High vacuum, cold gases and liquids, and solids are the principal insulating materials for superconducting apparatus. All these insulants have been claimed to show fairly good intrinsic dielectric performance under laboratory conditions where small scale experiments in the short term range are typical. However, the insulants must be integrated into large scaled insulating systems which must withstand any particular stressing voltage seen by the actual apparatus over the full life period. Estimation of the amount of degradation needs a reliable extrapolation from small scare experimental data. The latter are reviewed in the light of new experimental data, and guidelines for extrapolation are discussed. No degradation may be seen in resistivity and permittivity. Dielectric losses in liquids, however, show some degradation, and breakdown as a statistical event must be scrutinized very critically. Although information for breakdown strength degradation in large systems is still fragmentary, some thumb rules can be recommended for design. (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 56]
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