You must be in the UK... differences in lingo and codes may cause you to get little response here. rcd is the equivalent of an arc fault breaker I believe. These sense potential and potential change and trip prior to full ground fault.
In the case of a full ground fault a regular breaker would still trip... as current increases with load and trips the breaker. Much more flows to ground before tripping and it also takes longer to trip. Sounds like a silly answer but I think that was your question, no?