The paper summarizes some basic problems of on-line monitoring for electrical equipment in substation and points out that on-line monitoring is a necessary method in state maintenance.The paper introduces the basic situation of on-line insulation monitoring equipment in substation

and analyzes several untypical phenomena in on-line monitoring for electrical equipment based on a great quantity of on-line monitoring data.And this paper also analyzes some possible factors that will influence on-line monitoring data

such as poor stability of dielectric loss measurement

high operation fault rate of signal collection

data analysis not based on substation operation mode

and lack of necessary analysis and judgement in on-line monitoring system.