DC testing is not obsolete everywhere. It remains a valid method for paper-insulated lead-covered (PILC) cable, for some generator and motor insulation resistance tests, and for DC withstand of certain switchgear and converter equipment. The distinction is the dielectric: oil-impregnated paper tolerates DC stress and does not trap charge in the same way as a polymer. Applying a DC hipot to XLPE cable, however, is no longer defensible either technically or contractually.
DC hipot testing of XLPE cable is a legacy technique that conflicts with the physics of the dielectric it is applied to. It traps charge that later destabilises the cable, it extinguishes the partial discharge that would identify defects, it cannot detect water trees, and it is no longer recognised by the standards that govern cable testing. Its apparent advantages - light equipment and a simple pass/fail result - are now matched or exceeded by 0.1 Hz VLF sets that also deliver tan delta diagnostics.
The recommended practice for MV and HV extruded cable is a VLF withstand test at 3U₀ for 15 min to 60 min as the acceptance and maintenance test, supported by a tan delta measurement at 1.5U₀ to 2U₀ where condition assessment is required, in accordance with IEEE 400.2 and HD 620 / VDE 0276-620. For PILC cable, DC remains a valid alternative and should be retained where the asset base requires it.