Current Transformers - General Information

All instrument transformers contained in the first part of this catalogue are cast-resin transformers with the following characteristics:

  • free of maintenance
  • dry-type
  • installation in any position possible.

The features of the cast resin material used for construction guarantee

  • best insulation
  • great mechanical strength
  • high arc and temperature stability.

All instrument transformers are subject to a routine test according to the respective standards (e.g. DIN VDE, IEC).

Current transformers with a primary winding having change over terminals are provided with a plastic cover and a schematic circuit diagram to see the actual connection of the terminals.

The secondary terminals of the winding intended for calibration purposes can be sealed. Multi-core type current transformers can have up to 4 cores.


A capacitive voltage tapping is available for all support type current transformers. The voltage can be tapped from an additional terminal in the terminal box or a special socket above the terminal box according to ordered CT-type. The capacitance of the high voltage insulation C1 is connected to terminal C and can be used for simple voltage indication. It depends on type and number of cores in the transformer. If not used terminal C has to be earthed. The capacitive voltage tapping can be designed for HR-
or LRM-system (on customer’s request).

Current Transformers for Measurement

intended for connection with measuring instruments, integrating meters and similar devices (e.g. 15 VA Cl 0,5 FS 5).

The rated accuracy limit factor (FS) is the factor which, when applied to the rated primary current (IN), gives the r.m.s. value of the maximum current at which the current transformer complies with the accuracy requirements Fg.

FS 5, i.e. min. 10% composite error at 5 x IN FS10, like FS 5, but at 10 x IN.

Current Transformers for Protection

intended for use with electrical protection devices, (e.g. 15 VA 10P10). A current transformer for protection is classified in Class 10P and 5P expressing in terms the secondary terminal performance:

1) Accuracy class at rated current

2) Rated composite error at accuracy limit current in percent

The accuracy class 5P requires a higher demand in construction of the core as the accuracy class 10P. The accuracy limit factor is given by the figure behind the accuracy class designation 10P and 5P respectively. 10P10 (5P10): max. 10 (5)% composite error Fg at 10 x IN. For general purpose the accuracy class 10P10 is used.


Behaviour of Current Transformers in Case of Over Current