The job a PTC does that an RTD can't
A motor-winding PTC is a thermal switch, not a temperature sensor. Below its rated trip temperature (Tk) the device looks like a low resistance — typically below 100 Ω for a single element. Within a narrow window around Tk the resistance shoots up several orders of magnitude in a few kelvins. A protection relay watches for that step and disconnects the motor before insulation damage occurs.
An RTD could do the same job in principle — a transmitter reads the resistance, a controller compares it to a setpoint, and a relay fires. In practice the analogue control loop adds calibration drift, the firmware adds latency, and a single failed cable can leave the loop permanently below its trip point. A PTC fails safe: a broken sensor reads as infinite resistance, which the relay interprets as "overheated" and trips. That is why every European motor over a few kW ships with a PTC pocket cast into the windings, even when the rest of the protection chain is digital.
DIN 44081 — single element
DIN 44081 specifies a single PTC bead in a small insulating sleeve, terminated with two leads. It is the natural choice for single-phase motors — fans, small appliances, hermetic compressors — where one winding hot-spot is the worst-case failure point and a single sensor location is sufficient.
The single-element variant is also used as a one-per-phase sensor inside larger machines where the protection scheme uses three independent inputs to a multi-channel relay rather than a series chain. This buys per-phase diagnostics at the cost of three sensor cables.
DIN 44082 — triple element
DIN 44082 takes three of those same beads, wires them in series inside a single shrink-sleeve assembly, and brings six leads (or two leads after termination) out to the connection box. The classic build is one bead per stator phase — U, V and W — buried in the end-winding bundle at the slot exit.
The series connection makes the assembly behave like one big PTC whose trip happens as soon as any one bead crosses Tk. That is the whole point: a three-phase motor that runs hot on one phase only — for example because of a single-phasing fault, a stuck rotor on one pole, or an unbalanced supply — will reach insulation limit on one bead long before the average winding temperature looks alarming. A single bulk-temperature sensor would miss this failure mode entirely.
The R(T) curve — what every relay reads against
The standard does not specify a smooth curve; it specifies a handful of breakpoint resistances at fixed temperatures relative to the rated trip point Tk. A compliant PTC must lie inside the band at every breakpoint.
| Temperature | Single element (44081) | Triple element (44082) |
|---|---|---|
| -20 °C to Tk - 20 K | R ≤ 250 Ω | R ≤ 750 Ω |
| +25 °C | R ≤ 100 Ω | R ≤ 300 Ω |
| Tk - 5 K | R ≤ 550 Ω | R ≤ 1 650 Ω |
| Tk + 5 K | R ≥ 1 330 Ω | R ≥ 3 990 Ω |
| Tk + 15 K (U ≤ 7.5 V) | R ≥ 12 kΩ | R ≥ 12 kΩ |
The triple-element values are exactly 3× the single-element values up to the trip region, which is what you would expect from three identical beads in series. Above Tk+5 K the curves converge because the trip itself drives every bead into the high-resistance region — the assembly resistance is dominated by the hottest bead at that point.
Note the operating-voltage caveat on the Tk+15 K row: the standard requires the test to be done at U ≤ 7.5 V to avoid self-heating biasing the measurement. The same constraint applies to your motor-protection relay — most parts spec a maximum operating voltage of 2.5 V across the PTC chain to keep self-heating below 100 mW per bead.
Tk values and motor insulation class
DIN 44081/44082 cover trip temperatures from +60 °C to +180 °C in a standard ladder. The right Tk follows from the motor's IEC 60085 insulation class — see our article on insulation classes for the underlying logic. A Class F-insulated motor running B-rise (155 °C insulation limit, 130 °C target) is typically paired with a Tk = 145 °C PTC: hot enough that normal heavy loading doesn't false-trip, cold enough that the protection fires before the insulation cooks.
| Motor insulation class | Class limit | Typical PTC Tk |
|---|---|---|
| Class A | 105 °C | 90 °C |
| Class B | 130 °C | 110 – 120 °C |
| Class F | 155 °C | 140 – 150 °C |
| Class H | 180 °C | 160 – 170 °C |
| Class H+ | 200 °C+ | 180 °C |
What happened to the DIN documents
DIN 44081:1980 and DIN 44082:1985 were both withdrawn and superseded by DIN VDE V 0898-1-401:2020-03, which consolidates them into a single document and aligns the language with IEC 60738-1. The technical content — every breakpoint, every tolerance, every test condition — was carried over unchanged. A datasheet that says "to DIN 44081" or "to DIN 44082" today is, strictly speaking, claiming compliance with the consolidated VDE document; everyone in the supply chain still uses the older names because every motor-protection relay on the market reads against the same R(T) curve.
If you are writing a new product specification in 2026, prefer the language "compliant with DIN VDE V 0898-1-401, formerly DIN 44081/44082". That tells your buyer what they are getting and your auditor what document they are buying it against.
Where the protection relay fits
The other side of the protection chain is the evaluation relay. The KRIWAN INT69 is the most widely-cited reference part: it accepts up to nine series-connected PTC sensors to DIN 44081 / 44082, trips when the chain resistance crosses ~4.5 kΩ, and resets at ~2.75 kΩ. Both thresholds carry a ±20 % tolerance, which is why the standard's Tk+5 K breakpoint sits comfortably below 4.5 kΩ for both single and triple-element parts.
Relay behaviour itself is governed by DIN EN 60947-8, which defines control units for thermistor protection of rotating electrical machines. If you are evaluating a PTC against a non-INT69 relay, check the relay's trip/reset thresholds and the maximum chain resistance it can drive — some compact panel-mount relays only support up to three PTCs in series. See our article on the INT69-style evaluation circuit for a deeper walk-through of the protection-side hardware.
Practical specification checklist
- Topology — single PTC per phase wired to a multi-channel relay, or three PTCs in series per DIN 44082? The latter is cheaper, the former gives you per-phase diagnostics.
- Tk ladder value — pick from the standard 60 / 70 / 80 / 90 / 100 / 110 / 120 / 130 / 140 / 150 / 160 / 170 / 180 °C set. Custom intermediate values are technically possible but force you off the catalogue.
- Lead length and termination — match the motor's terminal-box geometry. Stranded tinned copper or nickel; PVC, silicone or PTFE jacket depending on hot-spot temperature.
- Sleeve / housing — heat-shrink for in-winding embedding; metal or ceramic housing for screw-mount on the stator iron.
- Dielectric rating — AC 2.5 kV is standard; for medium-voltage machines or wet-environment installations specify a high-voltage glass-encapsulated variant explicitly.
Jianlu's MZ6 series PTC thermistors are built to DIN VDE V 0898-1-401 (formerly DIN 44081/44082) across the full 60 – 180 °C Tk range, in single and triple-element variants, with custom lead length and choice of heat-shrink, silicone-resin, ceramic or metal housing. AC 2.5 kV dielectric is standard; ≥ 2.5 kV HV variant available on request.