The invention relates to a circuit breaker for electrical supply lines, particularly power supply lines or battery cables, for motor vehicles, which circuit breaker is comprised of a first connecting element (1), a second connecting element (3) connectable to said first connecting element (1), and a current path whereby in the conducting state of the circuit breaker a current passes between the first connecting element (1) and the second connecting element (3).The technical problem of providing a circuit breaker for electrical supply lines which can be manufactured and advantageously ensures an error-free operation is solved in that a pyrotechnic separating unit (5) is thermally actuatable by Joule heat emitted by at least one of said connecting elements (1, 3) and in that, the connection between the connecting elements (1, 3) can be released by means of the actuated pyrotechnic separating unit (5).