Every motor failure has a story. Most start the same way — insulation that gave out before the winding did. Heat, vibration, moisture, voltage spikes, chemicals. One or all of them. The question isn’t if insulation degrades. It’s how fast — and what’s driving it.
This article covers the five aging stresses, what each one actually does to insulation, and how the IEC 60085 thermal class system gives you a framework for managing the biggest one.
Table of Contents
The Job Insulation Is Doing
Keep conductors at different potentials separated. That’s it. The moment that barrier fails, you’ve got a fault. Insulation doesn’t fail suddenly — it degrades gradually, losing dielectric strength, becoming brittle or conductive, until it can’t do the job anymore.
Five stresses drive that process.
Stress 1 — Thermal (The Dominant One)
Heat is the primary killer. It accelerates chemical breakdown — oxidation, hydrolysis, polymer chain degradation — all of it speeds up with temperature. The relationship follows Arrhenius chemistry: roughly every 10°C rise halves insulation life. That’s the Montsinger rule of thumb, and while it’s not exact for every material, it’s close enough to take seriously.
This is precisely why IEC 60085 exists. The standard assigns thermal classes to insulation systems based on the maximum continuous temperature the system can sustain while still delivering acceptable service life. That rating comes from either real field experience — called the Assessed Thermal Endurance (ATE) — or comparative lab testing against a known reference material, called the Relative Thermal Endurance (RTE).
IEC 60085 is explicit: this standard applies where thermal stress is the dominant aging factor. That qualifier matters. It’s not one stress among equals — it’s usually the one running the clock.
Standard reference — IEC 60085:2007, Clause 1 (Scope): “This standard is applicable where the thermal factor is the dominant ageing factor.”
Stress 2 — Electrical
The voltage gradient across the insulation wall drives partial discharge — small ionization events inside voids or at interfaces that erode the material from the inside. You don’t see it happening. But over time, those discharges eat through the insulation wall until breakdown occurs.
This is a bigger concern in medium and high voltage machines, but it’s always present. Surge events and switching transients make it worse. Transient voltages can develop from internal or external sources and often occur during motor startup — the transient current frequency can be several times higher than normal winding current, putting extreme stress on the insulation.
VFD-fed motors are the clearest modern example. Inverter drives introduce fast voltage rise times that stress turn insulation and weak points, and higher voltage designs increase the risk of partial discharge if voids exist.
Stress 3 — Ambient
Everything the operating environment throws at the insulation — moisture, process chemicals, oils, cleaning agents, industrial contamination. Moisture reduces dielectric strength, increases leakage currents, and accelerates tracking.
Chemical attack changes the material properties of the insulation itself. A motor in a clean, dry, climate-controlled room ages completely differently from one running in a petrochemical plant, a food processing facility, or a marine environment.
Standard reference — IEC 60085:2007, Clause 4.2: “Apart from thermal factors, the ability of the EIS to fulfil its function is affected by many factors, such as electrical and mechanical stresses, vibration, deleterious atmospheres and chemicals, moisture, dirt and radiation.”
Stress 4 — Mechanical
Vibration, shock, and conductor movement during start events. Every time a motor starts, the conductors experience electromagnetic forces. Over thousands of starts, the insulation between conductors flexes, abrades, and cracks. Movement inside the slot, sharp lamination edges, and cyclic vibration can slowly cut through liners and sleeves.
Coil support systems, end-winding bracing, and varnish fill quality all exist specifically to fight this. A rewind that skips proper impregnation is leaving this stress unmanaged.
Stress 5 — Voltage Transients and Harmonics
Some standards group this under electrical stress. In practice it’s worth separating, especially for inverter-driven machines. The concern here isn’t steady-state voltage gradient — it’s the repeated impulse events from fast-switching drives and harmonic content that drives additional heating and dielectric stress beyond what the thermal class rating alone accounts for.
