A Project Report by :-
COURSE No. DIT-2, IRIEEN, NASIK
1.0 Earthing :- Earthing or earthing system is the total of all means and measures by which part of an electrical circuit, accessible conductive parts of electrical equipment (exposed conductive parts) or conductive parts in the vicinity of an electrical installation (extraneous conductive parts) are connected to general conductive mass of the earth, whose electric potential at any point is conventionally taken as zero. 2.0 Requirements of earthing :- Requirements of a good earthing system are :- a) Protection of buildings and installations against lightning. b) Safety of human/animal life by limiting touch and step voltages to safe values. c) Electro Magnetic Compatibility (EMC) i.e. limitation of electromagnetic disturbances to equipments and systems.
d) Correct operation of the electricity supply network and to ensure proper line voltage and good power quality with reduced harmonics. 3.0 Evolution of earthing :- In France in 1923 a Standard for electrical installations made earthing of frames a compulsory requirement, for casings of fixed and moving motors which may be touched in uninsulated area, in installations with a voltage greater than 150V and for fixed and portable electrical household appliances with a power rating greater than 4 kW. In 1927, a Decree stipulated the earthing of the transformer neutral in public distribution in France compulsory for Voltages above 150 V AC.
Thus the first earthing system came into existence in the form of the unearthed neutral. The Permanent Insulation Monitor (PIM), with three lamps (in three-phase) was used up to 1955.
The Lamp-Monitors continued till 1962, when alongside the Standard, the Decree of the 14.11.62 legalised the unearthed and the earthed neutrals. The development of Residual Current Device (RCD) or Short Circuit Protection Device (SCPD) or Earth Leakage Circuit Breaker (ELCB) and Lightning Arresters, revolutionized the earthing & protection. 4.0
Standards on earthing :- Following are some useful Standards to be referred :- *
IS:3043 – 1987 Reaffirmed 2006: Indian Standard Code of Practice for Earthing. *
IEEE Std. 142-1991: IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems *
IEEE Std. 80-2000 IEEE Guide for Safety in AC Substation Grounding. *
Indian Electricity Rules : 1956 & National Electric Code by BIS : SP : 1985 5.0
Effects of earth fault :- As shown in following diagrams :- a)
Supply is earthed, but metal-casing of equipment is un-earthed, causing probability of electrical shock to a person, touching it, in case of internal fault causing live wire coming in contact with the enclosure.
In this second instance, casing is earthed, allowing fuse to blow, making it safe. c)
In this third instance, location of fault within casing, goes undetected, but is safe.
So, following factors govern the protection by means of earth fault current :- i)
Rating of fuse or breaker.
Impedance of earth loop circuit of return path.
Impedance of the fault itself, e.g. arc having high impedance. iv)
Location of earth-fault on supply-side or neutral-side of load. v)
Continuity of earth wire between casing and equipment & system earth. 6.0
Types of earthing :- There are basically six types of earthing as described below :
1. equipment grounds,
2. static grounds,
3. systems grounds,
4. maintenance grounds,
5. electronic grounds and
6. lightning grounds.
Each of the above is explained below :-
Equipment Grounds :- An equipment ground is the physical connection to earth of non-current carrying metal parts. This type of grounding is done so that all metal parts of equipment that personnel can come into contact with, are always at or near zero (0) volts...
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