Good Lord!,
Why is this such a problem to Electricians that should know the Theory of a CT.
A CT (Current Transformer) like Greg said above, merely steps the current down, say from 400A down to say 3A, a level that the metering gear can use.
The big thing is here, is the Primary winding is a solid Busbar.
Forget about PT's (Potential Transformers) in this case this will only confuse the issue.
However CT's are normally a PoCo thing here because they form part of the PoCo metering equipment and are locked and sealed.
Oh and by the way, never open the Secondary side of a CT, when there is current flowing in the Primary
