There are two requirements for measuring CTs. They must:
– have a level of accuracy suitable to the application for the normal operating current;
– protect the devices in the event of a fault current.
The accuracy is defined by the accuracy class determining the permissible phase and current amplitude error over a range of 5% to 120% of the rated primary current
The standardized IEC accuracy classes are: 0.1 - 0.2 - 0.5 - 1 - 3 - 5.
Classes 0.5 and 1 are used in the majority of cases.
Class 0.2 is only used for precision metering.
Classes 0.1 - 3 - 5 are never used in medium voltage.
Specific case: wide range current transformers are devices that can be used permanently with a primary current of 120, 150 or 200% of the rated current. The rise in temperature and transformer accuracy are guaranteed.
Protection of metering devices in the event of a fault is defined by the safety factor SF
This is the ratio between the rated primary limit current ( Ipl ) and the rated primary current ( Ipn). I pl is the value of the primary current for which the secondary current error is equal to 10% (see Figure below).
The preferred SF values are 5 and 10.
Definition and standardized values as per IEC 60185 and DIN VDE 0414-1 are:
Measuring core rated output: 2.5 – 5.0 – 10 – 15 – 30 VA; burden output factor cos ø= 0.8
0.1 – 0.2 – 0.5 – 1: valid in the range of 25 % and 100 % of the rated burden.
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, e.g. for tariff energy metering)
3 – 5: valid in the range 50% to 100% of the rated burden
Label: measuring cores are identified by a combination of the rated output with the overcurrent limit factor and with the class, e.g.
15 VA class 0.5 FS 10
15 VA class 0.5 ext. 150%
(extended current measuring range)