A CT (current transformer) needs to clip over just the live lead (or just the neutral), so
that it measures the actual current. Clipping it over both live and neutral will measure
nothing since the sum of those currents is zero.
The most important thing to remember with a CT is that you must have a burden
resistor securely connected to its secondary - if you don't highly dangerous voltages
can be generated - so always go for a unit with a built-in burden resistor if you can.