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 recommended 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.) 

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 recommended 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.) 

There are no recommendations of N-PE voltage from safety standards.

However, there are recommendations for EMC (although not from standards) - which have, in some cases, been as low as 5 to 10 V.

There is a solution to this ... although not without cost ... which is to use transformers to permit the filters to perform as they would in a TN system. This is recommended in the standards for VSDs, and also in BS EN 60204-1. The transformer secondary neutral is earthed, thus forming TN-S circuits in the TT installation.

On a single phase system, 11v is probably on the borderline of being acceptable. In contrast, by the time it reaches 20-50 you almost certainly have an unrealised  fault somewhere. The thinking is as follows - the voltage drop in the LN loop may be right at the limit, as high as ~ 10% of supply (20 odd V) Half of this is lost on the live on the way out, and the other half in the neutral wire on the way back. TN-S would be the same, for the same reason. However this is neutral to true earth voltage - there may be an additional offset between true earth (an electrode far away from the near field of your local one and carrying negligible current) and your TT installation electrodes - if there are significant currents running into the electrodes then the metal work of the whole installation may also be a few volts off terra-firma earth voltage. It may be worth installing a test electrode, or even a garden fork or un-insulated screwdriver in the ground a few m from the building and seeing how far off the building earth voltage  that is.

Be aware in a TT system can go a bit wild if your user electrodes are lower resistance than the ones at the substation.  -  if this happens, when the fault to earth comes on, the voltage is divided between the series connection of your electrodes, and the substations transformers electrode(s).
Normally we assume the substation neutral voltage rises a few volts, and most of the 230V  or whatever, is dropped in the earth around your your local one, but it can be the other way about - so that the phase you have shorted to ground is nearer true earth if your earth is good enough, then the the whole transformer, neutral and everyone else's supply  bounces up to near 230V for the neutral and upto about 400V for the other phases. So while you have the test electrode in the lawn or flower beds, also check the phase voltage/voltages. you may not have a fault, but your neighbours just might.

Mike.