Technical terms of current transformers

Term	Explanation
Primary rated current I_pr(alternative symbol I_N)	Value of the rated current on the primary side
Secondary rated current I_sr	Value of the rated current on the secondary side
Secondary signal	With low power transformers, a voltage signal in the mV range (e.g. 333 mV) is output using an internal load.
Rated power S_r	Value of apparent power (in [VA]) that the transformer can deliver to the secondary circuit at secondary rated current and rated load
Rated frequency f_R	Value of the rated frequency
Accuracy class	Indication that the measurement deviations are within specified limits under prescribed conditions of use.
Rated insulation level U_m	Maximum voltage; the rms value of the highest phase-to-phase voltage for which a measuring transformer is rated in terms of its insulation. The value of the rated insulation level is specified in 3°increments: 1. maximum value of the conductor-to-conductor voltage for which the insulation of the transformers is designed; 2. Value of the rated short-duration power frequency withstand voltage (50 Hz, 1 min), with which the insulation safety of the devices is tested 3. Value of the surge voltage level (this specification is mostly unoccupied here, since according to IEC 61869/1 a specification is only required for transformers with a conductor-to-conductor voltage of > 1.2 kV)
Overcurrent limiting factor (FS)	Ratio of rated limiting current to primary rated current.
Rated continuous thermal current I_cth	Value of the continuous current in the primary winding at which the overtemperature does not exceed the value specified in the standard, with the secondary winding loaded with the rated load.
Rated short-time thermal current I_th	Value of short-time current for a limited time in the primary winding at which the overtemperature does not exceed the value specified in the standard, with the secondary winding loaded with the rated load.
Rated peak current I_dyn	Maximum value of the primary current whose electromagnetic force effect does not cause electrical and mechanical damage to the current transformer with short-circuited secondary winding.
“Open-circuit voltage” of low power current transformers (e.g. SCL transformer with 333 mV secondary signal)	Due to the low measuring resistance, there is no dangerous “open-circuit voltage” with low power transformers. It is therefore not necessary to short-circuit the secondary terminals.
“Open-circuit voltage” of typical current transformers (e.g. SCT transformers with 1 A and 5 A secondary signal)	WARNING Current transformers which are not directly connected to a load must be short-circuited on the secondary side for safety reasons! A current transformer operated open on the secondary side induces very high peak voltage values at its secondary terminals. The magnitudes of these voltages can reach values of up to several kilovolts, depending on the dimensioning of the current transformer, and thus represent a danger to persons and the functional safety of the transformer. For safety reasons, open operation should generally be avoided in order to prevent magnetization of the core iron during secondary open operation.
Grounding of secondary terminals	According to DIN VDE 0141 (01/2000) paragraph 5.3.4, current and voltage transformers for rated voltages from U_m= 3.6 kV must be earthed on the secondary side. For low voltage (U_m≤ 1.2 kV), grounding is not required if the transformer housings do not have any metal surfaces that can be touched over a large area.