CT
Current transformer
A current-measurement transformer used with metering and protection equipment.
SLD / Schematic
Protection functions · 87
Differential protection
A function comparing documented electrical quantities across a protection zone.
SLD / Schematic · 87
What it represents
Differential protection compares quantities associated with the boundaries of a protection zone according to the selected implementation. Equipment and application suffixes can distinguish transformer, busbar, line or other arrangements. The base number alone does not define the zone or measurements.
On a power drawing, keep this function reference connected to the selected equipment and its complete designation. A multifunction relay can implement several numbered functions. This entry explains the reference; it supplies no settings or commissioning procedure.
How to read it
- Identify the protected zone and each measurement boundary.
- Read CT associations, polarity and any device-specific compensation from the protection documentation.
Information to show or reference
- Identify the protected equipment or zone.
- Reference measurement boundaries.
- Link the relay application and trip destinations.
A distinction that matters
Transformer differential, busbar differential and other differential applications are not interchangeable merely because they share the base number 87.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
Public reference table of ANSI device-function numbers
Public manufacturer table used to check the base function-number name.
IEEE C37.2-2022: power-system device function numbers
Normative publication reference for the device-function convention.
Relion 620: functions, codes and symbols
Public examples of manufacturer-specific function names and suffixes.
About this graphic
This illustration was authored independently by Elora Grid to explain the concept. It is not reproduced from a customer drawing, an official standard graphic or a manufacturer CAD file. Its correspondence to an exact IEC or IEEE symbol identifier has not been certified.
SVG and DXF downloads contain this original artwork. DXF uses unscaled drawing units; a GA envelope supplies no product dimensions, clearances or fabrication information. Original artwork is available under CC BY 4.0. Third-party references retain their own terms.
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