Think of the CT as a step-UP transformer ... because that is what it actually is (stepping current down means stepping voltage up). The difference is that the primary (the conductor going through the window) is "wired" in series with another load (the entire metered load). You have a low number of turns on the primary (usually 1) and a high number of turns on the secondary (for a 600:5 that would be 120 turns). Actual voltage calculation depends on a lot of factors, and under load will drop low. But the turns ratio can determine what you could get in an open circuit condition.
A transformer reflects the impedance of the secondary back upon the primary at the given ratio. That's why a normal transformer primary will draw more or less current depending on the impedance connected to the secondary. As you reflect more impedance back against the current flow going through the CT window, the voltage drop at the window increases. It won't be infinite since it's not perfect coupling, so you can't stop the current in the conductor with an open CT. Potentially (no pun here ... this is lethal) you can get the supply voltage (how hard it will try to make sure the load current flows through the window) times the turns ratio of the CT.
Don't mess with live CT's unless you are specifically trained in the procedure.
