7. Current transformer

7.1 Principle

Current transformers fall into two categories: low-cost current transformers for fixed-frequency industrial measurement (50, 60 and 400 Hz) and so-called instrumentation current transformers. The latter have an output for easy connection to measuring equipment, and are shielded against external magnetic fields. The resistor that sets the transformation ratio is sometimes distributed to control the effects of stray inductances and capacitances in the high-frequency winding.

Instrumentation current transformers have a bandwidth of the order of 1 Hz to 20 MHz, with an accuracy of 0.1 to 0.5%. The high cut-off frequency can reach 300 MHz for current transformers optimized for frequency response.

The principle is shown in figure...