I find that hard to believe. The most marginal situation would be that of a fault current of exactly 30mA (without extension lead), corresponding to a L-E fault of approx. 8,000Ω (it's much more complicated with N-E faults, so I won't consider that possibility ). The CPC of even a 50 metre 1.25mm² extension lead would be (just) under 1Ω - so the chances of the fault being so marginal that it would operate the RCBO without the extension lead, but not with it (due to the extra 1Ω, would surely be incredibly small?You are missing the fact that there is a high resistance cpc in the wound extension lead which is limiting the fault current to earth so it no longer trips the RCBO.What am I missing?
Kind Regards, John.