in a  building continuously indicate  neutral to earth voltage  as 10 to 11 V. Main cause for this is harmonics of VFDs.

There could be a number of things happening here -  protective conductor currents from the installation will raise the potential on the consumer's electrode to some extent (but I'd be surprised if it reached tens of volts under normal circumstances). In the simple case BS 7671 puts a limit of 50V on that before protective devices (RCDs) are expected to trip.  Reducing the resistance of the consumer's electrode can help.

There's also voltage drop along the supply N - (which will appear as an increase of N voltage at the load end) - in the UK with a supply under the ESQCR the maximum drop would be 16% or 230V (or from 253V at the source to 216.2 at the intake or a drop of 36.8V - around half of which would typically be along the supply N - so max 18 or 19V. Plus any voltage drop within the installation - so maybe another half of 5% - so even in an entirely compliant situation you could in theory be looking at N at a load being more than 24V adrift from Earth.

Then the supply reference (star point) can be held away from true Earth by Earth current returning via the source electrode (which is also influenced but protective conductor currents, if normally to a small amount).

Typically things aren't quite as bad as that though - if the source is 3-phase then N currents and protective-conductor-currents may well cancel out to some degree (even if your installation only uses one of the phases).

   - Andy.

AJJewsbury said:

In the simple case BS 7671 puts a limit of 50V on that before protective devices (RCDs) are expected to trip. 

Just to be clear, there is no limit in BS 7671 on touch-voltage to the general mass of Earth in this regard ... only in terms of selection of protective devices and combined resistance of protective conductor and earth electrode alongside this. There is no limit to touch-voltage other than that provided for by supplementary protective bonding, which will be local touch-voltage, and limited to either 50 V AC/120 V DC (or 25 V AC/60 V DC in medical locations).

Just to be clear, there is no limit in BS 7671 on touch-voltage to the general mass of Earth in this regard ... only in terms of selection of protective devices and combined resistance of protective conductor and earth electrode alongside this.

Which are explicitly co-ordinated so that a touch voltage of >50V cannot persist - reg. 411.5.3 (ii).

The touch voltage can of course exceed 50V for very short durations, but that's the same with supplementary bonding where the fault current exceeds Ia.

   - Andy.

Just to be clear, there is no limit in BS 7671 on touch-voltage to the general mass of Earth in this regard ... only in terms of selection of protective devices and combined resistance of protective conductor and earth electrode alongside this.

Which are explicitly co-ordinated so that a touch voltage of >50V cannot persist - reg. 411.5.3 (ii).

The touch voltage can of course exceed 50V for very short durations, but that's the same with supplementary bonding where the fault current exceeds Ia.

   - Andy.

AJJewsbury said:

Which are explicitly co-ordinated so that a touch voltage of >50V cannot persist - reg. 411.5.3 (ii).

Not disputing that, although only true in some circumstances, such as RCDs in TT systems ... it's unlikely that a high touch-voltage will persist in other cases for faults in the installation, ignoring PE faults and leakage currents.

The reason for pointing this out, is that there's a common misunderstanding that BS 7671 actually serves to limit touch-voltage values (although as you correctly point out, for the most part, it's aims to limit the time those touch-voltages may persist).