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Exhibit 4 — Maintenance Requirements <br />Mobile Substation, Temporary Equipment, and Portable Cables Maintenance Intervals <br />4.1 Description <br />4.1.1 Mobile Substations and temporary equipment are vital to the overall <br />operations and maintenance functions of substation facilities. Due to <br />inherent safety risks and wear - and -tear, each unit requires special safety <br />and maintenance criteria. <br />4.2 Portable Cables: <br />4.2.1 Micro -Ohm Tests <br />❑ Measure shield resistance <br />4.2.2 DC Hi -Pot Tests <br />❑ Accepted industry test methods: <br />• Test cable per IEEE Std. 400 -1991 <br />• Test cable per Doble standard (Recommended) <br />Energization Test: Tests to be performed prior to energization <br />After verifying that all cables and potheads are in good physical condition, <br />energize cables for a minimum of 60 seconds prior to picking up load on <br />cables. NOTE: All personnel should maintain a safe working <br />distance from cables during this energization test. <br />4.3 Mobile Substation and Temporary Equipment Maintenance Activity Guidelines <br />(Intervals in months) <br />Mobile Substations and Temporary Equipment <br />Interval <br />Transformer: <br />DGA + mini iO (for units without hydrogen <br />12 <br />monitors) <br />DGA not necessary for units with hydrogen <br />N/A <br />monitors <br />Insulation /PF Tests <br />36 <br />Auxiliary Equipment OO <br />72 <br />LTC: <br />DGA +miniO <br />12 <br />Disassembly + Controls + Auxiliary Equipment OO <br />72 <br />Mobile - Memorandum of Understanding 4/28/2010 Page 8 of 11 <br />