Question
Explain dielectric strength and dielectric breakdown.

Answer

→"An insulating medium which retains its insulating property up to a maximum electric field is called the dielectric strength of the insulating medium."
OR
→"The minimum electric field which starts the ionization a given non-conducting medium is called its dielectric strength."
A dielectric medium loses its insulating property many times, due to applied electric field. This effect is called dielectric breakdown.
For air, dielectric strength is nearly $3 \times 10^6 V / m$. (or $3000 V / mm$ )

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