Temperature Monitoring Relays: PTC Thermistor and PT100
What is a temperature monitoring relay? A temperature monitoring relay is a DIN-rail device that reads a temperature sensor — a PTC thermistor embedded in a motor winding or an external PT100/PT1000 RTD — and switches an output contact when the reading crosses a set threshold, per IEC 60947-8 for thermistor protection. Miss the winding heat and the insulation cooks; catch it early and the motor trips before the varnish breaks down. This article covers PTC thermistor operation, PT100/PT1000 operation, how to choose between them, threshold and hysteresis behavior, wiring, and where each type gets specified.
How PTC Thermistor Protection Works
A PTC (positive temperature coefficient) thermistor is a small ceramic sensor embedded directly in the motor winding at the manufacturing stage, one per phase on most three-phase motors. Its resistance stays low and roughly flat until the winding reaches the sensor's reference temperature, then climbs sharply over a narrow band. The monitoring relay does not measure degrees; it measures resistance and compares it to fixed thresholds defined in IEC 60947-8.
Three resistance bands matter. Below roughly 250 Ohm, the relay reads a short-circuited sensor or sensor chain and can flag a fault. Around the reference resistance — nominally 3.3 kOhm — the relay trips on overtemperature. Above roughly 4 kOhm, the relay reads an open circuit (broken lead, disconnected sensor) and again reports a fault rather than treating it as "cold." That three-band logic is why PTC relays are simple and fast: they do not need calibration against an ambient baseline, they just watch resistance cross fixed points.
Three sensors are normally wired in series into one input, because winding heat can be localized (a shorted turn on one phase does not always heat the other two evenly). Series wiring means the relay trips on whichever phase gets hot first, which is the intent.
How PT100 and PT1000 RTD Monitoring Works
A PT100 (or PT1000) is a platinum resistance temperature detector. Unlike a PTC, its resistance changes smoothly and near-linearly with temperature across a wide range, which means the relay can display and switch on an actual temperature value, not just a pass/fail band. That is the core trade-off against PTC: PT100 gives a number on a display or a 4-20 mA output for trending; PTC gives a binary trip at one fixed reference point per sensor.
PT100 sensors are usually mounted externally — bolted to a bearing housing, strapped to a winding overhang, or inserted into a thermowell in a process line — rather than cast into the winding insulation. Because the relay is reading a continuous signal, the user sets the trip point (and often a separate warning point) in the relay configuration rather than relying on a sensor that is pre-built to one reference value.
Two- and three-wire RTD connections exist; longer cable runs need three-wire compensation so lead resistance is not read as extra heat. What we see in the field: three-wire PT100 wiring gets skipped on short runs inside the same panel, which is fine, but on a sensor 30-40 meters out on a pump or compressor skid, lead resistance error becomes real and shows up as a phantom few degrees of offset.
Formula: PTC Trip Resistance
Formula: PTC Sensor Trip Threshold — Source: IEC 60947-8
Rsensor ≈ Rref (nominal reference resistance ≈ 3.3 kΩ) triggers trip; Rsensor < Rshort or Rsensor > Ropen triggers sensor-fault output
| Symbol | Description | Unit |
|---|---|---|
| Rsensor | measured resistance of the series-connected PTC chain | Ω / kΩ |
| Rref | reference (trip) resistance defined in IEC 60947-8 | kΩ (≈3.3 kΩ) |
| Rshort | lower fault band, indicates shorted sensor or wiring | Ω (low, per relay design) |
| Ropen | upper fault band, indicates open circuit / broken lead | kΩ (high, per relay design) |
PTC vs PT100: Choosing the Right Sensor Type
The choice usually is not really a choice — it follows the motor. If the motor already has PTC sensors cast into the windings (standard on most IE3/IE4 industrial motors above a certain frame size), fit a PTC relay and stop there. Adding a PT100 relay to a motor that has no RTD sensor means retrofitting a temperature probe onto a bearing or winding overhang, which is a mechanical job, not just a wiring job.
PT100 earns its keep on process equipment where the operator wants a number: bearing temperature on a large pump, oil temperature on a gearbox, ambient temperature in a panel. Trending a temperature is more useful than a single trip point when the failure mode develops slowly — bearing wear, for instance, shows up as a temperature that climbs over weeks, and only a PT100 relay (or a PLC reading the RTD directly) lets someone act on the trend before it trips.
| Criteria | PTC Thermistor | PT100 / PT1000 RTD |
|---|---|---|
| Output type | Binary trip at fixed reference resistance | Continuous resistance/temperature, user sets trip point |
| Typical mounting | Cast into winding insulation at motor build | Bolted/strapped externally or in a thermowell |
| Best for | Motor winding overtemperature, fast thermal events | Bearings, gearboxes, process fluids, slow drift monitoring |
| Setup effort | Low — reference value is fixed by the sensor/standard | Higher — trip and warning points configured per application |
| Governing standard | IEC 60947-8 | No single dedicated IEC number; relay per IEC 60947-5-1 |
Trip Threshold, Hysteresis and Output Logic
PTC relays do not have an adjustable threshold in the usual sense — the trip point is set by the sensor's reference resistance, which the motor manufacturer chose to match the winding insulation class. What is configurable is usually the output logic: automatic reset once the winding cools back below the reset resistance band, or manual/latched reset requiring an operator to acknowledge the fault before restart.
