standards and guidelines for communication sites - Radio And ...
standards and guidelines for communication sites - Radio And ... standards and guidelines for communication sites - Radio And ...
DISSIMILAR METALS AND CORROSION CONTROL CHAPTER 4: EXTERNAL GROUNDING (EARTHING) TABLE 4-4 TOWER GROUND BUS BAR SPECIFICATIONS Item Specification Material Bare, solid Alloy 110 (99.9%) copper bus bar or plate of one piece construction. Should be tin-plated if installing on a galvanized tower. (See “Dissimilar Metals and Corrosion Control” on page 4-34 for information regarding tower corrosion related to copper bus bars.) Minimum Dimensions (ANSI-J-STD-607-A-2002 and NFPA 70-2005, Article 250.64) Mounting brackets Stainless steel Height: 51 mm (2 in.) Thickness:6.35 mm (0.25 in.) Length: Variable to meet the application requirements and allow for future growth. 305 mm (12 in.) is recommended as the minimum length. Conductor mounting holes Number dependent on number of conductors to be attached Holes to be 11 mm (0.4375 in.) minimum on 19 mm (0.75 in.) centers to permit the convenient use of two-hole lugs. Method of attachment of grounding electrode conductor Exothermic welding Irreversible crimp connection 4.5 DISSIMILAR METALS AND CORROSION CONTROL Although the type of metals used in a grounding (earthing) electrode system do not affect the resistance to earth of the grounding electrode system, consideration should be given to select a metal that is resistance to corrosion in the type of soil in which it will be installed. The two areas that should be considered regarding the corrosion resistance of a metal are the compatibility with the soil itself and possible galvanic corrosion effects when it is electrically connected to neighboring metals at the site. (BS 7430:1998) 4.5.1 CORROSION RELATED TO SOIL TYPE The compatibility of a metal with soil is determined by the chemical composition of the soil. The chemical composition factors associated with the corrosion of metals in contact with the soil are as follows: acidity or alkalinity (pH), salt content, soil porosity (aeration), and the presence of anaerobic bacteria. (BS 7430:1998 and TIA/EIA-222-F-R2003) The following list gives a general representation of the aggressiveness of soils, listed in order of increasing aggressiveness (BS 7430:1998): • Gravelly soils (Least Aggressive) • Sandy soils • Silty soils (loam) • Clays • Peat and other organic soils • Made up soils containing cinders (Most Aggressive) 4-34 68P81089E50-B 9/1/05
STANDARDS AND GUIDELINES FOR COMMUNICATION SITES DISSIMILAR METALS AND CORROSION CONTROL The least aggressive soils tend to be those having a high resistivity. The resistivity of soil can be measured, which provides an indication of corrosiveness under aerated conditions (BS 7430:1998). See “Performing Soil Resistivity Test” on page B-6 for measurement details. Soil with a resistivity below 2,000 ohm centimeters (Ω·cm) is generally considered to be highly corrosive (TIA/EIA-222-F-R2003). More details about the aggressiveness of soils can be obtained by measuring the redox (from the words reduction and oxidation) potential of the soil, which indicates the risk of corrosion due to the presence of anaerobic bacteria (BS 7430:1998). Test equipment required to measure redox potential is commercially available. The procedure required to test the redox potential can be found in ISO 11271:2002(E). A geotechnical firm may be required to measure the redox potential of the soil. General guidance on the corrosiveness of some grounding electrode system metals in relation to soil composition is given below in Table 4-5 (BS 7430:1998). A geotechnical firm may be required to determine all of the listed soil parameters. TABLE 4-5 CORROSION RESISTANCE PROPERTIES OF COMMON GROUNDING ELECTRODE SYSTEM METALS AS RELATED TO SOIL COMPOSITION Soil Parameter Grounding Electrode Metal Copper Galvanized Steel Austenitic Stainless Steel* < 700 Slightly Reduced Moderately Reduced Slightly Reduced Resistivity (Ω·cm) 700 to 4000 Slightly Reduced Moderately Reduced Generally Unaffected > 4000 Generally Unaffected Generally Unaffected Generally Unaffected < 200 Moderately Reduced