Current Transformers (CTs) can be used for monitoring current or for transforming primary current into reduced secondary current used for meters, relays, control equipment and other instruments. CTs that transform current isolate the high voltage primary, permit grounding of the secondary, and step-down the magnitude of the measured current to a standard value that can be safely handled by the instrument. To determine which CT is appropriate for a particular application, it is important to understand the following characteristics that are used to classify current transformers.


The CT ratio is the ratio of primary current input to secondary current output at full load. For example a CT with a ratio of 300 primary amps at full load and will produce 5 amps of secondary current when 300 amps flow through the primary. If the primary current changes the secondary current output will change proportionally. For example, if 150 amps flow through the 300 amp rated primary the secondary current output will be 2.5 amps (150:300 = 2.5:5).


The polarity of a CT is determined by the direction the colls are wound around the core of the CT (clockwise or counter- clockwise) and by the way the leads, if any, are brought out of the transformer case.

All current transformers are subtractive polarity and will have the following designations to guide proper installation: (H1) primary current, line facing direction; (H2) primary current, load facing direction; and (X1) secondary current.

Taking care to observe proper polarity is important when installing and connecting current transformers to power metering and protective relays.

Accuracy Class

Accuracy Class describes the performance characteristics of a CT and the maximum burden allowable on the CTs secondary. Depending on their Accuracy Class, CTs are divided into Metering Accuracy CTs or Relaying Accuracy CTs (Protection CTs). A CT can have ratings for both groups.

Metering Accuracy CTs are rated for specified standart burdens and designed to be highly accurate from very low current to the maximum current rating of the CT. Because of their high degree of accuracy, these CTs are typically used by utility companies for measuring usage for billing purposes.

Relaying Accuracy CTs are not as accurate as Metering Accuracy CTs. They are designed to perform with a reasonable degree of accuracy over a wider range of current. These CTs are typically used for supplying current to protective relays. The wider range of current allows the protective relay to operate at different fault levels.

Another part of the CT Accuracy Class is the maximum burden allowed for the CT. This is the load that may be imposed on a transformer secondary without causing an error greater than the stated accuracy classification. For Metering Class CTs burden is expressed as ohms impedance. For protection-class CTs burden is express as volt-amperes (VA). Protection-class CT burdens are displayed as the maximum secondary volts allowable if 20 times the CT rating were to flow through the secondary circuit (100 amperes with a five-ampere nominal CR secondary).

Current Transformer Shorting

CTs should remain shorted during installation until secondary wiring is complete. Figure 4 shows the termination of a multi- ratio CT on a shorting terminal strip.

A shorting screw inserted through the shorting bar ties isolated terminal strip points together. Any shorting winding effectively shorts the entire CT.

Use Caution When Installing a Current Transformer
  • Inspect the physical and mechanical condition of the CT before installation.
  • Check the connection of the transformer requirements for the instrument or the system requirements before connecting the CT.
  • Inspect the space between the CT phases, ground and secondary conductor for adequate clearance between the primary and secondary circuity wiring.
  • Verify that the shorting device on the CT is properly connected until the CT is ready to be installed. The secondary of the CT must always have a burden (load) connected when not in use.
NOTE: A dangerously high secondary voltage can develop with an open-circuited secondary.