There is no way in Hades a car alternator will put out 400v it's physically impossible. As for spikes every time you turn on the headlights, the heater, the wipers, or the radio you cause a momentary spike and nothing blows. That is one of the reasons the main feed is a fusible link because it will outlast these spikes where a fuse won't. The welders are putting out high amperage and that is what welds not voltage. Put the books down and grab a wrench and get real world experience.