Considerably Reduced Moderately Reduced Redox Potential (mV) 200 to 400 Slightly Reduced Slightly Reduced Generally Unaffected > 400 Generally Unaffected Generally Unaffected Generally Unaffected < 10 Generally Unaffected Generally Unaffected Generally Unaffected Moisture Content (%) 10 to 80 Slightly Reduced Moderately Reduced Slightly Reduced Dissolved > 80 Slightly Reduced Slightly Reduced Slightly Reduced Salts Moderately Reduced Moderately Reduced Slightly Reduced Chlorides Moderately Reduced Moderately Reduced Moderately Reduced pH Acidic < 6 Moderately Reduced Considerably Reduced Slightly Reduced Neutral 6 to 8 Generally Unaffected Generally Unaffected Generally Unaffected Alkaline > 8 Slightly Reduced Moderately Reduced Generally Unaffected Organic Acids > 8 Considerably Reduced Moderately Reduced Slightly Reduced * Austenitic stainless steel shall be formed from 18% chromium and 8% nickel (18/8 stainless steel), per UL 467-2004, section 9.2.6. Table based on information from BS 7430:1998. 68P81089E50-B 9/1/05 4-35
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STANDARDS AND GUIDELINES FOR COMMUNICATION SITES DISSIMILAR METALS AND CORROSION CONTROL<br />
The least aggressive soils tend to be those having a high resistivity. The resistivity of soil can be<br />
measured, which provides an indication of corrosiveness under aerated conditions (BS 7430:1998). See<br />
“Per<strong>for</strong>ming Soil Resistivity Test” on page B-6 <strong>for</strong> measurement details. Soil with a resistivity below<br />
2,000 ohm centimeters (Ω·cm) is generally considered to be highly corrosive (TIA/EIA-222-F-R2003).<br />
More details about the aggressiveness of soils can be obtained by measuring the redox (from the words<br />
reduction <strong>and</strong> oxidation) potential of the soil, which indicates the risk of corrosion due to the presence<br />
of anaerobic bacteria (BS 7430:1998). Test equipment required to measure redox potential is<br />
commercially available. The procedure required to test the redox potential can be found in ISO<br />
11271:2002(E). A geotechnical firm may be required to measure the redox potential of the soil.<br />
General guidance on the corrosiveness of some grounding electrode system metals in relation to soil<br />
composition is given below in Table 4-5 (BS 7430:1998). A geotechnical firm may be required to<br />
determine all of the listed soil parameters.<br />
TABLE 4-5 CORROSION RESISTANCE PROPERTIES OF COMMON GROUNDING ELECTRODE SYSTEM METALS AS RELATED<br />
TO SOIL COMPOSITION<br />
Soil Parameter<br />
Grounding Electrode Metal<br />
Copper Galvanized Steel<br />
Austenitic Stainless<br />
Steel*<br />
< 700 Slightly Reduced Moderately Reduced Slightly Reduced<br />
Resistivity (Ω·cm) 700 to 4000 Slightly Reduced Moderately Reduced Generally Unaffected<br />
> 4000 Generally Unaffected Generally Unaffected Generally Unaffected<br />
< 200 Moderately Reduced Considerably Reduced Moderately Reduced<br />
Redox Potential (mV) 200 to 400 Slightly Reduced Slightly Reduced Generally Unaffected<br />
> 400 Generally Unaffected Generally Unaffected Generally Unaffected<br />
< 10 Generally Unaffected Generally Unaffected Generally Unaffected<br />
Moisture Content (%) 10 to 80 Slightly Reduced Moderately Reduced Slightly Reduced<br />
Dissolved<br />
> 80 Slightly Reduced Slightly Reduced Slightly Reduced<br />
Salts Moderately Reduced Moderately Reduced Slightly Reduced<br />
Chlorides Moderately Reduced Moderately Reduced Moderately Reduced<br />
pH<br />
Acidic < 6 Moderately Reduced Considerably Reduced Slightly Reduced<br />
Neutral 6 to 8 Generally Unaffected Generally Unaffected Generally Unaffected<br />
Alkaline > 8 Slightly Reduced Moderately Reduced Generally Unaffected<br />
Organic Acids > 8 Considerably Reduced Moderately Reduced Slightly Reduced<br />
* Austenitic stainless steel shall be <strong>for</strong>med from 18% chromium <strong>and</strong> 8% nickel (18/8 stainless steel), per UL 467-2004, section 9.2.6.<br />
Table based on in<strong>for</strong>mation from BS 7430:1998.<br />
68P81089E50-B 9/1/05 4-35
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