# Thermistor motor protection relay

Canonical URL: https://eloragrid.ai/symbols/thermistor-motor-protection-relay
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: PTC
Drawing use: Schematic
Reference families: IEC, Manufacturer

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.

### Terms

- 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.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [EMR motor protection: public function comparison](https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/motor-relays.html)
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/)


## 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

## Important distinction

A thermistor relay measures winding temperature directly; a thermal overload relay models heating from current.

## Facts

- Sensor: PTC thermistors in motor windings
- Output: Trip contact in motor control circuit
- Preview: Independent educational concept; consult the applicable standard and project legend for the exact graphic

## Public references

- [EMR motor protection: public function comparison](https://www.eaton.com/us/en-us/catalog/electrical-circuit-protection/motor-relays.html) — Eaton. Public motor protection function comparison including thermal protection.
- [IEC graphical symbols for diagrams](https://tc3.iec.ch/tc-activity/graphical-symbols-for-diagrams/) — IEC. IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.