The life-saving.

 The life-saving, or circuit breaker is a protective device having the task of protecting the plant from direct and indirect contacts. Direct contact occurs when you touch parts of a generic system that are normally in tension, such as contact with a terminal non-isolated contact with a conductor, etc., is indirect contact occurs when you are in contact with parts of system that are not normally live, but which may be a potential non-zero due to a failure or breakdown of insulation of electrical equipment, such as the casing of an engine or an electrical appliance. Consider the operation of the differential, referring to fig1.
In it there is a toroidal magnetic core on which they are allocated two windings (coils) connected in series with the line conductors and the coil differential acting on the steering control, without ground fault, which in normal operation, coils are struck by the same current, the flow produced by the latter turns out to be balanced consenguenza will have no magnetic effect and the relay will not intervene. In the presence of a dispersion and then a ground fault is the flow of the two windings unbalanced (the sum is not zero) and the resulting magnetic flux activates the tripping relay. The main characteristics of the 'circuit breaker are: rated voltage (Vn), the rated current (In), the rated residual current of intervention (IDN). Idn values are: