DATA BULLETIN PEI 505
Porcelain enamel is defined as a substantially vitreous or glassy inorganic coating bonded to metal by fusion at a temperature above 800°f.
The inert, impermeable qualities of glass make a porcelain enamel coating a good electrical insulator. Using steel as the base metal, some selected areas of the metal may be left uncoated for electrical connections. Both electrical resistance and electrical conductivity may be achieved in porcelain enamel coating systems. The electrical resistance per unit area is a function of the thickness and the composition of the porcelain enamel. In addition to resistance, usually expressed as resistivity, other electrical properties of interest are dielectric strength, dielectric constant, and dissipation factor. These properties vary with temperature. In general, as the temperature increases, the resistivity and dielectric strength decrease, while the dielectric constant and dissipation factor increase. The dielectric constant and the dissipation factor also vary with frequency.
The dielectric strength of a porcelain enamel coating will vary from point to point due to surface variations and internal bubble structure of the coating. Dielectric strength of ordinary porcelain enamels will range from 200 to 500 volts per mil with an average of 4 to 6 mils total thickness. Increase and decrease due to total thickness are not directly proportional to that thickness. However, special porcelain enamel coatings with reduced bubble structure can be produced which have uniformly high voltage breakdown resistance. This strength can be greatly increased by changes in frit composition and alterations in the microstructure of the coating. While the bubble structure of a porcelain enamel coating can be greatly reduced, it cannot be eliminated. The best dielectric strength is obtained with a minimum of three thin coats of extremely dense "altered" porcelain enamel.
Volume resistivity varies from 10.13 to 10.16 ohm-cm at room temperature. Volume resistivity will decrease rapidly with an increase in temperature. In fact, ordinary porcelain enamel actually becomes a fairly good electrical conductor at or above its firing temperature.
At 400-cycles per second and at room temperature, the dielectric constant is in the range of 6 to 12. A sharp increase in values can be expected at temperatures above 250 degree F to 300 degree F (121 degree C to 149 degree C).
At 400-cycles per second and room temperature, the dissipation factor is about 1 or 2 %. Much higher values occur at temperatures above about 200 degree F (93 degree C) but decrease with increased frequency.