27
Undervoltage
A function responding to voltage below its configured operating criteria.
Protection functions
Protection functions · 81
Frequency protection
A function responding to a documented frequency condition.
SLD / Schematic · 81
What it represents
Frequency protection responds to frequency conditions according to the selected implementation. Underfrequency, overfrequency and rate-of-change functions can be distinguished by additional suffixes and device documentation. A generic 81 reference does not determine which variant is applied.
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
- Read the complete designation, not only the base number.
- Identify the voltage or other input from which the selected device derives its frequency measurement.
Information to show or reference
- State the applied frequency function.
- Reference the measurement and relay.
- Link any trip, alarm or load-control action.
A distinction that matters
Underfrequency, overfrequency and rate-of-change are different functions even when their designations share the number 81.
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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