Rated current
Rated or nominal current (earlier designation) is the value of the primary and secondary current cited on the rating plate (primary rated current, secondary rated current), for which the current transformer is dimensioned. Standardised rated currents are (apart from in the classes 0.2 S and 0.5 S) 10 – 12.5 – 15 – 20 – 25 – 30 – 40 – 50 – 60 – 75 A, as well as the decimal multiples and fractions thereof. Standardised secondary currents are 1 and 5 A, preferably 5 A.
Standardised rated currents for the classes 0.2 S and 0.5 S are 25 – 50 – 100 A and their decimal multiples, as well as secondary (only) 5 A.
Correct selection of the primary nominal current is important for the accuracy of measurement. Recommended is a ratio slightly beyond the measured / defined maximum load current (In).
Example: In = 1,154 A; selected transformer ratio = 1,250/5.
The nominal current can also be defined on the basis of the following considerations:
- Dependent on the mains supply transformer nominal current times approx. 1.1 (next transformer size)
- Protection (rated fuse current = CT primary current) of the measured system part (LVDSB, subdistribution boards)
- Actual nominal current times 1.2 (if the actual current lies considerably below the transformer or fuse nominal current then this approach should be selected)
Over-dimensioning the current transformer must be avoided, otherwise the accuracy of measurement significantly decrease especially with small load currents.
