You can consider:

• 3rd or 5th harmonics value of the total leakage current (applied voltage must not contain harmonics),
• the active losses,
• IR profile,
• nonlinearity factor,
• partial discharges (PD).

External pollution extremely influences the total leakage current and partial discharges.

you can consider:

* the non-linear resistance having V-I caractéristics of the varistor.

* the stray capacitances. 

*voltage drop of the dry band formation in the housing.

*both of internal and external leakage current.

*housing profil