CTs should be specified as follows :

RATIO : input / output current ratio
VA : total burden including pilot wires, see diagram below
CLASS : Accuracy required for operation
DIMENSIONS : maximum & minimum limits

The following factors affect CT prices :

• Specifying a higher VA or CLASS than necessary usually results in a higher cost.
• The cost generally increases as the CT's internal diameter increases.
• 1A CTs are usually more expensive than 5A.
• Fittings such as mounting feet are charged as extras. However, these costs are often more than recovered by the reduction in fitting costs on assembly.
• Should special accuracies or burdens be required, other than those listed on our standard lists, please contact our Design Engineers who may be able to suit your needs.

NOTE : It is important to ensure the secondary of the CT is not left disconnected while the primary supply is on. In this condition high-voltage spikes may be produced in the transformer secondary, often into thousands of volts, sufficient to degrade the transformer insulation.

• 0.1 or 0.2 for precision measurements
• 0.5 for high grade kilowatt hour meters
• 1.0 for commercial grade kilowatt hour meters
• 1.0 or 3 for general industrial measurements
• 3 or 5 for approximate measurements

• Moving iron ammeter 1-2VA
• Moving coil rectifier ammeter 1-2.5VA
• Electrodynamic instrument 2.5-5VA
• Maximum demand ammeter 3-6VA
• Recording ammeter or transducer 1-2.5VA

In addition to the general specification required for CT design, protection CTs require an Accuracy Limit Factor (ALF). This is the multiple of rated current up to which the CT will operate while complying with the accuracy class requirements.

In general the following applies:

• Instantaneous overcurrent relays & trip coils -
  2.5VA Class 10P5
• Thermal inverse time relays -
  7.5VA Class 10P10
• Low consumption Relay - 2.5VA Class 10P10
• Inverse definite min. time relays (IDMT) overcurrent - 15VA Class 10P10/15
• IDMT Earth fault relays with approximate time grading - 15VA Class 10P10
• IDMT Earth fault relays with phase fault stability or accurate time grading required - 15VA Class 5P10