(1) Optical current transformer (Oct) is a current sensor which is made of optical elements, such as optical glass and all optical fibers. 

(2) air core coil current transformer. Also known as Rogowski Coil current transformer. Hollow coils are often made by uniformly wound enamelled wires on a circular skeleton made of non-ferromagnetic materials such as plastics and ceramics, whose relative permeability is the same as that of air, this is a distinctive feature of the Hollow Coil as distinct from the current transformer with a core. 

(3) core coil type low power current transformer (LPCT) . It is a development of traditional electromagnetic current transformer. Designed as a high impedance resistor, the saturation characteristic is improved at very high primary current, the measuring range is expanded, and the power consumption is reduced, can Be non-saturated high-accuracy measurement up to the short-circuit current over-current, full offset short-circuit current, measurement and protection can be used together with a core coil of low-power current transformer, its output for voltage signal.