Before installation on overhead lines, quality of polymeric insulators must be approved through full design and type testing. Subsequently, manufacturers are responsible for performing sample and routine tests. Moreover, in many cases, users also demand special acceptance tests.

This edited paper, contributed to INMR by Jan Lachman and colleagues at EGU HV Laboratory, discusses key learning points from design and type testing these insulators over a period of 25 years. For example, long-term testing has shown that failures of polymeric insulators occur mostly during interface testing. Indeed, while the current IEC 62217 standard dealing with polymeric insulators specifies tests on interfaces to verify quality, experience has shown that composite insulators with poor housing adhesion sometimes still pass. For that reason, IEC 62217 is now under revision by IEC TC 36 / MT 19 with the goal of stricter criteria to increase capability of detecting defective insulators through interface testing.

In 1995, polymeric long-rod insulators from 6 manufacturers were installed on the same 110?kV line in the Czech Republic.shows the appearance of one of these insulators, with a silicone rubber housing, as revealed during inspections conducted in 2000 and 2006. All insulators evaluated during these inspections were in good to excellent condition with respect to electrical and mechanical properties, although some color change, chalking, minor surface cracking and decrease in hydrophobicity were recorded.

A long-rod polymeric insulator with EPDM rubber housing was installed on one tower of a 400 kV line in early 1990. Chalking of the housing was noted and all insulator samples had become fully hydrophilic. While there was no evidence of tracking or erosion, the lower part of a bottom shed showed cracks, most probably due to exposure to corona. An improper design of corona ring design was later identified.