Page 742 - Industrial Power Engineering and Applications Handbook

22/702 Industrial Power Engineering and Applications Handbook
r7 when determining the size of the ground conductor is to
V
R,=-!2 sustain the rated short-time ground fault current of the
I
system for the required duration, without damage to or
permanent deformation of the ground conductor and to
22.4 Metal for the grounding limit its temperature rise within permissible limits. It
conductor will also limit the voltage drop within 55 volts between
any two grounded points with which a human body may
come into contact. However, for all practical purposes,
Copper, aluminium, steel and galvanized iron are the the minimum size of conductor as determined below for
most widely used metals for the purpose of grounding. a required fault level will generally be adequate to limit
Choice of any of them will depend upon availability and the voltage drop within the safe limits. For more dctails
economics in addition to the climatic conditions (corrosion refer to IEC 60298.
effect) at the site of installation. In Table 22.3 we provide The ground conductor can be of aluminium, GI or
a brief comparison of these metals for the most appropriate copper, as discussed earlier. A humid or a chemically
choice of the metal for the required application. contaminated location is corroding in nature. Aluminium
has a rapid reaction and is fast corroding. At such locations,
22.4.1 Size of the grounding conductor use of GI or copper conductor would be more appropriate.
Table 22.4 suggests the ground conductor sizes for
This is a matter of system design and is different for LT aluminium conductor power cables for small and mcdium-
and HT systems, as discussed above. The main criterion rating feeders when aluminium is used for the ground

Table 22.3 Comparison of grounding metals

No. Churucteristics Copper Alum in iurn Steel Galvnnized irori
I 2 3 4
~ ~~ ~ ~~~~
1 Conductivity (96) 100 61 30-40 8.5
(for annealed copper) (for EC grade aluminium) (for copper-clad steel core) (for Zn-coated steel)
~ ~~ ~ ~
2 Resistance to High. Being cathodic Highly corrosive and is, Corrosive. Copper-clad High, and is extensively
corrosion with respect to other therefore, less preferred steel may be used to used for ground
metals, which may be compared to other overcome this deficiency connections and grids
buried in the vicinity metals, for underground
connections or ground
electrodes. For surface
connection\, however,
where it is less corrosive
and highly conductive,
compared to steel or steel
alloys it is preferred
3 Galvanic effect" Copper is a galvanic
metal and causes
corrosion, in the presence
of moisture, in nearby
metals, such as cable
sheathes, steel structure The are not galvanic but become anodic in the vicinity of copper and erode
and water, gas or drain
pipes, buried in its
vicinity. With all such
metals, it forms a
complete electrolytic
circuit and corrodes
them. Tinning may give
protection against its
galvanic effects but this
is an expensive
proposition
4 Approximate cost 100 50 IO 15
considerations (4) Therefore most appropriate
and economical
~ ~~ ~ ~ ~ ~ ~ ~~ ~
"Thia occurs when two dissimilar metals in an electrolyte have a metallic tie between them. There is a flow of electricity between the anodic
and cathodic metal surfaces, generated by the local cells set between dissimilar metals. One metal becomes an anode and the other a cathode
and causes an anodic reaction which represents acquisition of charges by the corroding metal. The anode corrodes and protects the cathode,
as current flows through the electrolyte between them.