Current transformers cannot measure cables with returning current in them also
Kind of obvious, but it means you cannot just clip them around a normal power cable, such as are used to power household devices that plug into wall sockets.
The primary winding of the CT is the wire carrying the current you want to measure. If you clip your CT around a two or three core cable that has wires carrying the same current but in opposite directions, the magnetic fields created by the two wires are equal and opposite and will cancel each other. Your CT will have no output
Read more: Link - CT sensors - Introduction | OpenEnergyMonitor