Underground plant includes all the telecommunications equipment installed in underground structures such as utility holes, Controlled Environment Vaults (CEVs), and ducts, along with associated hardware. Underground plant can be exposed to waters containing water-soluble salts of the native soil. Utility holes often show evidence of corrosion of support hardware and bonding ribbons that is caused by sulfate-reducing bacteria. The environment in utility holes and ducts can be made corrosive by man-made chemicals such as industrial effluent, fertilizers, and de-icing salts. Protective plastic coatings and cable jackets can rapidly deteriorate from leaking steam pipes present in many urban areas and from gasoline leaking from underground storage tanks.
The most aggressive contributor to corrosion of underground plant is dc stray current from electrified rail transportation systems, cathodic protection rectifiers, or welding and mining operations. Although such dc currents are mostly dealt with “after the fact” using protective systems (e.g., low resistance bonds, reverse current switches, cathodic protection), some of the protection has to be included at the manufacturing stage. This protection may include insulating covers on cable shields, or nonmetallic components or coatings for apparatus.