Dielectric breakdown

Dielectric breakdown is failure of an insulating regime under a sufficiently strong electric field, allowing substantial electric current through the material. For example, an electron freed from a chemical atom or molecule can become a mobile charge carrier. The small-field linear electric susceptibility model does not describe this conducting state.^[ref]

For an ideal uniform dielectric between large parallel plates, $Q=CV=(ϵA/d)(Ed)=ϵAE$. A prescribed breakdown field $E_{\mathrm{bd}}$ therefore limits charge independently of plate separation. Voltage capacity still grows with separation. This ideal result neglects field enhancement near edges and any dependence of the breakdown field on gap geometry or environmental conditions.

Dielectric strength characterizes the electric field at dielectric breakdown under specified test conditions. In an ideal uniform gap, it sets a voltage scale $V_{\mathrm{bd}}\simeq E_{\mathrm{bd}}d$.^[ref] The parallel-plate capacitor model consequently gives the stored energy density scaleThis is a model scale, not an operating rating: fringing field of a capacitor make local electric fields differ from $V/d$, while leakage and dielectric loss spend some supplied energy as heat. Comparing usable capacitor energy therefore requires insulation and breakdown performance as well as permittivity.