Electric Fields and Capacitors 101 3.17 Dielectric Strength and Working Voltage There is a maximum potential gradient that any insulating material can withstand before dielectric breakdown occurs. There are of course some applications where dielectric breakdown is deliberately produced e.g. a sparking plug in a car engine, which produces an arc between its electrodes when subjected to a p.d. of several kilovolts. This then ignites the air/fuel mixture. However, it is obviously not a condition that is desirable in a capacitor, since it results in its destruction. Capacitors normally have marked on them a maximum working voltage. When in use you must ensure that the voltage applied between its terminals does not exceed this value, otherwise dielectric breakdown will occur. Dielectric breakdown is the effect produced in an insulating material when the voltage applied across it is more than it can withstand. The result is that the material is forced to conduct. However when this happens, the sudden surge of current through it will cause it to burn, melt, vaporise or be permanently damaged in some other way Another way of referring to this maximum working voltage is to quote the dielectric strength. This is the maximum voltage gradient that the dielectric can withstand, quoted in kV/m or in V/mm. Worked Example 3.18 Q A capacitor is designed to be operated from a 400 V supply, and uses a dielectric which (allowing for a factor of safety), has a dielectric strength of 0.5 MV/m. Calculate the minimum thickness of dielectric required. A V 400 V; E 0.5 10 6 V/m EEAnsVddVd volt/metreso metremm 400050086..1 Worked Example 3.19 Q A 270pF capacitor is to be made from two metallic foil sheets, each of length 20 cm and width 3 cm, separated by a sheet of Tefl on having a relative permittivity of 2.1. Determine (a) the thickness of Tefl on sheet required, and (b) the maximum possible working voltage for the capacitor if the Tefl on has a dielectric strength of 350 kV/m.