What is the Method of test for power factor and  permittivity ?

Take samples of core of length not less than 5 m, from the complete cable and immerse them in water  for 24 h at room temperature with a length of about 250 mm at each end of the core projecting above  the water. Measure the power factor and capacitance of the insulation between the conductor and the  water with suitable equipment at any convenient frequency between 40 Hz and 62 Hz, at the rated  voltage to earth, U  , of the cable and at a temperature 20 ± 1 °C.

P=[41.4*C*LOG10(D/d)]/L

Where

P is the permittivity of the insulation;
C is the measured capacitance (in nF) of the sample;
D is the diameter (in mm) over insulation;
d is the diameter (in mm) over conductor;
L is the immersed length (in m) of core.

Source BS 6469-99.2