Paper Insulation

Paper is an excellent insulating material in the dry condition.  The difficulty is in keeping it dry.  This is because paper is extremely hygroscopic (it absorbs moisture).  Hence, paper should be kept out of contact with air.  Paper insulation is, therefore, enclosed in a sheath which protects it from moisture.  One of the common materials used for the sheath is lead.  These cables are known as Paper Insulated Lead Covered (PILC) cables. 

Paper insulation is usually impregnated with mineral oil or with other special compounds.  This prevents the formation of air pockets in the paper insulation.  Paper insulation can be used upto a temperature of 80 deg. C. 

However, the downside of paper insulation is the difficult jointing process.  Special jointing procedures and equipments are needed to ensure that the insulation remains sealed.  This makes it difficult to work with.  Hence, paper insulation has gradually given way to PVC and XLPE Insulation. 

Another form of paper insulation is the pressboard.  Pressboards are formed by pressing paper sheets together to a thickness of a few mm.  An impregnating material is used for binding.  These pressboards provide mechanical support along with electrical insulation.