Generally the accuracy limit factor of measuring transformers is related to amount of transformer magnetizing current , therefore working point of core magnetizing curve influence the error. Also working point of magnetizing curve is related to secondary load (Is.Zs) or voltage.
The primary current contains two components:
An exciting current, which magnetizes the core and supplies the eddy current and hysteresis losses, etc.
A remaining primary current component, which is available for transformation to secondary current in the inverse ratio of turns.
The exciting current is not being transformed and is therefore the cause of transformer errors. The amount of exciting current drawn by a CT depends upon the core material and the amount of flux that must be developed in the core to satisfy the output requirements of the CT. that is, to develop sufficient driving voltage required, pushing the secondary current through its connected load or burden. This can be explained vectorally in Figure below.
According to relevant standards, measuring accuracy defined for specified region of secondary current (burden). For tariff measuring class this region is very wide and is acceptable between 1%-125% rated burden.
Definition and standardised values as per IEC 60185 and DIN VDE 0414-1:
Measuring core rated output: 2.5 – 5.0 – 10 – 15 – 30 VA; burden output factor cosφ= 0.8
Classes: 0.1 – 0.2 – 0.5 – 1: valid in the range of 25 % and 100 % of the rated burden.
Classes: 0.2 s and 0.5 s: For special applications (electrical meters that measure correctly between 50 mA and 6 A, i.e. between 1% and 120% of the rated current of 5A)
3 – 5: valid in the range 50% to 100% of the rated burden