This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
Although the insulating material is spoken of as a non-conductor, it will not absolutely prevent the passage of current but allows an infinitesimal amount to pass, the actual amount varying with the thickness and quality of the dielectric. It is usual, therefore, to specify the insulation resistance to be not less than 600 megohms per statute mile. The minimum allowable thickness of dielectric for various sized cables is given in Column 6 of the table on page 180.
Several simple tests may be made to determine the quality of the rubber insulation on cables. Cut off about a foot of the cable to be tested, carefully remove the silk or braid and draw out the wires. Stretch the rubber to double its original length and clamp it in this stretched condition for 24 hours. When it is released it should not be more than 15 per cent longer than the original length. Another good test is to carefully weigh a piece of insulation rubber stripped from a cable, then burn it and weigh the resulting ash. If the rubber is of good quality the weight of the ash should not be more than 40 per cent. of the weight of the original piece of rubber.