Low Tension Current Transformers

Low Tension Current Transformers are used for measurement and/or protective applications in 660V Class. In Measuring Current Transformers (Refer TABLE- 1) main concern is the accuracy of the secondary current over a wide operating range. Typically the errors are guaranteed to be within specified limits. The nature of errors are current (Ratio) error and phase displacement. Metering current transformers are specified by (1) a burden and (2) accuracy class.

Protective Current transformers (Refer TABLE-II) are specified by (1) a burden (2) accuracy class and (3) accuracy limiting factor. In other words the errors of the protective current transformers at the rated full loaded current as well as at a current given by the accuracy limit factor times the full load current are specified. There exists a class of protective current transformers for special purpose application known as class PS CTs. These are mainly intended for use in applications where the required characteristics of the current transformers cannot be conveniently expressed in terms used for class 5P, 10P and 15P current transformers. The knee point voltage, exciting current and secondary winding resistance are so dependent on the protective gear involved. The manufacturers of protection gear stipulates the requirements in terms of minimum knee point voltage and maximum exciting current. The current transformers of class PS type are of low reactance type since the core is made up of jointless ring type (spirally wound toroidal type) cores and secondary turns are substantially evenly distributed along the whole length of the magnetic circuit. PS class current transformers are characterised by

(1) Normal ratio

(2) Knee point voltage (KPV)

(3) Exciting current at KPV or a stated percentage

(4) Secondary resistance at 750 C.


Current Transformer can be of Ring type, Rectangular type, or Resin moulded type. High quality cores are used for the construction. Computer aided designs are used for all ratings to improve quality and to satisfy customer requirements

A. Ring Type & Rectangular Type

Low Tension Current Transformers are tape insulated, toroidal type. They are available in both ring type and rectangular type as per consumer requirement. High quality core is used in the Current Transformers and the manufacturing is under strict supervision and high quality control.

B. Matching / Interposing Current Transformer

1) Interposing CT: CTs are either Resin cast or framed construction. They are accurate and suitable for the required matching ratio as specified by the customer.

2) Universal Matching CT: Suitable for matching any ratio. Nine tappings are provided and it can be used to match any ratio

C. Resin cast Low Tension CTs

Low Tension Resin cast CTs with terminals : ARI, AR2, AR3

Low Tension Resin cast CT with bar primary : AR4

Low Tension Resin cast CTs window type : AR-5, AR-6.