Voltage Level at Secondary Side When Open Secondary

E = I x R applies. But with an open circuit on the secondary, the impedence (the equivalent of R) is infinity. What happens is that the voltage on the secondary keeps rising trying to make the current go to where it was designed to, 5A. So E = I x R with R = ∞, then E ≈ ∞ until there is I of 2.5A flowing. So in theory there is no ceiling to the voltage level other than what it takes to jump the air gap you created by opening the secondary circuit and alow 2.5A of current to flow in the circuit. Could be 10s of thousands of volts. Extremely dangerous.

Check JR's post carefully. This is why you never never never open the secondary of a

CT under load.

And by the way, DO NOT attempt to measure it with a meter! I have seen meters fry in the attempt.

...and do not put any form of fuse, breaker, switch or other openable contact in the secondary circuit!