Residual Current Devices
Regulation 531.3 – Residual current devices (RCDs)
The use of RCBOs to protect final distribution circuits has been further strengthened with the inclusion of a new clause 531.3.2 (ii). This
specifically identifies the use of RCBOs in residential premises as a consideration for limiting the risk of unwanted tripping:
Regulation 531.3.3 provides clarity on the various types of RCD available and identifies how each type should be selected dependent upon the load equipment characteristics. This now clearly defines where Type AC RCD’s can be utilised.
The new regulations are as follows:
531.3.2
Unwanted Tripping
Residual current protective devices shall be selected and erected such as to limit the risk of unwanted tripping. The following should be considered:
(ii) the use of RCBOs for individual final circuits in residential premises. See also section 314 (Division of Installation).
(iii) in order to avoid unwanted tripping by protective conductor currents and/or earth leakage currents, the accumulation of such currents downstream of the RCD shall not be more than 30% of the rated residual operating current.
531.3.3
Types of RCD
RCD type AC shall only be used to serve fixed equipment, where it is known that the load current contains no DC components.
NOTE: Examples of fixed equipment with a load current containing no DC components can include but not be limited to electric heating appliances, and/or simple filament lighting neither containing electronic components.
Overview
The addition of the new clause 531.3.2.(ii) places greater emphasis on the use of RCBOs on individual final circuits to avoid unwanted tripping, by providing greater sub-division of RCD protected circuits as required within section 314.
Utilising RCBOs on individual final circuits also provides the most effective solution to the requirements of 531.3.2.(iii). This limits the accumulated PE currents to a maximum of 30% of the rated residual operating current, which equates to a maximum of 9mA on a 30mA RCD.
In comparison, using a Dual RCCB board would require that the total accumulated leakage current from all of the final circuits connected to
the RCCB do not exceed 9mA.
This requires an understanding of the potential leakage currents of any equipment connected to each of the protected circuits, some of which may be unknown during the initial installation, as well as any subsequent upgrades where further circuits are added onto the RCD or new equipment added to the installation.
Type AC RCDs are now only permitted for use on circuits serving fixed equipment where it is known that the load cannot generate any dc component, for example electric heating appliances or simple filament lighting.
This further promotes the use of Type A devices as the minimum standard, mandating them on final circuits such as those providing power to socket outlets or LED lighting.
