Daniel Castleton asks this question, which concerns a consumer unit DP disconnector, the protection devices involved, and whether the necessary discrimination can be achieved. It has been answered by the NICEIC (Theme – Distribution Equipment):
Question: In the case of a consumer unit DP disconnector fitted with 100mA RCCB protection and final circuits fitted with 30mA RCBO protection, should the disconnector be a time-delayed S Type?
Alternatively, could I even remove the 100mA facility and use only as an isolator?
Or is the original combination acceptable to achieve discrimination?
Answer: To answer your first question, if the main switch is a time-delayed RCD, this will offer better discrimination than a non-delayed 100mA RCD.
The second part to your question depends upon whether the main switch is required as an RCD - say in a TT system. If the distribution board in question has appropriate RCD protection for circuits in the form of (say) RCBOs, and they do not affect any downstream circuits with further RCD protection, then it may not be required (nor desirable) to have the main switch also as an RCD.
It very much depends upon the setup you have, and there are many different combinations that can achieve a good electrical design.
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