Electrical grounding pdf

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Through mathematical analysis, comprehensive explanations, and detailed figures, Analysis of Grounding and Bonding Systems explains the theory and the reasons behind basic ground-electrodes i. Download or read online Electrical Grounding and Bonding written by I. Get Electrical Grounding and Bonding Books now! Few topics generate as much controversy and argument as that of grounding and the associated topics of surge protection, shielding and lightning protection of electrical and electronic systems.

Poor grounding practice can be. Improving your skills in electrical grounding and bonding has never been easier! The best available estimate of this parameter may be used. A coefficient k accounts for the different striking distances to a mast, a shield wire, and to the ground. Equation Lightning strokes have a wide distribution of current magnitudes, as shown in Figure The EGM theory shows that the protective area of a shield wire or mast depends on the amplitude of the stroke current.

If a shield wire protects a conductor for a stroke current Is, it may not shield the conductor for a stroke current less than Is that has a shorter striking distance.

Conversely, the same shielding arrangement will provide greater protection against stroke currents greater than Is that have greater striking distances. Since strokes less than some critical value Is can penetrate the shield system and terminate on the protected conductor, the insulation system must be able to withstand the resulting voltages without flashover.

Stated another way, the shield system should intercept all strokes of magnitude Is and greater so that flashover of the insulation will not occur. Bus insulators are usually selected to withstand a basic lightning impulse level BIL. Insulators may also be chosen according to other electrical characteristics, including negative polarity impulse critical flashover CFO voltage. Flashover occurs if the voltage pro- duced by the lightning stroke current flowing through the surge impedance of the station bus exceeds the withstand value.

This may be expressed by the Gilman and Whitehead equation :. A method is given below for calculating the withstand voltage of insulator strings. Strokes less than Is are permitted to enter the protected zone since the equipment can withstand voltages below its BIL design level. This will be illustrated by considering three levels of stroke current: Is , stroke currents greater than Is , and stroke currents less than Is. First, let us consider the stroke current Is.

Substituting this result in Equation In , Lee developed a simplified technique for applying the electrogeometric theory to the shielding of buildings and industrial plants ; ; Orrell extended the technique to specifically cover the protection of electric substations The technique developed by Lee has come to be known as the rolling sphere method. For the following illustration, the rolling sphere method will be used. This method employs the simplifying assumption that the striking distances to the ground, a mast, or a wire are the same.

With this exception, the rolling sphere method has been updated in accordance with the revised EGM. Use of the rolling sphere method involves rolling an imaginary sphere of radius S over the surface of a substation. The sphere rolls up and over and is supported by lightning masts, shield wires, substation fences, and other grounded metallic objects that can provide lightning shielding. A piece of equipment is said to be protected from a direct stroke if it remains below the curved surface of the sphere by virtue.

Equipment that touches the sphere or penetrates its surface is not protected. The basic concept is illustrated in Figure Continuing the discussion of protection against stroke current Is , consider first a single mast. The geometrical model of a single substation shield mast, the ground plane, the striking distance, and the zone of protection are shown in Figure An arc of radius S that touches the shield mast and the ground plane is shown in Figure All points below this arc are protected against the stroke current Is.

This is the protected zone. The arc is constructed as follows see Figure A dashed line is drawn parallel to the ground at a distance S the striking distance as obtained from Equation An arc of radius S, with its center located on the dashed line, is drawn so the radius of the arc just touches the mast.

Stepped leaders that result in stroke current Is and that descend outside of the point where the arc is tangent to the ground will strike the ground. Stepped leaders that result in stroke current Is and that descend inside the point where the arc is tangent to the ground will strike the shield mast, provided all other objects are within the protected zone.

The height of the shield mast that will provide the maximum zone of protection for stroke currents equal to Is is S. If the mast height is less than S, the zone of protection will be reduced. Increasing the shield mast height greater than S will provide additional protection in the case of a single mast.

This is not necessarily true in the case of multiple masts and shield wires. The protection zone can be visualized as the surface of a sphere with radius S that is rolled toward the mast until touching the mast. As the sphere is rolled around the mast, a three-dimensional surface of protection is defined. It is this concept that has led to the name rolling sphere for simplified applications of the electrogeometric model. Consider a stroke current Is1 with magnitude greater than Is. Strike distance, determined from Equation The geometrical model for this condition is shown in Figure Arcs of protection for stroke current Is1 and for the previously discussed Is are both shown.

The figure shows that the zone of protection provided by the mast for stroke current Is1 is greater than the zone of protection provided by the mast for stroke current Is. Stepped leaders that result in stroke current Is1 and that descend outside of the point where the arc is tangent to the ground will strike the. Stepped leaders that result in stroke current Is1 and that descend inside the point where the arc is tangent to the ground will strike the shield mast, provided all other objects are within the S1 protected zone.

Again, the protective zone can be visualized as the surface of a sphere touching the mast. In this case, the sphere has a radius S1. The remaining scenario to examine is the protection afforded when stroke currents are less than Is. Consider a stroke current Iso with magnitude less than Is.

The striking distance, determined from Equation Arcs of protection for stroke current Iso and Is are both shown. The figure shows that the zone of protection provided by the mast for stroke current Iso is less than the zone of protection provided by the mast for stroke current Is.

It is noted that a portion of the equipment protrudes above the dashed arc or zone of protection for stroke current Iso. Stepped leaders that result in stroke current Iso and that descend outside of the point where the arc is tangent to the ground will strike the ground. However, some stepped leaders that result in stroke current Iso and that descend inside the point where the arc is tangent to the ground could strike the equipment.

This is best shown by. Stepped leaders for stroke current Iso that descend inside the inner protective zone will strike the mast and protect equipment that is h in height.

Stepped leaders for stroke current Iso that descend in the shaded unprotected zone will strike equipment of height h in the area. If, however, the value of Is was selected based on the withstand insulation level of equipment used in the substation, stroke current Iso should cause no damage to equipment. A typical substation, however, is much more complex. It may contain several voltage levels and may utilize a combination of shield wires and lightning masts in a three-dimensional arrangement.

The above concept can be applied to multiple shielding masts, horizontal shield wires, or a combination of the two. The arc of protection for stroke current Is is shown for each set of masts.

The dashed arcs represent those points at which a descending stepped leader for stroke current Is will be attracted to one of the four masts.

The protected zone between the masts is defined by an arc of radius S with the center. The protective zone can again be visualized as the surface of a sphere with radius S, which is rolled toward a mast until touching the mast, then rolled up and over the mast such that it would be supported by the masts.

The dashed lines would be the locus of the center of the sphere as it is rolled across the substation surface. Using the concept of rolling a sphere of the proper radius, the protected area of an entire substation can be determined. This can be applied to any group of different height shield masts, shield wires, or a combination of the two. Substations, however, have two or more voltage levels. The rolling sphere method is applied in the same manner in such cases, except that the sphere radius would increase or decrease appropriate to the change in voltage at a transformer.

It may be appropriate to select some minimum stroke current, perhaps 2 kA for shielding stations below kV. Such an approach is justified by an examination of Figure It will be found that. Therefore, this limit will result in very little exposure, but will make the shielding system more economical. Mousa describes the application of the revised EGM Sc is the critical striking distance as determined by Equation Arcs of radius Sc are drawn with centers at G1, G2, and W2 to determine.