Right. Essentially when you overvolt a bulb it's going to have a shorter lifespan. When you overvolt it enough it will blow instantly. It's the equivalent of being struck by lightning.

I don't think there's an equation for this though. I believe it's all empirical and you just have to fit the curve to the data.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman