The simplest way of improving protective-current-transformer performance during transients is to make the transformer's magnetising inductance relatively low. This method may diminish transient errors but always increases steady-state errors. If the maximum acceptable transient error is defined, the primary current that may be transformed during the transient within the error limits depends on the transformer's magnetising inductance. For certain values of the magnetising inductance, this current reaches its maximum. It enables an optimisation of the protective current transformer to be made by adjusting both the core cross-section and the airgap length to achieve the desired level of transient accuracy.