51
Time overcurrent
An overcurrent function with a documented time characteristic.
Protection functions
Protection functions · 50
Instantaneous overcurrent
An overcurrent function without an intentional operating time delay in its instantaneous application.
SLD / Schematic · 50
What it represents
An instantaneous overcurrent function detects current above its operating criteria without an intentional time delay in the instantaneous application. Actual operating time, implementation and any combined 50/51 labelling are documented by the manufacturer.
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 phase, residual or another measurement from the full designation.
- Separate the function reference from pickup and other settings.
Information to show or reference
- Reference CT or other current inputs.
- Identify the protected circuit and relay.
- Link the associated trip logic.
A distinction that matters
Instantaneous does not mean zero physical operating time. A 50 label does not supply pickup values or prove coordination with other protection.
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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