Firstly, you can not use a current transformer to measure voltage.
When you use a current transformer you need to connect a "burden resistor" across the output.
If you do not have a load connected across the output, then the output voltage of the current transformer will rise to dangerous levels, as the transformer tries to deliver current into an open circuit.
The devices shown as having a voltage output have a burden resistor already fitted.
The variant called current output does not have a resistor fitted, you can choose a value to fit externally, to give you the voltage that you want. This type has back to back zener diodes fitted to prevent the output voltage from reaching a dangerous value.
There is lots of useful information about current transformers at openenergymonitor.
The following is a quote from that site:
In general, a CT must never be open-circuited once it's attached to a current-carrying conductor.
A CT is potentially dangerous if open-circuited.
If open-circuited with current flowing in the primary, the transformer secondary will attempt to continue driving current into what is effectively an infinite impedance. This will produce a high and potentially dangerous voltage across the secondary 
Some CT's have built-in protection. Some have protective Zener diodes as is the case with the SCT-013-000 recommended for use in this project, or if the CT is of the 'voltage output' type, it has a built in burden resistor, thus, it cannot be open-circuited.