Abstract: The mistakes sometimes will be made by judging the insulating property of insulating flange with the electrometric method, and which originate from a important factor that the electrometric method does not involve the pipe/soil potential at the unprotected end; that is the earth resistance of pipe end unprotected by the insulating flange. Based on Ohm's law, the polarization equations for the pipe end unprotected by flange have been derived, which shows that the pipe/soil potential at the unprotected end of insulating flange depends on the insulating resistance of insulating flauge, its pipe/soil potential at the protected end, the natural potential at the protected end, and its earth resistance.