protective treatment will be used ¨ Affects the design of grounding system – advisable to locate area at lowest soil resistivity in order to achieve the most economical grounding installation ¨ n There are two method to measure soil resistivity: 4 -point method and 2 -point method.

n 4 -point measurement ¨ Also known as Wenner method ¨ Requires the insertion four equally spaced and in-line electrodes into the test area. ¨ Known current from constant current generator is passed between the outer electrodes, then potential drop (a function of resistance) is measured across the two inner electrodes

Solution

n For a single ground electrode, Professor H. R. Dwight of the Massachusetts Institute of Technology develop a formula to estimate earth rod resistance to earth:

Field measurement of constructed grounding system n Two point method (Ammeter-Voltage Method) ¨ This method measure the total resistance of the unknown and an auxiliary ground. ¨ Auxiliary ground resistance assume to be negligible in comparison with resistance of the unknown ground ¨ This method subject to large errors

n Three point method ¨ Involve use of two test electrodes with their resistances designated as r 2 and r 3 and with the electrode to be measured designated as r 1. ¨ The resistance between each pair of electrodes is measured and designated as r 12, r 13 and r 23 where r 12 = r 1 + r 2. Solving the simultaneous equations,

¨ Therefore, by measuring the resistance of each pair of ground electrodes in series and substituting these values in equation above, the value of r 1 can be determined. ¨ If the two test electrodes have substantially higher resistance than the electrode under test, the errors on the individual measurements will greatly magnified in final results. ¨ Spacing between the three electrodes should be more than 10 m

n Ratio method ¨ n This method compares the resistance of the electrode under test to that of a known resistance, generally the same electrode configuration as in the fall-of-potential. Fall-of-potential method Consist of passing a current through the station ground via a ground electrode C remote from the station, and measuring the voltage between the station ground and the remote from the station at P ¨ Term “remote” – very large electrode spacing where earth current density approaches zero. ¨

n References ¨ IEEE Guide for Safety in AC Substation Grounding (ANSI/IEEE Std 80 -1986) – Institute of Electrical and Electronics Engineers. ¨ J. Philip Simmons, Electrical Grounding and Bonding, Thomson Delmar Learning, 2005