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Considerable work has been done on the live detection of faulty elements in the insulators of overhead AC transmission lines but very little on their dc equivalents. One method that has proved successful for the AC case is the measurement of the electric field along the insulators, parallel to the axis and at the rims of the sheds (composite insulators) or of the insulators in a string (porcelain insulators). The DC transmission-line voltage will always include ripple; therefore, it was considered possible that the field due to the ripple component, the ldquoharmonic electric field,rdquo could be used in a similar manner. This has proven to be the case as demonstrated by the results of the experiments, both in the field and the laboratory, which are presented in this paper. Field measurements showed that the harmonic electric field could be measured under live conditions and provide repeatable results, both on composite insulators and on porcelain insulator strings. Laboratory measurements of the harmonic electric field were made along healthy composite insulators and some with artificial faults, and along strings of healthy porcelain insulators and strings with a faulty insulator included. The results of laboratory experiments showed that the faults could be identified and located effectively.