Quote:
'...Subjecting the cable to what is conceptually a very low frequency indeed AC (periodic smooth polarity reversals of voltage taking what from memory was hours per cycle) was found to be beneficial by conditioning the insulation to relieve dielectric stress at any voids or imperfections so that insulation breakdown was less likely to occur.'
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That's how you're meant to pressure-test XLPE (cross-layered polyethylene) high-voltage cable, and for similar reason (0.1Hz or something like) Although I was told on a HV SAP refresher course once that it was only because of imperfections in manufacturing that this was the case.
D.C. pressure-testing is good to a point but not recommended.