The Secondary of the potential Transformers are usually earthed. There are various reasons for this. The first reason is concerned with safety. Since, the primary winding of the potential transformer is connected to High voltage, there are chances for the high voltage, in rare conditions, to get transferred to the secondary. This can occur due to capacitance, insulation failure or any other damage. Such a scenario would be extremely dangerous. For this reason, the secondary is usually earthed at the star point for star connected transformers.
Another reason for earthing the secondary would be functional. For applications such as synchronization where two PT voltages need to be compared, the synchronizing equipment (dark lamp method and bright lamp methods) would function only if the star point of the PTs are grounded.
