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General Information

Current Transformers - General Specifications, Selection and Installation Information

Technical Specification

  • Reference Standards : EN60044–1
  • Housing : Self extinguishing thermoplastic classification VO to UL94* (*unless otherwise stated)
  • Temperature Range : Working –25 to +50˚C, relative humidity 85%
  • Frequency : 47…63Hz (400Hz on request)
  • System Voltage : 720V max.
  • Test Voltage : 3KV for 1 minute
  • Rated short-time current (l th) : 60 In (TAQ1, TAQ2 and TAQ6 30 In)
  • Dynamic short circuit current (l dyn) : 2.5 l th
  • Saturation Coefficient : <5 C for class 0.5/1/3
  • Terminal Markings : Primary P1 & P2 (K & L) : Secondary S1 & S2 (k & l)
  • Safety Factor (f.s.) : ≤5 for class 0.5 - 1 - 3

Options

  • Tropicalized execution : Working temperature –25 to +60˚C relative humidity >95%. Max conductor temperature 90˚C
  • System Voltage : 1.2KV
  • Test Voltage : 6KV for 1 minute

Important Note

  1. It is essential with certain instrumentation that the CT is physically positioned correctly on the conductor. P1 (K) must face the supply feeder, and P2 (L) must face the load. It is also important to ensure that secondary connections are made in accordance with instrument diagrams.
  2. The secondary terminals of the CT must NOT be open-circuited on load as dangerously high voltages may be present under these conditions. It is recommended that one side of the secondary windings is earthed, unless where specified otherwise on the equipment itself.

Application Note

When using CT’s with an aperture, it is possible to reduce primary ratio by passing multiple turns of the primary conductor cable through the aperture. The resultant ratio will be CT primary divided by the number of turns, e.g. a 200/5A CT with the primary conductor passed through the aperture twice will produce a CT ratio of 100/5A.

Secondary Lead Burden

When selecting a current transformer, it is important to consider the power absorbed by the cables connected between the CT secondary terminals and the measuring instrument. The resultant cable burden should be added to the equipment burden, and the total should not exceed the available VA of the CT. Where the current transformer is to be mounted remotely, a–/1A secondary is recommended.

For every 10˚C variation in temperature, the VA absorbed by the cables will increase by 4% (Ambient Reference Temp ˚C).

Cable burden chart for 5A current transformers.

Accuracy of a Transformer

The percentage of error, produced in a transformer, is established by IEC60044-1. In measurement transformers: 25% and 100% of nominal power. In protection transformers: 100% of nominal power.