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INTRODUCTION
  
INTRODUCTION






The most underground structures have
been electrically bonded incommon to reduce hazardous voltages associated with lightning and man-made fault currents or induced currents in the earth.
A common grounding system provides an economical and lower resistance to remote earth than does an individual earthing connection. This tends to ensure a low resistance return path for power system earth return currents and fault currents.
An additional benefit is minimizing earth potential gradients around individual earthing electrodes or elements.
It also tends to reduce step and touch voltages at the surface of the earth.
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Characteristics of a Good Grounding System
 •Good electrical conductivity that causes
Low resistance and electrical impedance.Withstanding high fault currents with no evidence of fusing or mechanical deterioration in the event of a foreseeable fault.
Energy is dissipated into the ground in the safest possible way.
High frequency lightning impulses will flow through the ground electrode path. in preference to any other.

 •Good corrosion resistance
Electrically interconnecting many dissimilar metals in the soil environment can lead to significantly increased corrosion rates on some of the underground structures.
In addition to its inherent high conductivity copper is usually cathodic with respect to
other metals in association with grounding sites.

 •Mechanically robust, reliable and ability to perform for atleast 40 years working life-time for a facility

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Selection of Conductors and Related Corrosion Problems


When materials such as black iron (BI) cast iron (CI) and ductile iron (DI) are interconnected. they are very close together in the electromotive series of metals and therefore, each would suffer very little additional corrosion by connecting to the other metal. When a dissimilar metal couple is created by connecting BI, Cl, or DI to copper or brass a significant corrosion cell is created.
Copper is electropositive with respect to all ferrous construction materials. In addition copper will not polarize readily as is the case for ferrous structures. Therefor accelerated corrosion is the result on ferrous structures whenever they are directly coupled to bare copper in the soil.
Tinning of copper has been tried by some utilities, that reduces the corrosion cell with respect to steel and zinc by about 5O% and practically eliminates this potential with respect to lead.
Most electrical engineers specify copper for grounding grid since it is the preferred material of choice for electrical conductivity. However, when copper is directly burried in the soil and completely isolated from other construction materials, it will corrode.
In acidic soil conditions, the corrosion rate of copper may be greater than that of iron or steel.
Aluminium has been used for ground grid less frequently. Though at first glance the use of aluminum would seem to be a natural choice for gas insulated substation (GIS), equipment since the enclosures are made of aluminum or aluminum alloys.
Steel has been used for ground-grid conductors in many european countries mainly for the benefit of eliminating most of the adverse effects of copper already mentioned.
Application of galvanized or stainless steel in combination with cathodic protection is typical.