OL NC
Motor overload trip NC contact
An overload trip NC contact opens on the documented overload relay operation.
Schematic
Symbols · PTC
Thermistor motor protection relay
Relay that monitors PTC thermistors embedded in motor windings and trips when their resistance rises at the rated temperature.
In plain English
What does Thermistor motor protection relay mean?
A thermistor protection relay is connected to a chain of PTC thermistors built into a motor's windings. When the windings reach the thermistors' rated response temperature, their resistance rises sharply and the relay opens its contact in the motor control circuit, protecting against overheating that a current-based overload cannot detect.
A simple example
Typical causes it catches are blocked ventilation, high ambient temperature and frequent starting. The symbol does not show whether the relay detects a short-circuited sensor loop, whether it latches, or how it is reset, which affect safety where automatic restart is possible.
What the drawings help you do
Follow the sensor loop to the motor terminal box. Check reset type and short-circuit detection. A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.
This particular reference
Relay that monitors PTC thermistors embedded in motor windings and trips when their resistance rises at the rated temperature. A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.
- Winding
- A coil of conductor inside an electrical machine or transformer.
- Normally open / normally closed
- The contact state in the normal condition defined for the drawing.
- Coil
- A wound conductor that can produce a magnetic field when energised.
Read the source: EMR motor protection: public function comparison · IEC graphical symbols for diagrams
Schematic · PTC
What it represents
A thermistor protection relay is connected to a chain of PTC thermistors built into a motor's windings. When the windings reach the thermistors' rated response temperature, their resistance rises sharply and the relay opens its contact in the motor control circuit, protecting against overheating that a current-based overload cannot detect.
Typical causes it catches are blocked ventilation, high ambient temperature and frequent starting. The symbol does not show whether the relay detects a short-circuited sensor loop, whether it latches, or how it is reset, which affect safety where automatic restart is possible.
How to read it
- Follow the sensor loop to the motor terminal box.
- Check reset type and short-circuit detection.
Information to show or reference
- Show sensor loop terminals
- State reset type
- Reference motor control trip contact
A distinction that matters
A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
EMR motor protection: public function comparison
Public motor protection function comparison including thermal protection.
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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