We have developed Aa new method for diagnosing porcelain insulators of overhead lines with voltages equal to 6/10 kV based on the analysis of partial discharges characteristics. The value of the breakdown voltage obtained by the partial discharges method with a 5% accuracy is in good correlation with the results obtained by the traditional methods, such as direct resistance measurement and others. Breakdown voltages were calculated for garlands without switching off the applied voltage. The developed technique can be useful from a practical point of view to identify the insulators, containing the so-called ’zero’ insulators, whose resistance is dozens of times less than that of the defect-free insulators.