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There is a **suspension insulator test **about the CFO (Critical Flash Over).The CFO voltage can be measured through different types of laboratory methods. The following is one of them.

The up and down method was used to evaluate the CFO voltage of an insulation structure and it is described in IEEE Standard 4-1995. There are four steps about this suspension insulator test:

1. Applying a lightening impulse with a magnitude less than the expected flashover voltage of an insulation structure.

2. Increasing the magnitude of the lightening impulse by a very small amount (approximately

3~5 % of the expected flashover voltage) for subsequent impulses until flashover occurs.

3. Applying a series of 20 impulses. Decreasing the prospective voltage by about 3~5% after each flashover and increasing the prospective voltage by about 3~5% after each withstand during the series of these 20 impulses.

4. Average the values of the 20 impulses to obtain the CFO voltage.

Through this suspension insulator test, we can know: For self-restoring insulation, normal (Gaussian) distribution is the best model for approximating the probability density function of disruptive surface flashover. Utilizing statistics for Gaussian distribution, the desired level of withstand can be calculated using the integral probability function, which is defined in equation.

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