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dielectric strength[‚dī·ə′lek·trik ′streŋkth]
(also electric strength), the intensity of a uniform electric field at which dielectric breakdown occurs.
When dielectric strength is determined, impulse measurements are usually made in order to prevent thermal breakdown. However, the voltage pulses used in such measurements should be long enough that the processes resulting in dielectric breakdown are not accompanied by overvoltages. The processes that result in dielectric breakdown are collision ionization, tunneling, or both.
At voltages higher than its dielectric strength, a dielectric becomes a conductor. In other words, the conductivity of a dielectric increases abruptly when the electric field intensity E reaches the breakdown intensity Ebr. The transition to the conducting state often results in the failure of a dielectric material owing to overheating.
Dielectric strength is a property of all gases, including metal vapors, and of solid and liquid dielectrics. For mica, quartz, and other good dielectrics, the dielectric strength may be as high as 106–107 volts per cm (V/cm). For purified and degassed liquid dielectrics, the dielectric strength may be as high as 106 V/cm. The dielectric strength of gases depends on pressure and other factors. For air under standard conditions and with a layer thickness of the order of 1 cm, the dielectric strength is about 3 × 104V/cm. For semiconductors, such as Ge and Si, the dielectric strength is of the order of 105 V/cm. However, at very low temperatures, where breakdown is caused by the collision ionization of impurities, the dielectric strength of Ge is of the order of 5 V/cm.