Why is a too high zs reading on a circuit okay if it’s protected by an Rcd?

[radiohead]

Interesting, A while ago I had conduit CPC fail on 2 circuits with 50-150ohm Zs, I pulled through a new CPC for one of them, and for the cooker ran a new bit of T&E as was close to CU.

Although I believe this is the best way to fix it as now has a nice low Zs and meets disconnection time for the MCBs, But in theory if the circuits were on an RCD or upgraded to be on an RCD then could have been left like this as the 150ohms is enough to meet disconnection due to the RCD.

If it was to later have an EICR though I cant imagine many people passing it without a code as the r1+r2 would be high

This is where experience and common sense kicks in (hopefully)....if such a reading was obtained on a TN system clearly alarm bell should ring despite the presence of an RCD, and if a TT was in place you would hope an experienced person would verify that the high reading was due to the Ra and not a wiring issue.
Experience electricians recognise when a problem exists regardless of the regulations.