A PTC (Positive Temperature Coefficient) thermistor is embedded directly in the stator winding and paired with a motor protection relay input that monitors its resistance; when the sensor crosses a defined trip threshold the relay de-energises the contactor and latches a fault [S1][S4].
The scheme is distinct from a thermal-overload relay, which senses stator current via a bimetallic strip rather than copper temperature; PTC protection therefore catches cooling-fan loss, blocked ventilation, repeated starting and high ambient even when current stays inside nameplate [S3].
PTC Sensor Behaviour and the DIN 44081 Resistance Curve
A PTC thermistor stays at a low, near-constant resistance through normal operation and then rises sharply once its reference temperature (NAT, Nominelle Ansprech Temperatur) is reached, mimicking a switch [S2][S3]. At 25°C a single sensor typically reads 50–100 Ω; three series sensors sit at roughly 150–300 Ω cold, jump to ≈1.65 kΩ at NAT, and overshoot the 3.0–4.0 kΩ trip band a few degrees above NAT [S2][S4]. A three-in-series DIN 44081 PTC chain is the default winding configuration on IEC-frame motors, and the relay injects a small sense current through the pair of input terminals, not through the motor power leads [S3][S5].
This sharp non-linear step is what makes PTC ideal for a binary protection channel. It is not an analog temperature transmitter: the relay does not resolve winding temperature across the operating range, it only resolves "below NAT" versus "above NAT plus margin" [S6]. For actual winding temperature trending, a PT1000 RTD or KTY84 silicon sensor is the correct choice and requires a different input module [S3].
Insulation Class, NAT Selection and the Trip Threshold
Motor insulation class sets the ceiling; NAT must be selected below the class limit with a safety margin so the relay trips before insulation degrades. Common pairings are Class A 105°C, Class E 115°C, Class B 130°C, Class F 155°C and Class H 180°C; motor builders usually fit PTC NAT 10–20 K below the class limit, so a Class F motor typically carries 130°C or 145°C sensors [S2][S4]. The relay's trip threshold is hard-wired to the DIN 44081 nominal response resistance, not user-adjustable in most basic units; the panel builder's real engineering choice is NAT selection at the motor, not threshold tweaking at the relay [S2][S4].
For hazardous-area motors (Zone 1 / Zone 21), the same PTC chain is used but the relay itself must be ATEX-certified, as on the ABB CM-MSx.xx range, which carries ATEX approval for installation in potentially explosive atmospheres [S1][S5]. In that case the sensor lead routing, the type-tested PTCs and the relay form a single certified protective chain, and the user cannot substitute an uncertified safety relay into the loop [S1].
Comparison of Winding-Temperature Sensing Options
Four practical options show up on motor nameplates and VFD cards. Bimetallic thermal overload relays: cheapest, single-phase friendly, trip class 10/20/30, no winding sensing, no pre-alarm [S3]. PTC thermistor chain (DIN 44081): single binary trip point, ATEX-capable, low cost, no analog trend, fits a dedicated protection relay input [S1][S3][S4]. KTY84 silicon sensor: analog, linear over ≈−50 to +150°C, requires VFD or analog-input module, cannot drive a simple relay directly [S3]. PT1000 RTD: analog, ±0.3°C class A accuracy, four-wire for best accuracy, used where insulation-grade trending or condition monitoring is required [S3].
The decision rule from a process-engineer perspective: if you only need a trip and an interlock, PTC on a relay is the cost-minimising choice. If you need a pre-alarm, a temperature trend on the HMI, or post-trip diagnostics, an RTD feeding a microcomputer protection device or the VFD's analog input is the correct tool, and PTC alone will not give you that data [S3][S4].
Wiring Rules, Sensor Counts and the Short-Circuit Detection
Each PTC sensor is a two-terminal device, and the relay measures resistance through the same two wires that carry the sense current, so a four-wire Kelvin connection is not used; long cable runs therefore appear in series with the sensor resistance [S3][S4]. A practical ceiling of roughly 1 km of two-core shielded cable is typical before the cold resistance drifts into the trip band, and the relay's "dynamic interrupted wire detection" function injects a periodic pulse to confirm the loop is continuous rather than relying solely on steady-state current [S5]. Short-circuit monitoring of the sensor circuit is a separate diagnostic, because a low-resistance short across the input would otherwise mask an overtemperature event and silently disable protection [S1][S5].
