Earthing System – Material Selection, Life and Corrosion

An ideal earthing system especially the earth electrodes shall  offer long  life almost equal  to the

life of a building or at least 30 to 40 years. But often the system degrades and need maintenance, replacement or improvement. In general  maintenance  of  earthing LV D GLIÀFXOW WDVN IRU   WKH HQJLQHHU HVSHFLDOO\ LQ DQ

industrial  and  commercial  installation.  In  most  of the

cases corrosion is the main reason for deterioration of earthing system.

IS 3043-Code practice of earthing explains: The possibility on damage to cables and other underground VHUYLFHV DQG VWUXFWXUDO PHWDOZRUN LQ WKH YLFLQLW\ RI HDUWK-

electrode  due to electrolytic action  between  dissimilar

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for earth-electrodes is selected. Materials compatible with other metal structures in the vicinity should be VHOHFWHG RU RWKHU UHPHGLDO DFWLRQ WDNHQ

IS 62305 – 3 elaborates the subject as: It should also be remembered that reinforcing steel rods in concrete generate the same magnitude of galvanic potential as copper conductors in soil. A further problem arises from electrochemical corrosion due to galvanic currents. Therefore, when steel in concrete is connected to steel in soil,   a   driving   galvanic   voltage   of approximately

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soil and the wet concrete dissolve steel in soil. Earth electrodes in soil should use copper, copper coated steel or stainless steel conductors where these are connected to steel in concrete.

Earth electrodes in a large building consists of vertical and horizontal conductors interconnected, foundation steel, buried pipes etc. It is a common practice in India to use Copper for some application (eg transformer neutral) and GI for other (eg transformer body). Often

 

these electrodes are interconnected. Due to the awareness of bimetallic corrosion the interconnections are made with BI-Metals. But the corrosion due to galvanic  effect  is  never considered or neglected due to un awareness

Corrosion is an  electrochemical  process  in  which  one metal corrodes preferentially  to  another  when both metals are in electrical contact, in the presence of an electrolyte.

Metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal acts as anode and the other as  cathode.  The electro potential difference between the dissimilar PHWDOV LV WKH GULYLQJ IRUFH IRU DQ DFFHOHUDWHG DWWDFN RQ

the anode member  of  the  galvanic  couple.  The anode metal dissolves into the electrolyte, and deposit collects on the cathodic metal.

Electro Potential of metals (Anodic Index) as per DIN VDE 0151 / 06.86
Copper, solid or plated 0.0 to – 0.1 V
Steel in concrete – 0.1 to – 0.4 V
Hot-dip, galvanized steel (GI) – 0.9 to – 1.1 V

From the chart it is clear that Copper surface is  cathodic in nature, where as GI is anodic. GI dissolves (or corrodes and disappear) after some time if it is in contact with copper. Steel inside concrete foundation is DOVR FDWKRGLF OLNH FRSSHU. +HQFH FRUURVLRQ RI *, KDSSHQ

due to steel in foundation. Galvanic corrosion is one of the main reason for deterioration of GI earth electrodes reducing the life, which need frequent maintenance or replacement.

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