Electricity to-day is playing an ever – increasing role in the lives of almost everyone in the civilized world – both in urban and in rural areas. Unfortunately it’s use brings with it a degree of danger, on the one hand due to the susceptibility of the human and animal frames to the effects of stray currents and on the other hand to the possibility of defects in electrical apparatus, whether in the supply authorities system or on the consumer’s premises.
Any method of providing protection against this danger will never be in use except in an emergency as no current flows in the protective system when the installation is sound. It is, therefore, imperative to ensure that the initial installation is as perfect as in humanly possible and that where possible, tests should be made to verify that the protective apparatus continues to be efficient.
In all methods of providing protection against the dangers associated with the use of electricity, earthing plays a very important role. By earthing we mean making a connection to the general mass of earth. It’s use is so widespread that at practically every point of the supply network, from the generators to the apparatus on the consumer’s premises, earth electrodes will be found. An unfortunate feature of the use of earthing is that little thought and study is given to the theoretical aspect of the problem and so electrodes are used which have no justification in theory, much less in practice.
Earthing is also of major importance in our efforts to increase the reliability of the supply service, as it helps to provide stability of voltage conditions, preventing excessive voltage peaks during disturbances, and also as a means of providing a measure of protection against lightning. In this latter application, the earthing system may be distributed along a high voltage line, the individual electrodes being connected together through an over head “ground” wire which acts as a form of shield to the live conductors underneath. On the other hand, the individual steel towers provide an earth in themselves, which may be reduced still further by connecting other electrodes in parallel and so providing a low-resistance path to earth for lightning surges at each tower.
For protecting buildings and transformer stations from lightning strokes, lightning arrestors are often used which must also be provided with a low earth-resistance connection to enable the very large currents encountered, to be effectively discharged to the general mass of the earth.
There are, indeed, numerous other uses for which earthing is adopted; each one demanding from the system used, specific characteristics which must be given careful consideration. It is, however, generally accepted that provided the installation has a low resistance to earth, it will carry out its functions satisfactorily. What then has to be decided in most cases is how low this resistance must be required in each case and to give as much information as possible on the technique of obtaining such low-resistance earths as are deemed necessary.
It is common knowledge that Earthing, and especially Bonding, plays a most dominating role when it comes to personnel safety and prevention of fire and explosions but what is seldom appreciated is that EARTHING plays a fundamental role in preventing over-voltage conditions. Hence it imparts on the short and long term LIFE of electrical equipment. At the low cost of implementation there is no measure that is most cost effective.
Earthing can be divided into Neutral earthing and Equipment earthing. Neutral earthing deals with the earthing of system neutral to ensure system security and protection,where as equipment earthing deals with earthing of non- current carrying parts of equipment to ensure safety to personnel and protection against lightning.
Depending on the type of installation i.e, generating station, H.V. substation, transformer centre, pole/tower and consumer installations, suitable earthing system has to be designed duly taking into consideration, the various requirements such as fault current, limiting of earth potential rise, safety of nearby communication circuits and safe body currents etc.
The effect of electric current passing through vital organs of the body depends on magnitude, duration and frequency of current. The most dangerous consequence is a heart condition known as ventricular fibrillation, which results in stoppage of blood circulation.
a) Effect of magnitude of current :- The threshold of perception is a current of 1 m.A. Currents in the range of 1-6 m.A are known as ‘ let go current ‘ because these currents, though unpleasant, do not impair the ability of a person, holding an energised object to release it. Currents in the 9-25 mA range may be painful and impair the ability to release energised object. Still higher currents make breathing difficult. However, if the current is less than about 60 mA, the effects are not permanent and disappear when current is interrupted. Currents higher than 60 mA may lead to ventricular fibrillation, injury and death.
b) Effect of duration of current: The magnitude of 50 HZ tolerable current is related to duration. According to tests reported by Dalziel, 99.5% of persons of 50 Kg weight can withstand the current given by equation.
IB = 0.116 / ^t
Where IB is the rms value of body current in amperes and ‘t’ is the time in seconds. If the weight of body is 70 Kg., the equation for tolerable current is
IB = 0.157 /^t
These equations are valid for 0.03 < t < 3 seconds.
Effect of frequency :- The tolerable currents mentioned above are for 50 - 60 Hz. It has been found that human body can tolerate about 5 times higher direct current. At high frequencies (3000 – 10000 Hz) still higher currents can be tolerated.
The soil resistivity depends on the following factors:-
For a H.V. station earthing the two important factors to be considered are Earth potential rise and safe touch and step potentials.
It is to be noted that limiting the step and touch potentials to safe value is more important than attaining a low value of the resistance. However the earth resistance of the sub-station has to be brought down to the lowest possible level. The safe value of earth resistance for any sub-station depends upon not only the level of fault current and the resistance but also on the vicinity to communication stations.