What is insulation resistance? 5 things to know
Maintaining the right level of insulation resistance is vital in many industries which use machinery that operates at high voltages. A whole host of external factors can have an effect on the performance of insulation – general wear and tear, equipment damage, moisture, corrosive vapours, and of course – prolonged exposure to extremely high temperatures. From a design perspective, it’s very important that you understand the requirements of your high voltage application, the most important insulation material resistance of course being electrical resistance. However, in these applications, temperature and fire rating also have an important role to play.
1) Insulation resistance classes
Most electrically conductive materials also have high thermal conductivity. When selecting an insulation solution consideration should be given to materials such as mica which are not as thermally conductive.
The International Electrotechnical Commission (IEC) has a classification system for insulation materials based on the temperatures it can withstand, which are as follows:
| IEC 60085 Thermal class[3] |
Maximum hot spot temperature allowed |
Relative thermal endurance index (°C)[3] |
Typical materials |
| 90 | 90 °C | >90 – 105 | Unimpregnated paper, silk, cotton, vulcanized natural rubber, thermoplastics that soften above 90 °C[5] |
| 105 | 105 °C | >105 – 120 | Organic materials such as cotton, silk, paper, some synthetic fibers[6] |
| 120 | 120 °C | >120 – 130 | Polyurethane, epoxy resins, polyethylene terephthalate, and other materials that have shown usable lifetime at this temperature |
| 130 | 130 °C | >130 – 155 | Inorganic materials such as mica, glass fibers, asbestos, with high-temperature binders, or others with usable lifetime at this temperature |
| 155 | 155 °C | >155 – 180 | Class 130 materials with binders stable at the higher temperature, or other materials with usable lifetime at this temperature |
| 180 | 180 °C | >180 – 200 | Silicone elastomers, and Class 130 inorganic materials with high-temperature binders, or other materials with usable lifetime at this temperature |
| 200 | 200 °C | >200 – 220 | As for Class B, and including teflon |
| 220 | 220 °C | >220 – 250 | As for IEC class 200 |
| 240 °C | Polyimide enamel or Polyimide films | ||
| 250 | 250 °C | >250 | As for IEC class 200. Further IEC classes designated numerically at 25 °C increments. |
Source: https://en.wikipedia.org/wiki/Insulation_system
Class 155 is the most commonly used set of materials for motor windings – allowing a maximum hotspot temperature of 155 Degrees C.
Pure mica can withstand temperatures of up to 1000 degrees. The IEC place mica within the 130 class, meaning pure mica insulation has an allowable hotspot operating temperature of 130 degrees. Mica can be bound with other materials capable of resisting high temperatures to create a more effective insulation material which pushes the insulation into the higher classes and allowing for a higher hotspot temperature. Our mica laminate boards are commonly used in a range of industries as insulation for winding elements.
2) Thermal resistance and deterioration
When continually operating at high temperatures, insulation materials can deteriorate rapidly if not monitored and maintained. It’s important to note the maximum temperature threshold of your insulative material and ensure that your equipment is within this threshold. When a temperature threshold is exceeded, the insulative materials deteriorates at an accelerated rate – with the level of deterioration approximately doubling for every increase of 10 degrees C. Class F insulation uses half of its mechanical strength and effectiveness after 20,000 hours (2.5 years) at its rated temperature. In order to protect and even extend the usable life of your insulation, it’s vital that you carefully monitor and adjust the operating temperature where necessary and possible.
3) Measuring winding insulation resistance
As we’ve already discussed, monitoring and maintaining the effectiveness of your winding insulation is vital to the safe operation of your equipment. Deterioration is inevitable and non-reversible, and it’s important that you know when insulation needs to be replaced.
Resistance is measured using a megohmmeter (a high resistance ohmmeter). The megohmmeter applies 500 or 1000 V of DC to the windings and ground of the motor and calculates the resistance. There are many factors contributing to the effectiveness and safety of your insulation, but here’s an overview of ranges in resistance and how they indicate the performance of your insulation.
| Insulation resistance value | Insulation level |
| 2 Megohm or less | Bad |
| 2-5 Megohm | Critical |
| 5-10 Megohm | Abnormal |
| 10-50 Megohm | Good |
| 50-100 Megohm | Very good |
| 100 Megohm or more | Excellent |
Source: https://electrical-engineering-portal.com/how-to-measure-insulation-resistance-of-a-motor
A drop in insulation resistance can be sudden or gradual, which is why it’s absolutely vital to measure regularly and maintain to prevent potential accidents or damage to your equipment.
4) Fire resistance of insulation materials
According to a study by OBO Bettermann, in Germany alone, electricity was the number one cause of fire in 2014, accounting for 33% of all fires. When it comes to selecting insulation materials for electrical appliances, you should consider:
- Fire integrity
- Resistance to combustion
- Flame retardance
The British Standard EN 13501-1 has a number of classifications for materials based on their reaction to fire. They are as follows.
| Classification | Definition | Description |
| A1 | Non-combustible | No contribution to fire |
| A2 | Limited combustibility | Very limited contribution to fire |
| B | Combustible | Limited contribution to fire |
| C | Minor contribution to fire | |
| D | Medium contribution to fire | |
| E | High contribution to fire | |
| F | Easily flammable |
Mica is and therefore falls within class A1. Because it is naturally non-flammable and non-hazardous, it is an effective choice for insulating electrical appliances and helping to reduce the risk of fire.
5) Mica as an electrical insulator
Because of its unique electrical and thermal properties, sheet mica is commonly used in the electronic and electrical industries. In addition to this, it is also highly flexible and durable, allowing it to be cut, punched, stamped and machined into a variety of shapes for a range of applications. Block mica is commonly used an electrical insulator in electronic equipment. High-quality muscovite film mica is used as a dielectric in capacitors, and lower grade film mica is used in transmitting capacitors.
Electrical insulation solutions
Do you have an insulation challenge around electrical equipment and could use some expert advice? Elmelin have been providing mica-based insulation solutions for over 100 years. We work with a range of industries including automotive, aerospace, foundry and steel to provide flexible and stable insulation capable of withstanding extremely high voltages and high temperatures. If you’d like to learn more, get in touch.