When dielectric material loses its resistivity and permits very large current to flow through it, then the phenomenon is called dielectric breakdown. There are many reasons for dielectric breakdown and different mechanisms involved in this phenomenon are
- Intrinsic breakdown
- Thermal breakdown
- Discharge breakdown
- Electrochemical breakdown
- Defect breakdown
- The breakdown voltage per unit thickness of the material is called dielectric strength. When the applied electric field is large, some of the electrons from the Valence band crosses over to the conduction band across the forbidden gap. Then it will give large conduction currents. The literature of electrons is called field emission and breakdown is called intrinsic breakdown or Zener breakdown. When the electric field is 10⁶ v\cm. The electrons are accelerated to high velocity, then collisions happen, then more electrons and holes released and covalent bonds can be broken. This breakage of covalent bonds increases with time and dielectric breakdown occurs. This type of breakdown is called an avalanche breakdown. This occurs in the samples because it requires large fields. This occurs even at low temperatures.
- A thermal breakdown occurs when the rate of heat generation is greater than dissipation. Energy due to dielectric loss appears as heat. The temperature of dielectric increases, resulting in local melting. Once melting starts, that particular region becomes highly conductive, enormous current flows the breakdown occurs. This occurs at very high temperatures for A.C fields, the breakdown occurs at low field strengths. This is only for a few milliseconds.
- Discharge breakdown is classified as external or internal. The external breakdown is caused by a corona discharge. Such discharge is observed at sharp edges of electrodes. Deterioration accompanied by the formation of carbon. Damaged areas become conducting leading to power arc and complete failure to the dielectric. Internal breakdown occurs when the insulator contains occluded gas bubbles. When large bubbles are present, this occurs even at low voltages
- The chemical and electrochemical breakdown is related to thermal breakdown. In an insulating material, temperature raises the chemical deterioration. When the temperature rises, the mobility of ions increases the leakage current also increases and this leads to dielectric breakdown. The insulation resistance decreases and finally results in a breakdown. Insulating material becomes oxides and dielectric properties decrease. Organic materials working at high temperature lost their stability.
- If the surface of the material has defects such as crackles and porosity then impurities such as dust at these discontinuities leading to a breakdown. Also if the defect in the form of strain, that region will also break on the application of fields.