Understanding Insulator Breakdown Voltage and Dielectric Strength

The electrons in insulating materials are tightly bound, making them excellent barriers to free electron flow. Yet, even the best dielectrics cannot withstand unlimited voltage. When a sufficient electrical field is applied, any insulator will eventually undergo breakdown, allowing current to pass through.

Unlike conductors, where current increases linearly with voltage for a given resistance, the current–voltage relationship in insulators is highly nonlinear. Below a critical threshold—known as the breakdown voltage or dielectric strength—almost no current flows. Exceeding this threshold triggers a rapid surge of current.

Dielectric strength is the voltage required to force current through an insulating material, effectively “breaking” it. Post‑breakdown, the material’s molecular structure may be altered, and the insulator might no longer retain its original properties. Typically, a localized puncture forms at the site of current flow.

Material thickness influences breakdown voltage. While specific dielectric strength is often expressed in volts per mil (1/1000 inch) or kilovolts per inch, the relationship is not strictly linear: a three‑fold increase in thickness yields slightly less than triple the dielectric strength. For rough estimates, using the voltage‑per‑thickness rating remains acceptable.

* Materials listed are specially prepared for electrical use.

Key Takeaways

• A high enough voltage can liberate electrons from insulators, forcing current through.
• The minimum voltage required to “break” an insulator is called the breakdown voltage or dielectric strength.
• Increasing thickness raises breakdown voltage, all else being equal.
• Dielectric strength is usually expressed in volts per mil or kilovolts per inch.

Related Worksheets

• Conductors and Insulators Worksheet