What is CFO (Critical Flash Over)?

The CFO voltage of an insulation structure is defined as the crest value of a standard impulse for which the insulation exhibits 50% probability of withstand (or 50% probability of failure).

insulator

The breakdown in the air gap or flashover on the insulation material’s surface is caused by a physical phenomenon called electronic avalanche. An effective electronic avalanche possesses a statistical characteristic. As a result, the insulation strengths of different dielectrics such as gas, liquid, or solid exhibit a statistical nature.

 

For the purpose of insulation coordination, the voltage-withstand strength of the insulation structure is quite often expressed in terms of, Basic Insulation Level (BIL). For self-restoring insulation, such as gas dielectrics, BIL is understood by most utilities as the crest value of a standard impulse for which the insulation exhibits 90% probability of withstanding (or 10% probability of failure).

 


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