IEC 60141-3:1963 pdf download 充油和气压电缆及其附件的试验 - 第3部分:275kV以下交流电压用外部气压(气压)电缆及其附件 Tests on oil-filled and gas-pressure cables and their accessories - Part 3: External gas-pressure (gas compression) cables and accessories for alternating voltages up to 275 kV