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Insulator Spark Leakage (Flashover)

Flashover is the phenomenon where sparks are discharged between the terminal and metal shell of the spark plug. It occurs when the voltage across the plug itself is 25,000 V to 30,000 V. Photo 1: Center electrode tip breakage
1) If the spark gap has widened as a result of terminal wear, or if the compression pressure is high as in a turbo engine, a higher voltage is required between the electrodes and flashover becomes more likely to occur. In practice, flashover does not normally occur because the flashover voltage is blocked by fitting a plug cap that makes a good seal with the insulator.
2) However, if the seal part of the plug cap deteriorates (it hardens over time), or if water droplets get inside the plug cap, flashover becomes more likely, so periodic maintenance of the plug caps is necessary.
Reference: Flashover leaves black streaks on the insulator. These streaks are formed as the rubber of the cap seal is melted by a spark discharge and then deposited on the insulator.
See Photo 1.

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