What is Breakdown Voltage?

Breakdown voltage, sometimes also called dielectric strength or striking voltage, is the quantity of electrical force required to transform the electrical properties of an object. Most commonly, it is used with respect to insulators. The breakdown voltage is the minimum voltage necessary to force an insulator to conduct some amount of electricity. This measurement is meaningful only in relation to an existing system; it is the point at which a material defies the operator's expectations for how it will function.

Insulators, by definition, conduct no electricity. Breakdown voltage is the point at which a material ceases to be an insulator and becomes a resistor; that is, conducts electricity at some proportion of the total current. Insulators are characterized by atoms with tightly bound electrons. The atomic forces holding these electrons in place exceeds most outside voltages that might induce electrons to flow. This force is finite, however, and can always potentially be exceeded by an external voltage, which will then cause electrons to flow at some rate through the substance.

All else being equal, the quality of an insulator increases along with its breakdown voltage. Hence, porcelain, which has a dielectric strength of around 100 kilovolts per inch, is a mediocre insulator. Glass, which breaks down at 20 times the voltage that porcelain does, is much better.

Diodes also have a breakdown voltage. Simple diodes are intended to conduct electricity only in one direction, referred to as "forward." At a sufficiently high voltage, however, the diode can be made to conduct electricity in "reverse." Some diodes, called avalanche diodes, are intended for this type of use. At low voltages, they conduct electricity in one direction only. At a specific point, they conduct it just as effectively in the other direction. This distinguishes them from insulators and other diodes, which, even above the breakdown level, maintain relatively high resistance. Not surprisingly, triodes and other specialized electronics components also break down at a certain point and begin to conduct electricity along the path dictated by a sufficiently high voltage.

In practice, determining a material's exact breakdown voltage is difficult. A specific number attached to this quantity is not a reliable constant like a melting point; it is a statistical average. Consequently, when designing a circuit, one should make sure that its maximum voltage is well below the lowest breakdown voltage of any of the materials involved. An electrical system is only as good as the smallest breakdown voltage of one of its components.