The very application calls for special designs. The internal inductance must be minimum possible so that the rapid absorption of energy at a high di/dt does not cause an internal voltage rise as L(res) X di/dt.

Since it is a protective device, it requires that no internal fuses be employed or else it will not prevent a rapid surge voltage build up. Thirdly it must withstand high di/dt or dv/dt rates without internal ionisation and an early failure. The internal conducting parts must have adequate electrical and mechanical capacities.

The basic elements are wound with a very low loss electrical grade, biaxially oriented polypropylene film & or other suitable solid dielectric and a high purity annealed aluminium foil. Leads are connected to the aluminium foil and brought out. The elements are connected in series and parallel. There are no internal fuses. The assembly is insulated for full line voltage insulation and put within heavy duty, all welded M. S. containers. The containers and all M.S. parts, which come in contact with oil, are surface-treated. The lids are fitted and the containers are subjected to heat and very high vacuum. This ensures thorough removal of moisture contained within the sub assembly. The oil is similarly dried and put under vacuum. The containers are impregnated with oil-under-vacuum and when impregnation is complete, vacuum is released and the opening in the lid is sealed. They are impregnated with very special grade capacitor oil, which is biodegradable and thus constitutes no hazard to environment. The units are taken out and subjected to test, as per IS-11548. The units are degreased and given one coat of primer and two coats of plastic emulsion type of enamel paint, generally of grey colour.