CT's Secondary Terminal Earthing

Depends upon the application.

More information needed.

Good Luck, Old Salt

Generally, standard utility practice is to ground the CT secondary circuit at one point (and one point only!). The location varies: Some do it at the CT terminals, some at the first point the circuit enters a relay/control panel, etc. If the circuit is not grounded, there is the possibility of it having an extremely high induced voltage to ground during some operating conditions. Even with the ground, certain fault conditions could lead to overvoltages that would require additional protection (See Chapter 7 in the Art & Science of Protective Relaying free download from GE).

Note especially that a CT should NEVER be energized while the secondary is open-circuited! Any CT secondary must be shorted and grounded if unused, or else wired into a complete circuit when energized, to avoid catastrophic failure of the CT and danger to personnel and equipment.

I agree with Old Salt. It depends.

For example:

CT's used in current feedback for DC drives (measuring current in AC lines)were usually always ungrounded and connected to a full bridge diode rectifier circuit.

In a simple metering circuit like a door mounted ammeter it could be grounded.

However in a sophisticated circuit like a GE MultiLin Motor Protection System you should follow the installation instructions for the instrument.

A current transformer secondary terminal must be grounded before it is operated or put to use in an ac circuit.If it is not grounded,dangerously high voltage may build up,causing explosion in the CT.

Your thinking about open-circuited CTs developing dangerous voltages.

Yes and yes (it depends on the application).

See previous threads on CR4 as this question has been answered a few times before.