5.5 Rated output
5.5.201 Rated output values
The standard values of rated output for measuring classes, class P and class PR are:
2,5 – 5,0 – 10 – 15 and 30 VA.
Values above 30 VA may be selected to suit the application.
NOTE For a given transformer, provided one of the values of rated output is standard and associated with a standard accuracy class, the declaration of other rated outputs, which may be non-standard values, but associated with other standard accuracy classes, is not precluded.
5.5.202 Rated resistive burden values
Standard values for rated resistive burden in it for class TPX, TPY and TPZ current transformers are:
The preferred values are underlined. The values are based on a rated secondary current of 1 A. For current transformers having a rated secondary current other than 1 A, the above values shall be adjusted in inverse ratio to the square of the current.
NOTE For a given transformer, provided one of the values of rated resistive burden is standard and associated with a standard accuracy class, the declaration of other rated resistive burdens, which may be non-standard values, but associated with other standard accuracy classes, is not precluded.
5.6 Rated accuracy class
5.6.201 Measuring current transformers
5.6.201.1 Accuracy class designation for measuring current transformers
For measuring current transformers, the accuracy class is designated by the highest permissible percentage of the ratio error at rated primary current and rated output.
5.6.201.2 Standard accuracy classes
The standard accuracy classes for measuring current transformers are: 0,1 – 0,2 – 0,2S – 0,5 – 0,5S – 1 – 3 – 5
5.6.201.3 Limits of ratio error and phase displacement for measuring current transformers
For classes 0,1 – 0,2 – 0,5 and 1, the ratio error and phase displacement at rated frequency shall not exceed the values given in Table 201 where the burden can assume any value from 25 % to 100 % of the rated output.
For classes 0,2S and 0,5S the ratio error and phase displacement at the rated frequency shall not exceed the values given in Table 202 where the burden can assume any value from 25 % and 100 % of the rated output.
For class 3 and class 5, the ratio error at rated frequency shall not exceed the values given in Table 203 where the burden can assume any value from 50 % to 100 % of the rated output. There are no specified limits of phase displacement for class 3 and class 5.
For all classes, the burden shall have a power-factor of 0,8 lagging except that, when the burden is less than 5 VA, a power-factor of 1,0 shall be used, with a minimum value of 1 VA.
NOTE In general the prescribed limits of ratio error and phase displacement are valid for any given position of an external conductor spaced at a distance in air not less than that required for insulation in air at the highest voltage for equipment.
5.6.201.4 Extended burden range
For all measuring classes, an extended burden range can be specified. The ratio error and phase displacement shall not exceed the limits of the appropriate class given in Table 201, Table 202 and Table 203 for the range of secondary burden from 1 VA up to rated output. The power factor shall be 1,0 over the full burden range. The maximum rated output is limited to 15 VA.
5.6.201.5 Extended current ratings
Current transformers of accuracy classes 0.1 to 1 may be marked as having an extended current rating provided they comply with the following two requirements:
a) the rated continuous thermal current shall be the rated extended primary current.
b) the limits of ratio error and phase displacement prescribed for 120 % of rated primary current in Table 201 shall be retained up to the rated extended primary current.
The rated extended primary current shall be expressed as a percentage of the rated primary current.
5.6.201.6 Instrument security factor
An instrument security factor may be specified.
Standard values are: FS 5 and FS 10
5.6.202 Protective current transformers
5.6.202.1 General
Three different approaches are designated to define protective current transformers (see Table 204). In practice, each of the three definitions may result in the same physical realization.
