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Theory The CFO of any insulating material or system is defined as the crest value of a standard impulse (under specified conditions) for which the insulation exhibits 50% probability of withstand (or 50% probability of failure). The CFO can be measured through different types of laboratory methods. 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. The second more commonly used method is the Regression method. In this method, one starts with a Voltage V, that is sufficiently low, so that no breakdown occurs. 10 pulses are applied and then number of breakdowns, if any are noted. Next, the voltage is increased by 10% and 10 pulses are applied and the number of breakdowns noted. The experiment is repeated by progressively increasing the voltage, till the breakdown occurs for all the applied pulses. At each voltage level, the probability of breakdown is calculated as No of Breakdowns/ No of pulses applied. A graph of Probability of breakdown vs the applied voltage is plotted. The voltage corresponding to 50% is extracted from the graph. This value is reckoned as the critical flash over voltage for the given insulation. References:
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