E.3.1 Determination of intrinsic bulk conductivity by direct measurement

This can be performed by sensitive measurement of current through a sample of the dielectric, when a fixed potential of up to 500V is applied. The experimental arrangement for a planar sample is shown in Figure E-11. Note that a guard electrode is used to eliminate surface conductivity effects. For cables, the use of a coaxial version of the cable, eliminates the need for a guard electrode. See Figure E-12.

Figure E-11: Basic arrangement of apparatus for measuring dielectric conductivity in planar samples

Figure E-12: Arrangement for measuring cable dielectric conductivity and cross-section through co-axial cable

The test is continued until the recorded current has reached an equilibrium value. Note that the industry standard of measuring conductivity 60s after application of the potential produces a result that is contaminated by polarization effects. It is the equilibrium value that is the current due to intrinsic bulk conductivity.

The test reports the conductivity at the lowest operating temperature of the equipment (generally -10°C), although this can be done by carrying out the tests at a pair of somewhat higher temperatures (up to 50°C) and extrapolating down to -10°C.

The test is carried out on a sample of dielectric that is the same as being used in the design, i.e. same parameters, for example colour and thickness.