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A guide to specifying current transformers (CTs)

Supplier: Dyne Industries By: DYNE Industries Pty Ltd
20 December, 2016

Current transformers are generally toroidal – sometimes rectangular – transformers which are used to monitor a current in one circuit (primary) and deliver a proportional current in a second (secondary) circuit.

There are 2 main applications for CTs

  • Measurement
  • Protection

In Australia there are 2 main standards for CTs

  • AS1675-1986
  • AS60044.1-2007

When CTs are designed, made and tested to one of these standards they do not automatically comply with the other. Therefore it is important to understand the two standards, to use the correct standard in each application and to be consistent at the quoting and ordering stage. The difference may be in:

  • The construction of the CT,
  • The way the rating is stated on the nameplate,
  • The routine testing of the CT, or
  • The way the documentation of testing is provided. Appendix B of AS1675 gives a guidance to the application of CTs and is very useful reading.

Measurement CTs

Both standards require the current and VA to be specified.

AS1675 has classes M and ME. ( Measurement, Measurement Extended). Two tables in the standard show the error limits at various percentages of the rated current. The accuracy classes are 0.1, 0.2, 0.5, 1, 2, and 5 percent.

AS60044.1 has classes M, ME, and S. ( Measurement, Measurement Extended and Special). Three tables in the standard show the error limits at various percentages of the rated current. The accuracy classes are 0.1, 0.2, 0.5, 1.0, 3, and 5 percent.

The size of the CT is significantly affected by the differing test requirements of the 2 standards. Under AS1675 the CT is to be tested more stringently, requiring better performance at the low current (2.5 or 5% of rated current) than AS60044.1. Under AS1675 the maximum difference in performance between various percentages of load is set – not so in AS60044.1.

Therefore it is important to specify the standard to which the CT is to be made and the correct nomenclature and accuracy class in line with that standard.

The instrument security factor and limiting instrument current given in AS60044.1 provide for the maximum current the CT will deliver to the instrument before saturation of the CT core.

Read the rest of this guide here