My understanding is that you record the relevant time on the certificate.
So if an RCD is being used only for fault protection i.e. meeting disconnection times where Zs is too high for MCB. Then the relevant time will be 1x.
If an RCD is providing supplementary protection (which is usually the case for 30mA units) then the relevant figure is the 5x reading.
So in the case of a 17th Edition board on a TN-S or TN-C-S system the RCD's will be supplementary so 5x is the important one.
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