IDMT
Inverse-time characteristic qualifier
Qualifier showing that a measuring relay's operating time falls as the measured quantity rises above its setting.
Schematic / SLD
Symbols · t
Definite-time delay qualifier for measuring relays
Qualifier added to a measuring relay symbol to show that operation follows a fixed time delay after pick-up.
In plain English
What does Definite-time delay qualifier for measuring relays mean?
This additional drawing mark is added inside or beside a measuring relay box to show that the relay operates a fixed time after its measured quantity crosses the threshold. Combined with a quantity such as I> or U<, it tells the reader the element is definite-time rather than instantaneous or inverse-time.
A simple example
The additional drawing mark states the type of delay, not its value. The delay setting itself must appear in the settings table, and an element shown without any time additional drawing mark should not automatically be assumed to be instantaneous unless the legend says so.
What the drawings help you do
Read the additional drawing mark together with the quantity and threshold sign. Take the actual delay value from the settings record. A definite-time additional drawing mark gives a fixed delay; an inverse-time additional drawing mark gives a delay that shortens as the quantity increases.
This particular reference
additional drawing mark added to a measuring relay symbol to show that operation follows a fixed time delay after pick-up. A definite-time additional drawing mark gives a fixed delay; an inverse-time additional drawing mark gives a delay that shortens as the quantity increases.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Schematic
- A drawing showing function and connections rather than physical appearance.
Read the source: IEC graphical symbols for diagrams · Autodesk IEC-60617 preview: Operating-device and function qualifiers
Schematic / SLD · t
What it represents
This qualifier is added inside or beside a measuring relay box to show that the relay operates a fixed time after its measured quantity crosses the threshold. Combined with a quantity such as I> or U<, it tells the reader the element is definite-time rather than instantaneous or inverse-time.
The qualifier states the type of delay, not its value. The delay setting itself must appear in the settings table, and an element shown without any time qualifier should not automatically be assumed to be instantaneous unless the legend says so.
How to read it
- Read the qualifier together with the quantity and threshold sign.
- Take the actual delay value from the settings record.
Information to show or reference
- Pair with quantity and threshold
- State delay setting
- Confirm legend convention
A distinction that matters
A definite-time qualifier gives a fixed delay; an inverse-time qualifier gives a delay that shortens as the quantity increases.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
IEC graphical symbols for diagrams
IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.
Autodesk IEC-60617 preview: Operating-device and function qualifiers
Public IEC-context qualifier family showing how qualifiers modify a base symbol.
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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