2) If star point is kept open, then some floating voltage always remains there which may result in improper measurement.
3) For the sake of protection purpose, where in worst case if one of the secondary is opened, the induced high voltage will be grounded directly if star point is grounded
A CT is a separate source of power in an electrical system. For safety, each separate source of power is grounded, to minimize the voltages which can occur (in some cases, non-grounded "separately-derived" systems are permitted, but CT's are not one of these). In a three-phase metering system, you would need three CT's, one for each phase conductor. The required grounding of one side of each CT constructs a "wye" or "star" arrangement. Since you are measuring the current flow through each CT, instead of using them as a source of power, this "star" configuration is incidental to their purpose.
As said by the earlier respondent, the CT can produce quite high voltages if it is open-circuited while in use. However, if properly connected to metering equipment, the voltages it generates will be quite low. Indeed, the proper way to wire a CT is with a terminal block which includes a shorting jumper (or bar, link, screw, etc) which is installed before any meter is disconnected, and removed only after a meter is connected.
Metering CT
Re: Metering CT
