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Laboratory studies were conducted on gapped silicon-carbide arresters and gapless metal-oxide surge arresters (MOAs) gathered from 24 kV networks in Finland. A total of 410 arresters of 21 different types from eight different manufacturers were investigated. A total of 246 silicon-carbide arresters (age from 14 to 38 years) and 164 MOAs (age from 4 to 15 years) were studied at the High Voltage Laboratory of Tampere University of Technology. The results of the measurements on MOAs were compared to the measurements conducted earlier on 16 new, unused MOAs of the same types as those taken from the field. A portion of 16.7% (41 specimens) of the silicon-carbide arresters failed in the ac voltage withstand test and 34.5% (85 specimens) did not pass the lightning impulse sparkover test. The current impulse test of the silicon-carbide arresters resulted in a lot of damage. Due to the damage and failures in the electrical tests the authors recommend the electricity companies in Finland to exchange 7 types of silicon-carbide arresters to new MOA types in order to reduce the risk of arrester failures and to improve the protection levels in MV networks. All the ten MOA types studied were generally in good condition after being used 4-15 years in networks. Only three specimens (out of 164) could be evaluated to be faulty. Comparisons between the MOAs gathered from the networks and the unused MOAs showed that the protection levels of the MOA types studied had remained stable and the ac durability of these MOA types had not remarkably weakened during the use in networks.