The Thermal Class System — What the Numbers Mean
IEC 60085 gives the industry its common language for thermal rating. The thermal class is a single number: the maximum recommended continuous operating temperature in degrees Celsius for the insulation system. Here’s the full table directly from the standard:
| Thermal Class (°C) | Letter | ATE/RTE Range (°C) |
|---|---|---|
| 90 | Y | ≥90 <105 |
| 105 | A | ≥105 <120 |
| 120 | E | ≥120 <130 |
| 130 | B | ≥130 <155 |
| 155 | F | ≥155 <180 |
| 180 | H | ≥180 <200 |
| 200 | N | ≥200 <220 |
| 220 | R | ≥220 <250 |
| 250 | — | ≥250 <275 |
Standard reference — IEC 60085:2007, Table 1, footnotes:
- Letter designation may be added in parentheses, e.g. Class 180 (H). Where space is limited, the product TC may elect to use only the letter designation.
- Thermal classes above 250°C increase by increments of 25 and are designated accordingly.
Class F (155°C) is now the standard for most general-purpose motors. Class H (180°C) is used for higher duty cycles or where the designer wants thermal margin built in. Using a higher insulation class with a lower temperature rise class allows a significant increase in insulation lifetime.
The Part Most Articles Get Wrong: System vs. Material
This is where a lot of published content stops short — and where real mistakes happen in the field.
The thermal class on the nameplate belongs to the insulation system, not the individual materials inside it. IEC 60085 is direct on this point.
Standard reference — IEC 60085:2007, Clause 4: “The description of an electrotechnical device as being of a particular thermal class does not mean, and must not be taken to imply, that each EIM used in its construction is of the same thermal endurance.”
“Therefore, the thermal capabilities of an EIM shall not be deduced from the thermal class of an EIS of which it is a component.”
Here’s why that matters in practice. In a complete insulation system, surrounding materials can protect a lower-rated component — allowing it to function in a higher-class system than its individual rating suggests. But the reverse is also true: chemical incompatibility between materials can reduce the effective thermal class of the whole system below what any single component would indicate.
Standard reference — IEC 60085:2007, Clause 4: “The protective character of other EIM used in the system may improve the performance of an individual EIM allowing its use in an EIS with a thermal class greater than the thermal endurance of the individual EIM. On the other hand, problems of incompatibility between EIM may decrease the appropriate thermal class of the system below the thermal endurance of the EIM.”
You can spec Class H wire enamel, Class H slot liner, and Class H varnish — and still end up with a system that doesn’t perform as Class H if those materials interact badly or if the impregnation process is compromised.
This is critical during rewinds. Mixing insulation materials from different suppliers, substituting “equivalent class” materials, or cutting corners on varnish impregnation can silently downgrade the effective thermal class of the rewound machine — even if every component label reads correctly.
How the Five Stresses Compound
They don’t add up — they multiply. A motor running hot AND experiencing vibration AND exposed to moisture isn’t aging at the sum of those stresses. The interactions between them accelerate degradation faster than any single factor would predict.
Thermal, electrical, ambient, and mechanical stresses rarely act in isolation. That’s why condition assessment in harsh environments needs to account for all of them, not just the temperature trend on your monitoring system.
What to Do With This in Practice
Check your winding temperature data against the nameplate thermal class. A Class F motor in a 40°C ambient with a 105K temperature rise is sitting right at its design limit — that’s normal per the design intent. If actual winding temperatures are consistently above that, the insulation is aging faster than the designer planned.
Watch for the stress combinations. High temperature plus high starts per day plus a wet environment is a very different situation from any one of those factors alone. The thermal class tells you the ceiling — the operating conditions and stress mix determine how fast you’re approaching it.
Standards Referenced in This Article
| Standard | Scope |
|---|---|
| IEC 60085:2007 Ed. 4.0 | Thermal evaluation and designation of electrical insulation — primary reference for thermal classes |
| IEC 60216-1 | Ageing procedures and evaluation of test results for insulating materials |
| IEC 60216-5 | Determination of Relative Thermal Endurance Index (RTE) |
| IEC 60505 | Evaluation and qualification of electrical insulation systems |
| IEC 61857 (all parts) | Procedures for thermal evaluation of EIS |
| IEC 61858 | Thermal evaluation of modifications to established wire-wound EIS |