Why did I get 50v when I turned the fuse-board off?

Double pole MCB or RCBO in the CU would probably be helpful as both the the L and N are disconnected.  HOWEVER there could be a rise in potential on the E due to fault/s or PE/FE (Functional Earth) due to electronic in the instalation.  A lot of electronic equipment intentionally leak to Earth to function correctly.  

I'm not sure how you can have a high ze and a high pfc?

Perhaps diverted neutral currents or If tnc--s maybe a fault in pen conductor.

I think more information is required like type of earthing system,method of isolation ie: single or double pole?

Maybe reversed polarity, needs proper inspection I think.

Was this 50V to the system CPC 'earth' or the actual terra-firma earth under your feet, or to a temporary  in the yard/ gaerden. (bare shafted screwdrivers and garden forks make easy 'croc clip') temporary electrodes for such sanity tests.)
If there is a large offset between building CPC and terra-firma, there is a fault, either in the building or possibly the DNO network or substation outside that needs looking at.

 I presume the PSSC is the L-N loop and the high Ze/Zs is the L-E loop - is that right ? How is the building earthed and when was that last verified.

It may be trivial but actually it may be quite serious - be careful what you touch simultaneously.
Mike.

What's the Earthing system? A fair voltage difference between N and PE is perhaps expected on TT systems (hence the requirement for DP isolation on TT). There shouldn't be a significant difference on TN-C-S (PME) though - as the two should be one and the same at the cut-out. TN-S would be a bit in the middle.

What sort of instrument are you using and are there any live parts remaining in the vicinity - capacitive coupling can sometimes result in an apparently significant voltage appearing on isolated conductors which can be read by high impedance (electronic) voltmeters, but disappears as soon as a relatively low impedance (e.g. a human) shunts it. Probably not the case if you felt something though.

No engineer should get a shock it’s they turn off the mains.

One of the reasons all main switches in single phase domestics are double pole! (You should always be testing for dead in any event).

Some DNO faults can give nasty effects - damaged N or PE can be dragged up to a higher voltage by connected loads (either normal L-N currents or earth leakage) and uncleared L-PE elsewhere on the network can result in N and/or PE being a lonq way from the nominal 0V they should be.

   - Andy.