That is another attraction of adding a separate CU so that you can select individual circuits to protect. It leave the option open to move one back to the old CU if required while hunting faults.
If the old install is tripping a 100mA RCD with leakage then it really ought to be fixed as a matter of some urgency anyway since it indicates that there is probably a serious insulation break down failure somewhere.
I don't agree with that as a general statement... you need to take into account the likely users of the installation, and the uses it will be put to.
Just because a shock does not actually kill you it does not mean that you escape unscathed. The RCD protected installation will in many cases reduce the severity of a range of injuries that can be caused by electric shock including burns, arc flash injury, and muscular spasm injury (and secondary injuries resulting from said spasms). Not to mention the pain and distress that shocks can cause.
The number of serious shocks resulting from (mis) using *appliances* is actually quite large - and it is here that 30mA (or lower) threshold RCD protection will make a big impact on prognosis for recovery from the shock.
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