I thought that went without saying!Add "without giving any thought to the circuit design".substitute "the person who created the part of the installation in question"!
If you're thinking about loop impedance, I'm not sure that is particularly relevant in this context. You previous said that the loop impedance was "too low [I presume you meant "too high" ] for the OPD to operate, but I presume you meant that it was too high for the magnetic part of an MCB to operate (i.e. so as to achieve, say, the 'disconnection times' required by BS7671). However, it is surely the case that the thermal part of an MCB (or a fuse) should operate before the cable comes to any harm (even if that takes some time), regardless of the loop impedance? If that were not the case, then such devices would be useless for 'overload' (as opposed to 'fault') protection, wouldn't they? After all, an OPD only "knows" what current is going through it, and for how long, and how that compares with what the cable can tolerate - it knows nothing about the impedances (or voltages) that are resulting in that current.There is a third possibility John. Think about what the OCPD needs to operate...
Kind Regards, John