Dielectric strengths values

In the following tables, the dielectric strength values are for room temperature and normal atmospheric pressure, unless indicated otherwise. 

The study of the aP coupling is advantageous in poly( -alkyl methacrylate)s due to the accessibility to the crossover region by both dielectric and mechanical spectroscopies and to the fact that both secondary and main processes are associated with high dielectric strength values, in contrast to a variety of other materials where the P relaxation is much less intense,. 

As the thickness of an insulator increases, so does the voltage necessary to cause breakdown. However, this relationship is not Unear. Importantly, the voltage gradient (volts/mil) at breakdown decreases as the thickness of the sample increases. Therefore, it is necessary to know the sample thickness in comparing dielectric strength values (usually given in volts/mil). 

Dielectric relaxation and dielectric strength of polypropylene Table 2 Dielectric strength values on PP samples of different thickness... 

Dielectric strength decreases with the increase in the temperature of the specimen. If the design calls for the use of the product at various temperatures, the dielectric strength values at anticipated temperatures should carefully be evaluated. Interestingly enough, the dielectric strength of the material below room temperature is constant and independent of temperature change (5). 

The rate of voltage application significantly alters the test results. As mentioned earlier in this chapter, dielectric strength values obtained using the step-by-step method are lower than those obtained by the short-time test. 

Mechanical stress tends to reduce the dielectric strength values substantially. 

In designing plastic parts for electrical applications, the dielectric strength values must be studied in detail. If the parts are designed based on values derived from published literature without proper attention to the effects of thickness, moisture, temperature, mechanical stress, and actual use conditions, the results could prove disastrous. The actual use conditions of the insulating materials are quite different than the conditions of the dielectric strength test and, therefore, the real emphasis must always be on the performance of the material in actual service. 

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