Flexible Current Clamps are based on a totally different physical principle than traditional CT current transformers. The main part is called a Rogowski coil, named after German physicist Walter Rogowski. It is an air-core coil wound around a conductor with a current. The current flowing through the measured conductor causes centric magnetic field lines, which due to self-induction phenomenon induces an electromotive force at the end of the coil. This voltage is proportional to the rate of current change in the conductor, and not to the current itself. As a Rogowski coil does not have a core, there can be no core saturation effect making them ideal for measuring large currents.
How do flexible current clamps work?