CT vs VT: reading instrument transformers on drawings
Distinguish current and voltage measurement, then follow the ratio, polarity, terminal and secondary-circuit references.
Start with the measured quantity
A CT provides a current measurement related to its primary conductor. A VT, also called a potential transformer or PT, provides a voltage measurement related to its primary winding. Both are instrument-transformer concepts, but they have different circuit roles.
Use the equipment label and electrical context to identify the intended role. A simplified transformer-like drawing should not be interpreted without those references.
Read more than the generic mark
The generic representation does not supply the ratio, accuracy, burden, number of cores or complete secondary arrangement. Public manufacturer information shows that current and voltage transformer families have model-specific connections and characteristics.
The SLD locates the measurement in the power system. The schematic and device documentation describe the secondary circuit, terminals and connected measurement or protection equipment.
Keep testing arrangements specific to the application
Current-transformer test facilities require an appropriate selected terminal or test assembly. Phoenix Contact publishes transformer terminals with specific measuring, test and short-circuit features. A generic disconnect terminal should not be assumed to have those same properties.
This guide identifies drawing information to check; it is not an instruction for isolating or disconnecting a live measurement circuit. Operating and testing procedures come from the selected equipment and authorised work documentation.
Make the document links easy to follow
Identify the transformer and connected meter or relay, then cross-reference the terminals and any test facility. Use the documented polarity and ratio information consistently between the SLD, schematic and equipment schedule.
When comparing a CT and VT reference, compare their meaning and secondary-circuit purpose rather than treating graphical resemblance as an interchangeable asset. Product selection and exact standard graphics remain separate decisions.
Related references
Public sources
Public current- and voltage-transformer product families. Ratings, terminals and dimensions depend on the model.
Current transformers, secondaries and conductor indications; vendor block identifiers are distinct from IEC symbol identifiers.
Power transformers, voltage transformers, winding counts, screens and autotransformers.
Current-transformer test terminals and leading short-circuit arrangements. Use the selected product's instructions for any intervention.
Independently authored educational guidance. Original diagrams; no customer drawings or arrangements. Last editorial update: 13 October 2026.