Dielectric strength

Dielectric strength of a dielectric is the maximum electric field that the dielectric can withstand with its insulating properties intact. In other words, the dielectric strength equals the maximum electric field that can exist in a dielectric without electrical breakdown.

If the magnitude of the electric field in the dielectric exceeds the dielectric strength, then the insulating properties of the dielectric break down and the dielectric begins to conduct. As an example, if the dielectric strength of air is exceeded, sparks appear between the two wires, due to ionization of atoms and recombination with electrons in the air, similar to the process that produced corona discharge.

For a parallel plate capacitor, for any given separation d, the maximum voltage that can be applied to a capacitor without causing a discharge depends on the Dielectric strength of the dielectric.

Maximum voltage: V_max = E_max. d
=> V_max = Dielectric strength. d

Physical capacitors have a specification called by a variety of names, including working voltage, breakdown voltage, and rated voltage. This parameter represents the largest voltage that can be applied to the capacitor without exceeding the dielectric strength of the dielectric material in the capacitor.

Consequently, when selecting a capacitor for a given application, you must consider the capacitance of the device along with the
expected voltage across the capacitor in the circuit, making sure that the expected voltage will be smaller than the rated voltage of the capacitor.

Dielectric Constants and Dielectric Strengths of various materials at room temperature – Approximate values

Material	Dielectric Constant K	Dielectric Strength (106 V/m)
Air (dry)	1.00059	3
Bakelite	4.9	24
Fused quartz	3.78	8
Mylar	3.2	7
Neoprene rubber	6.7	12
Nylon	3.4	14
Paper	3.7	16
Paraffin-impregnated paper	3.5	11
Polystyrene	2.56	24
Polyvinyl chloride	3.4	40
Porcelain	6	12
Pyrex glass	5.6	14
Silicone oil	2.5	15
Strontium titanate	233	8
Teflon	2.1	60
Vacuum	1	–
Water	80	–