The rated secondary current of the current transformer shall be determined to ensure that the actual load in normal operation reaches about 60% of the rated value, at least not less than 30%. When the actual load current is less than 30%, a multi-ratio current transformer with a tapped secondary winding or a 0.5S, 0.2S class current transformer should be used.
Current transformers with S-class such as 0.2S and 0.5S are mainly used for connection with special watt-hour meters. These watt-hour meters are between 0.05-6A and 1%-120% of rated current 5A. Accurate metering under current.
Both 0.2S and 0.2 are CTs of the same precision. However, the S-class can achieve the accuracy requirement in the light load ("10%"), and the CT without the S-class can reach the accuracy requirement when the load reaches 30%! In particular, the 0.2S class is mainly used in applications where the load variation range is relatively large, and sometimes it is almost empty. In other words, a 0.2-level current transformer can achieve an accuracy level of 0.2 when the current in the circuit reaches 10% of the rated current, and a wide-range 0.2S-level current transformer when the current in the circuit reaches 1% or more. Can achieve an accuracy level of 0.2.
The definition of S grade: 0.2S grade can not exceed plus or minus 0.75% at 1% rated current, and the current error of 0.2 grade at 5% rated current cannot exceed plus or minus 0.75%.
s indicates that the measurement accuracy is maintained in the case of a small current. Although 0.2 and 0.2S are an accuracy level, at a low current, a 0.2-level transformer cannot guarantee accuracy.
The structure of the current transformer is relatively simple, and consists of a primary winding, a secondary winding, a core, and a frame, a casing, a terminal, and the like insulated from each other. The working principle is basically the same as that of the transformer. The number of turns (N1) of the primary winding is less, and it is directly connected in series with the power line. When the primary load current (i1) passes through the primary winding, the alternating magnetic flux induced is proportionally reduced. Secondary current (i2); the number of turns (N2) of the secondary winding is large, and the closed circuit is formed in series with the secondary load (Z) of the current coil of the instrument, relay, transmitter, etc., as shown in Fig. 1.
Since the primary winding has the same ampere-turns as the secondary winding, I1N1=I2N2, the current transformer rated current ratio: 
. In the actual operation of the current transformer, the load impedance is small, and the secondary winding is close to the short-circuit state, which is equivalent to a short-circuiting transformer.
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