The third type of devices with built-in RCD is the so-called 'RCD-fork'. For effective security devices also include differential automata, which are a combination of circuit-breaker with RCD (like "two in one '). Differential automatic works in both cases as leakage current to earth and short-circuit and overload. Their benefits apply in the case where the installation of two separate devices in the switch is not enough space. Price to automatic differential exceeds the price of the RCD.
The operating principle of the RCD RCD action based on the work of a differential current transformer. The total magnetic flux in the core is proportional to the difference between the currents in conductors are the transformer primary current. Under the influence of voltage in the secondary circuit flows a current proportional to the difference between the primary currents. This current and activates the trigger. In the normal mode, the resultant magnetic flux is zero, the current in the secondary winding of the transformer differential is also zero. When you touch a person to open the conductive parts of the body or to power consumers, by insulation breakdown which occurred at the phase conductors through the RCD than the load current flows an additional current – the current leakage. For CT it is a differential (difference). If the current exceeds value of the threshold element of the starter, the latter is activated and acts on the actuator. In order to control proper operation (efficiency) provides RCD circuit test (the button "Test"), which works by artificially creating a differential current switch-off.