Current transformers must be manufactured according to their application in the circuit in which they are operating. Thus, current transformers are classified for measurement and protection.
The current transformers used in current or energy measurement are devices capable of transforming the load currents in the ratio of Ip / 5 generally, allowing the recording of values by the measuring instruments without being in direct connection with the primary circuit the installation.
Sporadically, current transformers with several cores are constructed, some for the measurement of energy and others, suitable for the protection service. However, the concessionaires normally specify in their standards separate units for their billing measurement, and the installation designer should reserve an independent unit for protection, when applicable.
In addition to representing high safety to operators and readers, current transformers are intended to protect measuring instruments against overloads or overcurrents of very high values. This is possible because its core is specified to enter saturation for currents greater than the rated current times the overcurrent factor.