TT system neutral to earth voltage

Question

Can anyone help me to identify the recommended maximum voltage levels in-between neutral and earth of TT system with relevant standard.

AJJewsbury · Accepted Answer

Minimum would be 0.0V. Maximum could be almost anything (up to Uo). The classic problem with TT systems is that during an earth fault, all the fault current returns via the source electrode, multiplied by the soil resistance, can give a very substantial potential difference between true Earth and the transformer's star point (i.e. supply N) - the exact figure depending on the resistance of the consumer's electrode (and any parallel paths - e.g. bonding to extraneous-conductive-parts) as much as the source electrode itself. If the fault isn't cleared promptly (e.g. due to a faulty RCD or just because it's on the distribution network) that N can reman at a hazardous voltage for a long time. Hence all isolation devices need to open N as well as L on TT systems. 
 - Andy.

Gihan Kavinda Baddevithana · Answer

Actually, in a building continuously indicate neutral to earth voltage as 10 to 11 V. Main cause for this is harmonics of VFDs. Hence , I want to know is there r ecommended maximum voltage levels in-between neutral and earth of TT system (with relevant standard reference). (In above mistakenly I wrote a minimum now I'll correct same.)

TT system neutral to earth voltage

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