PT100 relays behave like other measuring relays in this catalog: adjustable threshold, adjustable hysteresis (the gap between trip and reset temperature that stops the output chattering as the reading sits right at the setpoint), and an adjustable trip delay to ride through a brief spike. Fail-safe (normally-energized) output logic means loss of the auxiliary supply also drops the output, which most panel builders treat as the default for motor protection duty.
Wiring and Installation Considerations
PTC sensor loops run at low voltage and low current from the motor terminal box to the relay input; standard control cable is enough, but keep the run short and away from power cabling where practical since the relay input is sensitive to induced noise on long parallel runs next to VFD output cables. Three sensors in series into one input is standard; some ranges support two independent PTC loops for dual-winding or star-delta motors.
PT100 wiring needs a decision on two-wire versus three-wire connection before the cable is pulled, since three-wire needs an extra core for lead-resistance compensation. This depends on how long the run is and how tight the temperature accuracy requirement is — a rough bearing-temperature trend for maintenance planning tolerates two-wire error that a process-critical reading would not.
Both sensor types feed into a relay output contact (SPDT or DPDT change-over) that then drives whatever the panel design calls for: a contactor coil to drop the motor, a PLC digital input for a soft interlock, or an alarm horn. The relay itself does not interrupt motor current — that is the contactor's job downstream of the relay contact.
Where Temperature Monitoring Relays Are Used
PTC relays are specified anywhere a motor already ships with embedded thermistors: pumps, fans, compressors, conveyor drives, and most industrial motors above a few kilowatts. It is standard practice to wire the PTC loop into the motor starter's control circuit so a winding fault drops the contactor directly, independent of thermal overload relay settings, since overload relays protect against sustained overcurrent, not a localized insulation hot spot.
PT100 relays show up on rotating equipment where bearing condition matters — large pumps, blowers, gearboxes — and on process skids where a fluid or ambient temperature needs a documented setpoint, such as panel interior temperature for cooling fan control or oil temperature on a hydraulic power unit. See our motor protection and thermistor monitoring guide for how temperature monitoring fits alongside phase-loss and current protection on the same motor.
This is one function inside a broader family covered in the monitoring relay engineering guide, which also covers voltage, current, level and phase-sequence monitoring. For the threshold, hysteresis and delay behavior shared across all monitoring relay types, see hysteresis, trip delay and latching in monitoring relays. Browse the full range of monitoring and control relays or pair temperature protection with motor protection circuit breakers and standard thermal overload relays for full motor branch protection.
Frequently Asked Questions
Can a PTC relay replace a thermal overload relay?
No. A PTC relay reads winding temperature directly and catches localized hot spots such as a shorted turn or blocked ventilation; a thermal overload relay reads line current and catches sustained overcurrent across the whole circuit. Motors with both risks specified normally carry both protections.
What happens if a PTC sensor lead breaks?
The relay reads the resistance climbing past the open-circuit band, above the trip reference, and reports a sensor fault rather than assuming the motor is cold. Most relays trip the output on this condition rather than ignoring it, since running with a disconnected sensor removes protection entirely.
Why does my PT100 relay show a different temperature than a handheld thermometer?
Check the wiring mode first — a two-wire connection on a long cable run adds lead resistance that the relay reads as extra temperature. Three-wire compensation removes most of this error. Sensor placement also matters: a probe strapped to a bearing housing reads housing temperature, not internal bearing temperature.
Do PTC relays need an auxiliary power supply?
Yes, typically. Unlike some three-phase supply monitors that draw power from the measured lines, PTC and PT100 temperature relays are usually powered from a separate 24V DC or 110/230V AC auxiliary supply, since the sensor signal itself carries no usable power.
Can one relay monitor both PTC and PT100 inputs?
Some ranges offer multi-input modules that accept either sensor type on configurable channels, but single-function relays dedicated to one sensor type are more common and usually cheaper for a single-motor application. Check the specific model's input configuration before assuming multi-sensor capability.
Conclusion
PTC and PT100 solve the same problem — catching a motor or bearing before it overheats — with different trade-offs. PTC is simple, fast, and matched to the motor by the manufacturer; it trips at one fixed reference point and needs no field calibration. PT100 trades that simplicity for a continuous, configurable reading that supports trending and a chosen setpoint, at the cost of external mounting and wiring decisions that affect accuracy. Specify PTC where the motor already carries embedded sensors and a straightforward trip is enough; specify PT100 where the application needs a number, not just a flag.
