Breakdown Voltage and Dielectric Strength
Breakdown Voltage and Dielectric Strength
An insulator or dielectric is a substance within which there are no mobile electrons necessary for electric conduction. However, when the voltage applied to such an insulator exceeds a certain value, then it breaks down and allows a heavy electric current (much larger than the usual leakage current) to flow through it. If the insulator is a solid medium, it gets punctured or cracked. The disruptive or breakdown voltage of an insulator is the minimum voltage required to break it down. Dielectric strength of an insulator or dielectric medium is given by the maximum potential difference which a unit thickness of the medium can withstand without breaking down.
In other words,
The dielectric strength is given by the potential gradient necessary to cause breakdown of an insulator. Its unit is volt/metre (V/m) although it is usually expressed in kV/mm.
For example, when we say that the dielectric strength of air is 3 kV/mm, then it means that the maximum potential difference which one mm thickness of air can withstand across it without breaking down is 3 kV or 3000 volts. If the potential difference exceeds this value, then air insulation breaks down allowing large electric current to pass through.
Dielectric strength of various insulating materials is very important factor in the design of high voltage generators, motors and transformers. Its value depends on the thickness of the insulator, temperature, moisture, content, shape and several other factors.
Doubling the thickness of insulation does not double the safe working voltage in a machine because:
The relation between the breakdown voltage V and the thickness of the dielectric is given approximately by the relation
V = At2/3
where A is a constant depending on the nature of the medium and also on the thickness t. The above statement is known as Baur’s law.
Safety Factor of a Dielectric
It is given by the ratio of the dielectric strength of the insulator and the electric field intensity established in it. If we represent the dielectric strength by Ebd and the actual field intensity by E, then safety factor k = Ebd /E
For example, for air Ebd = 3 × 106 V/m. If we establish a field intensity of 3 × 105 V/m in it, then,
k = 3 × 106/3 ⋅ 105 = 10.
Dielectric Constant and Strength
| Dielectric Constant and Strength | ||
| Insulating material | Dielectric constant or relative permittivity (er) | Dielectric Strength in kV/mm |
| Wood | 2.5-7 | — |
| Vacuum | 1 | infinity |
| Teflon | 2 | 20 |
| Rubber | 2.5-4 | 12-20 |
| Quartz | 4.5-4.7 | 8 |
| PVC | 3.7 | 50 |
| Porcelain | 5-6.7 | 15 |
| Polyurethane | 3.6 | 35 |
| Polyethylene | 2.3 | 40 |
| Paraffin wax | 1.7-2.3 | 30 |
| Paper | 1.8-2.6 | 18 |
| Nylon | 4.1 | 16 |
| Mylar | 3 | 400 |
| Mineral Oil | 2.2 | 10 |
| Micanite | 4-5-6 | 25-35 |
| Mica | 4-8 | 20-200 |
| Marble | 7 | 2 |
| Glass | 5-12 | 12-10 |
| Epoxy | 3.3 | 20 |
| Bakelite | 5 | 15 |
| Asbestos | 2 | 2 |
| Air | 1.0006 | 3.2 |
Read article –potential difference
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