Classification of RCDs according to the wave form of the earth leakage currents they are sensitive to:
▪ AC type, for alternating current only
▪ A type, for alternating and/or pulsating current with DC components
▪ B type, for alternating and/or pulsating current with DC components and continuous fault current.
AC type RCDs are suitable for all systems where users have sinusoidal earth current. They are not sensitive to impulsive leakage currents up to a peak of 250 A (8/20 wave form) such as those which may occur due to overlapping voltage impulses on the mains (e.g.: insertion of fluorescent bulbs, X-ray equipment, data processing systems and SCR controls).
A type RCDs are not sensitive to impulsive currents up to a peak of 250 A (8/20 wave form). They are particularly suitable for protecting systems in which the user equipment has electronic devices for rectifying the current or phase cutting adjustment of a physical quantity (speed temperature, light intensity etc.) supplied directly by the mains without the insertion of transformers and insulated in class I (class II is, by definition, free of faults to earth).These devices may generate a pulsating fault current with DC components which the A type RCD can recognize.
B type RCDs are recommended for use with drives and inverters for supplying motors for pumps, lifts, textile machines, machine tools etc., since they recognize a continuous fault current with a low level ripple.
Type AC and type A RCDs comply with IEC/EN 61008/61009; B type for RCDs is not mentioned in these reference standards: an international standard has been introduced in 2007 an it specifies additional requirements for B type RCDs.
This new standard, IEC 62423, can only be referred together with IEC 61008-1 (for RCCBs) and IEC 61009-1 (for RCD-blocks and RCBOs), this means that B type RCDs have to be compliant to all the prescriptions of IEC 61008/9.