Current Transformers (CT) and High Voltages

Current transformers typically have a very high number of turns on the secondary and a single wire or turn for the primary. They are called current transformers because of they way they are used. The secondary is driven into a relatively low impedance keeping the reflected primary impedance very low as well so the voltage drop on the primary is minimal and you get a nicely scaled voltage on the secondary.

When the secondary is open a few things happen. First it will start to act more like a voltage transformer as long as the primary inductance supports some voltage. Then as the core stores energy. The stored energy will be released as soon as the current stops flowing. Energy is Volts * Time * Current. In an open circuit the current is zero so the voltage can get very high. The Voltage * Time on the secondary inductance will reach a point where the flux in the core is cancelled and it may oscillate until all of the energy is dissipated or the energy may be dumped back to the source - those characteristics depend on the what the circtuit driving the primary looks like.