Why doesn't earthing always protect against transient overvoltage?

[F.ono]

I've been reading about earthing of electrical systems. The book that I'm reading states that transient overvoltage occurs mainly on no earthed systems, but it doesn't explain why. Could someone enlighten me?
It also says that overvoltage can degrade equipments, damage electronic components... but I don't understand what happens physically. Could someone explain?

 

[jedishrfu]

http://en.wikipedia.org/wiki/Ground_(electricity )

The wikipedia article mentions lightining as a possible source of overvoltage, line surges, and contact with higher voltage lines.

See "AC Power wiring installation"

 

[yungman]

I've been reading about earthing of electrical systems. The book that I'm reading states that transient overvoltage occurs mainly on no earthed systems, but it doesn't explain why. Could someone enlighten me?
It also says that overvoltage can degrade equipments, damage electronic components... but I don't understand what happens physically. Could someone explain?

I cannot agree with that totally. Trancient has very high frequency components, a cable to Earth ground do little about that. For safety, yes the earthed ground will be a lot better, but for protect the electronics, not always work.

All semi conductors have break down voltage. If you over drive input or output or even power pin of the device, you cause big current surge and damage the components. I work with HV, Earth ground do nothing to protect the instrument, it burn the components unless there is good protection.