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I'd be interested to know - can you point me in the right direction? In the meantime, can you tell me why, for example, a Ze of say 20 ohms is acceptable for a TT but not for a PME

I'd just like to know why there's such a huge disparity between 200 ohms (or whatever it is) and 0.35 ohms. It seems leniency is given to TT just because it can't get as good a result as a PME. In reality, isn't it the case that all you really need is a sufficient path to earth to be able to...

What different characteristics?

So sometime in the past, someone decided what would be an acceptable minimum Ze for safety. Surely this figure should be regardless of the type of earthing system used?

Why is the permitted Ze higher for a TT system than PME? Is it because TTs are required to have RCD protection? If so, if an RCD is present in a PME system, can we ignore the figures in the OSG for PME and refer to the higher TT Ze permitted figure?