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.

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.