BS 7663:1993

This British Standard describes three methods for the measurement of permittivity and of dissipation factor of electrical insulating materials in the form of sheets or rigid circular tubes at frequencies between 50 Hz and 1 MHz, under normal ambient temperatures and conditions. The methods described are applicable to sheet materials in the thickness range 0.3 mm to 5.0 mm and for tubes at least 75 mm long.

The methods are:

• an air substitution method, commonly called the “Lynch” method (method A)
• a two fluid immersion method (method B)
• a single fluid immersion method (method C).

Methods A and B are suitable for materials of permittivity up to 10.0. For materials of dissipation factor below 250 × 10^–6 the precision achieved by these methods may not be acceptable.

Method A requires accurate knowledge of the specimen thickness and should be used in all cases when this information is available. If the thickness is not accurately known, methods B or C may be used subject to the limitation that method C is suitable for materials for which a liquid matching its permittivity to within 0.1 is available. Method C is suitable for measurement of polyolefin sheets whose permittivity sufficiently closely matches that of silicone oil of kinematic viscosity of 1 cSt to 2 cSt¹).

Methods B and C are not suitable for materials with an open cell structure.

Method B is suitable for measurements on straight cylindrical tubes of circular cross section.

Three methods are described at frequencies between 50 Hz and 1 MHz.