Sensors in series add: three PTCs in series produce roughly three times the resistance at every point on the curve, so a 60 Ω cold part becomes ≈180 Ω cold, and the trip band shifts from 3.0–4.0 kΩ for a single sensor to roughly 9–12 kΩ for three in series, which is why the relay's trip table is indexed by sensor count [S2][S4]. Most modern relays auto-detect 1/3/6 sensor configurations through a single setup parameter, but the panel builder must still verify the number of sensors actually fitted to the motor before commissioning [S2][S4].
Failure Modes and Troubleshooting Logic
The three recurrent field failures are well documented. Open sensor or broken lead: the relay sees infinite resistance, recognises the interruption through its dynamic wire-break detection, and trips on sensor fault rather than waiting for an overheat that cannot be measured [S5]. Shorted sensor or lead: resistance collapses toward zero and the relay sees a cold reading, which is dangerous because overtemperature is masked; periodic short-circuit detection in the relay is the only thing that catches this case [S1][S5]. Misconfigured NAT: a Class B motor fitted with Class F NAT sensors will trip well below its thermal capability, costing production hours; class mismatch in the other direction (Class F motor, Class B sensors) risks no trip before insulation damage [S2][S4].
For commissioning, the field check is straightforward: measure resistance at the relay terminals with the motor cold (expect 50–300 Ω for a healthy chain), apply a calibrated heat source to confirm the trip, and verify the fault latches non-volatile so a power cycle does not erase the event [S1][S4][S5]. For deeper system-level grounding, the industrial relay family also covers phase, earth-fault and locked-rotor channels, and a correctly specified thermal channel slots in alongside those without cross-interference when the wiring separation rules (thermistor cable away from VFD power cable) are respected [S1][S4]. For related motor-circuit material decisions, see enameled magnet wire supply 2026 for the wire side of the same thermal-budget question.
Selection Criteria and Specifications Worth Checking
Five specifications separate a credible relay from a near-miss on a real plant. Control supply range: 24–240 V AC/DC on a single unit removes the need for a dedicated power supply and matches DCS-backed cabinets and standalone panels alike [S5]. ATEX/IECEx certification: required where the motor sits in a classified zone, and the certificate must cover the relay plus the sensor chain, not the relay alone [S1][S5]. Output configuration: a dry contact rated for the contactor coil inrush (typically ≥6 A, AC-15) for direct switching, or an active signal to a relay module or PLC for remote monitoring [S4]. Remote reset and non-volatile fault memory: both are necessary if the starter is unattended and you need to know why the line stopped at 02:14 on a Sunday [S5]. Connection technology: push-in or screw, with push-in cutting cabinet wiring time on volume builds [S5].
Two integration details often missed at purchase: first, the relay must be supplied with a sense voltage that does not itself heat the PTC, because self-heating shifts the trip point, and most products specify a sense current well under 1 mA for this reason [S3][S4]. Second, on VFD-driven motors the PTC input must be wired to the relay, not just to the drive's analog input, because many drives will only warn on PTC fault rather than latch a stop; the relay gives you the hard interlock the drive may not [S3][S4].
Standards, Sourcing and Trackable Signals
The relevant technical standards are DIN 44081 / DIN 44082 for the PTC chain characteristics and ATEX 2014/34/EU for hazardous-area certification; motor-side thermal limits are set by the insulation-class definitions in IEC 60085, and these are the documents a specifier should cite in the motor data sheet, not invented clauses [S1][S2][S4]. ABB, Eaton, Schneider, KEB and Pilz all publish PTC-input modules against this same framework, and the cross-vendor interoperability is real because the curve is standardised at the sensor level, not the relay level [S1][S2][S3].
Two trackable signals to watch going forward: a unified move toward IO-Link or Modbus on the relay's PTC channel, so the trip threshold and sensor count are read into the asset record rather than keyed in by hand, and a tightening of ATEX/IECEx marking expectations so that retrofit motors in legacy plants can be brought under the same certified chain without a full rewire [S1][S4][S5]. For panel builders sizing the upstream components, stainless steel targets derating and K1 sensor sizing is unrelated, but the discipline of cross-checking temperature class with derating is the same engineering reflex.