Circuit breakers do not prevent electric shock. They will only trip if defects occur that are acute enough to heat up wiring within the walls of the home. Do not ever count on a circuit breaker to protect your life because they will not do so.
GFCI or ground fault current interupters are designed to turn off prior to your getting a fatal electric shock. One should use the test button on them once a month or so to exersize the circuit and avoid failure that can result in electric shock.
AFCI or arc fault current interupters are new circuit breakers designed to stop house fires from occuring. They are expensive and new to the market.
Smart use of GFCI and AFCI circuit breakers can be a major safety upgrade to older homes that lack these more modern protective devices.
Installation of GFCI and AFCI should be left to electrical contractors. As a home inspector I see many homes a week where well intended home owners created far more hazards than they solved.
