Most cable engineers agree with Zog's assesment of DC high potential testing.
That said, the theoretical DC leakage current can be calculated if you have good data for the specific insulation resistance of the cable insulation in Megohms-km and data on the cable geometry.
IR = K log(D/d) Where K = specific insulation resistance, D= diameter over insulation and d= diameter under insulation.
IR is inversely protportional to length. A 500 meter cable will have twice the IR of a 1km cable.
Typical values of K in Mohm-1000ft: PVC = 2000 , XLPE = 20,000, EPR = 20,000 - 50,000.
Leakage current will be = V/IR, if we neglect a few other efffects and hold the DC for a long period of time, which can damage the cables.
My field testing experience is that calculated theoretical leakage curent and actual test results are orders of magnitude apart mostly due to leakage at the terminations and test connections.
AC "leakage" is the cable charging current. I= 2 pi f C e, where f= test frequency, C= Capacitance per km, e= applied voltage phase to neutral, kV.
Cable capacitance can be estimated by C= 7.35 x (SIC) / log(D/d) in picofarads per foot.
SIC is the dielectric conatant of the insulation: = 3.5-8.0 for PVC, = 2.8-3.5 for EPR, 2.3-6.0 for XLPE.
