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C AT H O D I C P R O T E C T I O N Stray Alternating Current Problems in Concrete Power Poles FIguRE 4 (a) Insulators mounted on a metal cross arm, and (b) the location of rebar in a destroyed concrete pole, which is very close to a hole for mounting a metal pin. As observed, one of the rebar is tangent to the holes. FIguRE 5 Large stones alone must not be used as backfll in a mounting cavity. (a) Incorrect mounting and (b) a correct mount. durability of the concrete. The concrete Thus, the total volume of pores in the cover cracked and then rebar corrosion CCS pole should be much less than the started. RCS pole. For this reason, the RCS pole absorbed much more water than the Solutions Used CCS pole. Therefore, the reduction of electrical resistance of the RCS pole in Concrete Quality the rain should be much greater than in Two types of concrete power poles the CCS pole, and high stray AC should were used in the study area: circular cross occur only in the RCS poles. Also, with section (CCS) and rectangular cross sec- a lower total volume of capillary pores, tion (RCS). Based on feld observations, concrete permeability decreases.3 For the CCS poles had good durability without this reason, the CCS poles had good any stray AC cracks or rebar corrosion. durability. The RCS concrete poles were The RCS poles were manufactured replaced with CCS poles. by pouring concrete into a mold. The water/cement ratio selected was ~0.6. Insulator Mounting The CCS poles were manufactured by Insulators can be mounted directly or centrifugal casting and water was re- indirectly on power poles. In the direct moved by centrifugal force before curing. method, insulators are mounted on a 38 MATERIALS PERFORMANCE May 2013 power pole without a cross arm; but in the indirect method, cross arms are used to mount insulators on power poles.4 Cross arms are made of metal, wood, or pultruded polymer composite. It is much more diffcult for current to creep from the surface of insulators mounted on nonmetal cross arms than insulators mounted on metal cross arms. In the area of study, metal cross arms had been used for mounting insulators on the power poles. The metal cross arm was connected to the concrete pole with several metal pins. As shown in Figure 4, the pins passed close to the rebar. In rainy conditions, the concrete became saturated with water and the empty space between the metal pins and concrete flled with water. The resistance between the power lines and ground was reduced considerably and the probability of current creep on the surface of the insulators increased. Therefore, the metal cross arms were replaced with polymer composite cross arms. Concrete Power Pole Mounting The method of mounting the concrete poles in the ground is another important factor that infuenced the occurrence of stray AC. Increasing the resistance of the backfll and increasing water drainage around the concrete pole decreases the probability of current leakage from power NACE International, Vol. 52, No. 5