531.2.9 Where, for compliance with the requirements of the regulations for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger. the characteristics of the devices shall be such that the intended discrimination is achieved.
This means except for the supplies to caravans and boats a 30ma 40ms RCD will be supplied by one three times the size and will supply one 1/3 of the size.
Where the CU is protected by a 100ma delayed RCD then you will need RCBO fitting to circuits with buried cables at less than 50mm where the special cable has not been used. Supply to bathroom, and sockets under 20A.
If the first RCD was not twin pole then a twin pole RCD would be needed for a supply to a TT system like a caravan or a boat.
But in general other than the special circumstances listed above one would not feed one 30ma 40ma RCD with another 30ma 40ms RCD. It would not comply with 531.2.9 which I have quoted.
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