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B - Connection to the MV publicdistribution network1 Supply of power at mediumvoltageIn order to ensure adequate protection of equipment against abnormally-mediumshort term power-frequency overvoltages, and transient overvoltages caused bylightning, switching, and system fault conditions, etc. all MV equipment must bespecified to have appropriate rated insulation levels.A "rated insulation level" is a set of specified dielectric withstand values coveringvarious operating conditions. For MV equipment, in addition to the "highest voltagefor equipment", it includes lightning impulse withstand and short-duration powerfrequency withstand.SwitchgearFigure B2 shown below, lists normal values of “withstand” voltage requirementsfrom IEC 62271-1 Standard. The choice between List 1 and List 2 values of tableB2 depends on the degree of exposure to lightning and switching overvoltages (1) ,the type of neutral earthing, and the type of overvoltage protection devices, etc. (forfurther guidance reference should be made to IEC 60071).BRated Rated lightning impulse withstand voltage Rated short-durationvoltage (peak value) power-frequencyU (r.m.s.withstand voltagevalue) List List 2 (r.m.s. value)To earth, Across the To earth, Across the To earth, Across thebetween isolating between isolating between isolatingpoles distance poles distance poles distanceand across and across and acrossopen open openswitching switching switchingdevice device device(kV) (kV) (kV) (kV) (kV) (kV) (kV)3.6 20 23 40 46 10 127.2 40 46 60 70 20 2312 60 70 75 85 28 3217.5 75 85 95 110 38 4524 95 110 125 145 50 6036 145 165 170 195 70 8052 - - 250 290 95 11072.5 - - 325 375 140 160Note: The withstand voltage values “across the isolating distance” are valid only forthe switching devices where the clearance between open contacts is designed to meetrequirements specified for disconnectors (isolators).Fig. B2 : Switchgear rated insulation levelsIt should be noted that, at the voltage levels in question, no switching overvoltageratings are mentioned. This is because overvoltages due to switching transients areless severe at these voltage levels than those due to lightning.TransformersFigure B3 shown below have been extracted from IEC 60076-3.The significance of list 1 and list 2 is the same as that for the switchgear table, i.e.the choice depends on the degree of exposure to lightning, etc.(1) This means basically that List 1 generally applies toswitchgear to be used on underground-cable systems whileList 2 is chosen for switchgear to be used on overhead-linesystems.Highest voltage Rated short duration Rated lightning impulsefor equipment power frequency withstand voltage(r.m.s.) withstand voltage (peak)(r.m.s.) List 1 List 2(kV) (kV) (kV) (kV)y 1.1 3 - -3.6 10 20 407.2 20 40 6012 28 60 7517.5 38 75 9524 50 95 12536 70 145 17052 95 25072.5 140 325Fig. B3 : Transformers rated insulation levelsSchneider Electric - Electrical installation guide 2009© Schneider Electric - all rights reserved