Grounding And Safety Techniques Ms. Tahoora Qureshi, Asst. Professor Department: B.E. Electrical Engineering Subject: Elements of Power System Semester: IV eCourseware@AIKTC Teaching Aids Service by KRRC Information Section

INTRODUCTION Electrical earthing or grounding: Earthing or grounding  system is an electrical circuit which connects a part of electrical system to the earth or ground. Types of earthing or grounding system: by function Equipment Grounding: System or neutral grounding:

INTRODUCTION What is earth & ground As per IEC 60364- Earth: The conductive mass of the Earth, whose electric potential at any point is Conventionally taken as zero As per IEEE standard 80:2000-“Ground– A conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth.” So ground refers to the equipment grounding system, Such as metallic raceways, cable armor, enclosures, cabinets, frames, building steel, and all other non-current carrying metal parts of the electrical distribution system.

INTRODUCTION Why earthing or grounding is needed: The main aim of earthing is to maintain a zero potential or zero voltage of all non-current carrying parts of electrical system which has the probability of electrified by some fault. If by some means the non-current carrying parts are electrified which is not earthed, the person touching it will receive a lethal shock. So by earthing the non-current carrying parts are connected to earth and maintained at earth potential. It also prevents static charge buildup. Also the earthing is used to release the fault current from the electrical system, which are describe below in the section of types of earthing.

INTRODUCTION Objectives of earthing or grounding Fixing the potential of live conductors with respect to the earth in normal operation, Limiting voltage between the non-current carrying parts in electrical system ( such as frames of electrical equipment) and the earth should an insulation fault occur. Implementing protection devices which remove the risk of electric shocks or electrocution of person; In low voltage network limiting rises in potential due to Medium Voltage faults.

TOLERABLE LIMITS OF BODY CURRENTS

SOIL RESISTIVITY

SOIL RESISTIVITY

SOIL RESISTIVITY

RESISTIVITY OF DIFFERENT SOILS

EFFECT OF MOISTURE ON SOIL RESISTIVITY

EFFECT OF SALT ON SOIL RESISTIVITY

EFFECT OF TEMPERATURE ON SOIL RESISTIVITY

STEP AND TOUCH POTENTIALS Step Potential: Voltage between the feet of a person When current is flowing from the tower to the earth ground, the ground potential rises at the tower and a voltage gradient will occur based on the resistivity of the soil, resulting in a potential difference between two points on the ground. This is called a Step Potential as it can cause voltage between a person’s feet.

STEP AND TOUCH POTENTIALS Touch Potential: Voltage between energized object and feet of a person If the ground connection between the tower and the soil is high resistance (common with some soil conditions), the tower itself (and any conductive item touching the tower) can be energized. Touch potential is the voltage between the energized object and the feet of a person in contact with the object.

STEP AND TOUCH POTENTIALS

MEASUREMENT OF EARTH RESISTANCE Fall of potential method

MEASUREMENT OF EARTH RESISTANCE Earth tester

MEASUREMENT OF SOIL RESISTIVITY

EQUIPMENT GROUNDING The process of connecting non-current-carrying metal parts (i.e. metallic enclosure) of the electri-cal equipment to earth (i.e. soil) in such a way that in case of insulation failure, the enclosure effectively remains at earth potential is called equipment grounding. (i) Ungrounded enclosure

EQUIPMENT GROUNDING (ii) Enclosure connected to neutral wire.

EQUIPMENT GROUNDING (iii) Ground wire connected to enclosure.

SYSTEM GROUNDING The process of connecting some electrical part of the power system (e.g. neutral point of a star-connected system, one conductor of the secondary of a transformer etc.) to earth (i.e. soil) is called system grounding.

SYSTEM GROUNDING

NEUTRAL GROUNDING The process of connecting neutral point of 3-phase system to earth (i.e. soil) either directly or through some circuit element (e.g. resistance, reactance etc.) is called neutral grounding.

ADVANTAGES OF NEUTRAL GROUNDING (i) Voltages of the healthy phases do not exceed line to ground voltages i.e. they remain nearly constant.   (ii) The high voltages due to arcing grounds are eliminated. (iii) The protective relays can be used to provide protection against earth faults. In case earth fault occurs on any line, the protective relay will operate to isolate the faulty line. (iv) The overvoltages due to lightning are discharged to earth. (v) It provides greater safety to personnel and equipment. (vi) It provides improved service reliability. (vii) Operating and maintenance expenditures are reduced.

METHODS OF NEUTRAL GROUNDING Solid or effective grounding Resistance grounding Reactance grounding Peterson-coil grounding

SOLID GROUNDING When the neutral point of a 3-phase system (e.g. 3- phase generator, 3-phase transformer etc.) is directly *connected to earth (i.e. soil) through a wire of neg- ligible resistance and reactance, it is called solid grounding or effective grounding.

RESISTANCE GROUNDING When the neutral point of a 3-phase system (e.g. 3-phase generator, 3-phase transformer etc.) is connected to earth (i.e. soil) through a resistor, it is called resistance grounding.

REACTANCE GROUNDING In this system, a reactance is inserted between the neutral and ground as shown in fig. The purpose of reactance is to limit the earth fault current. By changing the earthing reactance, the earth fault current can to changed to obtain the conditions similar to that of solid grounding.

ARC SUPPRESSION COIL GROUNDING OR RESONANCE GROUNDING When the value of L of arc suppression coil is such that the fault current IF exactly balances the capacitive current IC, it is called resonant grounding.

VOLTAGE TRANSFORMER EARTHING

GROUNDING TRANSFORMER

MEASUREMENT OF TOWER FOOTING RESISTANCE

MEASUREMENT OF TOWER FOOTING RESISTANCE