High － voltage single － core cable is widely used in urban transmission network，but it will produce a different degree of floating potential when its grounding system is abnormal． Based on the cause analysis of the burning failure of the outer sheath of 110 kV high － voltage AC cable，the possible floating potential of actual cable line is discussed and studied． The floating potential after the aluminum sheath grounding disappears is proportional to the line voltage level and is related to the grounding state of the graphite ( semi － conductive) layer of the cable． When the direct grounding is disconnected and the graphite layer is not effectively grounded，the floating voltage of graphite layer of the cable may reach more than 90% of the phase voltage，and the high floating voltage will inevitably lead to the high － frequency charge and discharge of the graphite layer on the weak ground insulation，and the constantly ablation of the outer sheath，causing fire and other serious consequences．