21
Distance protection
Uses measured voltage and current to assess an electrical distance or impedance characteristic.
Protection functions
Symbols · K
Minimum-impedance measuring relay
Measuring relay that compares voltage with current and operates when the apparent impedance falls below a set value.
In plain English
What does Minimum-impedance measuring relay mean?
A minimum-impedance relay divides measured voltage by measured current and operates when the result falls below its setting, indicating a fault close enough to fall inside its reach. It is used as generator and transformer backup protection and is the basic idea behind distance protection.
A simple example
The relay needs both CT and VT inputs, so the symbol should always be read with both connections. It does not show the characteristic shape on the impedance plane, any directional element, or the loss-of-voltage blocking needed to avoid operation when the VT supply fails.
What the drawings help you do
Confirm both voltage and current inputs are shown. Check for VT fuse-failure blocking. A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.
This particular reference
Measuring relay that compares voltage with current and operates when the apparent impedance falls below a set value. A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.
- Winding
- A coil of conductor inside an electrical machine or transformer.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Terminal
- An identified point where a conductor connects.
Read the source: AutoCAD Electrical: Miscellaneous monitoring relays · IEC graphical symbols for diagrams
Schematic / SLD · K
What it represents
A minimum-impedance relay divides measured voltage by measured current and operates when the result falls below its setting, indicating a fault close enough to fall inside its reach. It is used as generator and transformer backup protection and is the basic idea behind distance protection.
The relay needs both CT and VT inputs, so the symbol should always be read with both connections. It does not show the characteristic shape on the impedance plane, any directional element, or the loss-of-voltage blocking needed to avoid operation when the VT supply fails.
How to read it
- Confirm both voltage and current inputs are shown.
- Check for VT fuse-failure blocking.
Information to show or reference
- Show CT and VT inputs
- State reach and delay
- Reference blocking logic
A distinction that matters
A minimum-impedance relay uses voltage and current together; an overcurrent relay uses current alone.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
AutoCAD Electrical: Miscellaneous monitoring relays
Public AutoCAD Electrical relay listing that names the Minimum Impedance Relay block; the illustration is independently drawn.
IEC graphical symbols for diagrams
IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.
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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