U LOSS
Loss-of-voltage monitoring relay
A loss-of-voltage relay changes its output for a documented absence-of-voltage condition.
SLD / Schematic
Symbols · K
Minimum-voltage measuring relay
Measuring relay that operates when voltage falls below a set level while some voltage is still present.
In plain English
What does Minimum-voltage measuring relay mean?
The minimum-voltage relay form operates when the measured voltage falls below its setting. It is used for motor undervoltage tripping, load shedding and transfer schemes, where a sustained voltage dip indicates a supply problem rather than a momentary disturbance.
A simple example
The symbol shows the voltage quantity and the less-than threshold but not the time delay, which phases are monitored or whether operation is blocked on VT fuse failure. Without a blocking input a blown VT fuse can cause an unwanted trip, so the supervision arrangement should be cross-referenced.
What the drawings help you do
Check which phase voltages are monitored. Look for VT supervision or fuse-failure blocking. A minimum-voltage relay responds to a reduced voltage; a loss-of-voltage relay responds to absence of voltage.
This particular reference
Measuring relay that operates when voltage falls below a set level while some voltage is still present. A minimum-voltage relay responds to a reduced voltage; a loss-of-voltage relay responds to absence of voltage.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Phase
- One of the related AC waveforms in a power system.
- Signal
- Electrical information passed between devices, rather than necessarily the main power supply.
Read the source: AutoCAD Electrical: Voltage monitoring relays · IEC graphical symbols for diagrams
Schematic / SLD · K
What it represents
The minimum-voltage relay form operates when the measured voltage falls below its setting. It is used for motor undervoltage tripping, load shedding and transfer schemes, where a sustained voltage dip indicates a supply problem rather than a momentary disturbance.
The symbol shows the voltage quantity and the less-than threshold but not the time delay, which phases are monitored or whether operation is blocked on VT fuse failure. Without a blocking input a blown VT fuse can cause an unwanted trip, so the supervision arrangement should be cross-referenced.
How to read it
- Check which phase voltages are monitored.
- Look for VT supervision or fuse-failure blocking.
Information to show or reference
- State threshold and delay
- Show VT source
- Reference blocking input
A distinction that matters
A minimum-voltage relay responds to a reduced voltage; a loss-of-voltage relay responds to absence of voltage.
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: Voltage monitoring relays
Public AutoCAD Electrical relay listing that names the Minimum Voltage Protection 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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