The current transformers for measuring service have a core made of high permeability magnetic material. This means that they have little excitation current, small losses and low reluctance. They work under conditions of low magnetic induction (about 0.1 tesla).
They enter into saturation as soon as the magnetic induction grows to (0.4 to 0.5) tesla. This corresponds to a primary current growth of about four times its nominal value. gates 7/5VX1500
This is the maximum current that will be reflected in the secondary (4 x I2N) in case of saturation, even if the primary current exceeds that order of magnitude and reaches excessive values. In this case it is not necessary for the measuring devices connected to the secondary circuit to withstand large current currents, such as the instruments connected to the protection circuit.
This feature also prevents the secondary current distorted by the core saturation from reaching the instruments, which could generate measurement errors for billing purposes.
The CT core for protection is made of magnetic material that does not have the same magnetic permeability as the CT for measurement. Its core goes into saturation for very high flux values (high magnetic induction). This corresponds in practical terms to a primary current of about 20 times its nominal value.