The insulation-class ladder
IEC 60085 defines thermal classes by the maximum allowable hot-spot temperature at which the insulation system is expected to deliver an acceptable service life — conventionally 20 000 hours under otherwise-ideal conditions. The five most common classes:
| Class | Max hot-spot | Typical insulating materials | Use today |
|---|---|---|---|
| A | 105 °C | Cotton, silk, paper, varnished | Rare; small legacy motors |
| E | 120 °C | Synthetic enamels (older) | Very rare in modern builds |
| B | 130 °C | Mica, glass fibre, asbestos with bonding | Common for older industrial |
| F | 155 °C | Class B materials with higher-grade resin | Dominant in new builds |
| H | 180 °C | Silicone resin, glass with silicone bond | High-temperature industrial, hermetic |
| N, R, S | 200, 220, 240 °C | Polyimide, ceramic-bond | Special / aerospace / traction |
The number is the hot-spot limit — usually a few degrees above the average winding temperature, because the inside of the slot is hotter than the outside. Standard practice is to design for the rated rise plus an ambient allowance of 40 °C; that is why the headline class temperature matters but the per-degree margin in operation matters more.
The F/B convention
A great many modern industrial motors are built with Class F insulation materials but rated for Class B temperature rise. The notation is often written “F/B” on the nameplate. The practical meaning: the materials inside the slot will tolerate 155 °C, but the motor is allowed to rise only to the Class B limit of 130 °C in normal operation.
The 25 °C of unused thermal headroom roughly doubles the insulation service life. This follows from the Arrhenius equation — every 10 °C reduction in continuous operating temperature roughly halves the rate of insulation chemical degradation. A motor that runs 25 °C below its insulation limit lasts approximately 4 - 8× longer than the same motor running at the limit. For premium-efficiency industrial motors that get a 20-year service expectation, the F/B convention is the only practical way to deliver it without paying for Class H insulation that the application does not need.
Mapping to PTC trip temperatures
A PTC thermistor embedded in the winding has to trip before the insulation hits its class limit but late enough to avoid nuisance trips on legitimate heavy loading. The rule of thumb is to set Tk at 10 - 15 °C below the insulation class limit.
| Insulation class | Class limit | Typical PTC Tk | Comment |
|---|---|---|---|
| A | 105 °C | 90 °C | Rare today |
| B | 130 °C | 120 °C | Legacy industrial |
| F | 155 °C | 140 - 150 °C | Dominant; F/B builds often use 140 °C to protect the B-rise margin |
| H | 180 °C | 160 - 170 °C | Industrial high-temp, hermetic compressor |
| N+ | 200+ °C | 180 °C | Traction, aerospace |
See our companion article on DIN 44081 / 44082 PTC selection for the catalogue ladder of Tk values. The standard set covers 60 - 180 °C in 5 - 10 °C steps, so almost every common motor insulation class lands on a stock part.
RTD-based protection — what changes
An RTD-based scheme reads continuous temperature instead of a binary trip. The same insulation-class ladder still defines the protection thresholds, but the controller can implement a smarter strategy: an alarm at insulation-class − 20 °C, a trip at insulation-class − 10 °C, with optional load-curtailment in between. For drive-fed motors this lets you ride through transient overloads without nuisance shutdowns.
Embedded RTDs also enable life-prediction algorithms — the controller integrates "degrees-above-rise-limit hours" over the motor’s service life and reports remaining insulation life. This is increasingly common in premium industrial drives and is the reason embedded fluoropolymer Pt100 sensors are showing up in motors that traditionally only carried PTCs.
What about transformer windings?
The same IEC 60085 ladder applies to transformer windings — especially dry-type and resin-cast transformers. The trip-temperature mapping is similar but transformer designers often add a separate oil-temperature limit (for liquid-filled units) or a top-of-winding RTD (for dry-type). For dry-type transformers serving critical loads the standard practice is to embed three RTDs — one per phase in the highest-current section — and run them to a multi-channel monitor.
Bottom line for spec writers
- Find the motor’s nameplate insulation class.
- Check whether it’s rated for class-limit operation or F/B style.
- For F/B, pick Tk 10 - 15 °C below the insulation limit, not the rise limit — the headroom is there to protect against transient overload, not as the routine operating point.
- For protected duty cycles with frequent stalls or starts (cranes, hoists, compressors), bias Tk a few degrees higher to avoid nuisance trips, but never above the class limit.
- For continuous high-availability service (pumps, fans), bias slightly lower to extend insulation life.