What's the difference between live tank and dead tank circuit breakers?

With live tank circuit breakers, the enclosure that houses the contacts is energized, ie. "live". Dead tank circuit breaker's contact enclosures are not energized and are connected to the ground grid. Live tank breakers are usually less expensive but you have to have separate current transformers. 
See ABBhttp://abb.com/global/abbzh/abbzh251.nsf!OpenDatabase&db=/GLOBAL/ABBZH/ABBZH262.nsf&v=1C616&e=us&c=822916941B02F40741256A14004684D0
Use menu on left and select Circuit Breakers.   
See also 
http://www.tde.alstom.com/Transmission/Pages/AirInsulatedSwitch.html






Circuit breakers can be classified as live tank, where the enclosure that contains the breaking mechanism is at line potential, or dead tank with the enclosure at earth potential.   (from http://en.wikipedia.org/wiki/Circuit_breaker)