For many years the manufacturers of electrical appliances and other electrical equipment have been using electronic components to improve the performance of their products,increase comfort and save energy.
Loads such as washing machines with variations in spin speed, variable-speed tools, thermostats and dimmers operate at currents with varying wave shapes (pulsating currents with DC components, inverted currents, levelled currents).
There are three different types of currents.
Type I, inverted current with DC components, with value continuously greater than zero, caused by:
▪ three-phase current
▪ median point and three-phase current
▪ jumper connection
▪ unidirectional rectification with inductive and capacitive leveling
▪ Villard type voltage doubling.
Type II, pulsating current with DC components sometimes with zero value, caused by ohmic load with:
-unidirectional rectification without leveling
-single-phase jumper connection with or without leveling
-regulation of the symmetrical and asymmetrical phase operating angle (dimmers, RPM meters).
Type III, pulsating current with DC components passing through zero caused by inductive loads with:
▪ unidirectional rectification without leveling
▪ single-phase jumper connection with or without leveling
▪ symmetrical and asymmetrical regulation of the phase operating angle (dimmers, RPM meters).
If there is a fault current to earth after an insulation fault on live parts supplied with rectified current, the contact voltages are the same size as in alternating current. Standard RCDs, which are designed to operate with alternating current at 50-60 Hz, are insensitive to fault currents with DC components. Non-tripping of a RCD when there are fault currents with DC components may have two consequences:
▪ it is dangerous for people and equipment (electrocution or fire)
▪ it causes desensitivation of RCD due to excessive polarization of the transformer core that is no longer able to send the necessary power supply to the releaser (see picture, hysteresis cycle 1).
To avoid this problem, type A RCDs must be used; thanks to the specific technology of the residual current transformer toroidal cores, the supply level is increased to a value sufficient to trigger the releaser or tripping mechanism (see picture, hysteresis cycle 2).
The sensitivity of the tripping mechanism is further increased by its connection to an electrical circuit sensitive to the wave shape of the current. In this way the tripping of the RCD is assured for any unidirectional pulsating wave shape even in case of overlapping of a DC component up to 6 mA.