Fundamentals of material suitability for electrical insulation according to IEC 60664
There are certain rules for insulation materials used in low voltage applications. These are defined in standards. The (decisive) basic standard is IEC 60664, which specifies minimum requirements for voltages up to 1000VAC / 1500VDC and nominal frequencies up to 30 kHz.
The following list explains a few terms that should be considered by designing a device considering the point of view of the insulation material. All of them are mentioned in IEC 60664 and together they give a specific set of requirements depending on the application and location of the electrical devices.
Any contamination with gases, liquids, or solids that can reduce the electrical resistance or isolation capability of a separation line, is recognized by the standard as contamination. There are four different degrees of contamination. Pollution degree 4 is no longer acceptable for insulation, which is a safeguard measure.
Function of isolation
Pure galvanic isolation for the proper functioning of the device, no protection against accidental contact.
Insulation for protection against electric shock, first protective barrier against personal injury
Insulation consisting of basic insulation and an additional insulation
Insulation in addition or also independent of the basic insulation as a further protective measure against electric shock (principle of redundancy in case of failure)
A single insulation measure, but which corresponds to the protection of a double insulation, usually achieved by a much stronger material thickness
Comparative tracking index CTI
The CTI value is a measure of the sensitivity of an insulation surface to electrical tracking. Pollution and moisture can form an electrically conductive creepage path on the surface, which destroys the insulation material in the long term.
The shortest distance between two conductive parts through air, which is determined by the rated impulse voltage (results, inter alia, from the overvoltage category)
The shortest distance between two conductive parts along a solid material. The length of this section in the standard IEC 60663 depends on the degree of soiling, the material which is used (cti) and the overvoltage category.
There are 4 overvoltage categories that define the highest possible peak voltage in the circuit. An essential feature on the primary side is the proximity to the power grid (overvoltage category II describes devices that are connected to the house installation such as portable tools, etc.)
Rated surge voltage
The rated surge voltage results from the overvoltage category and rated voltage range. For the current overvoltage category II, the rated surge voltage is 2.5 kV.
Frequency of voltage
The dielectric strength of insulating materials depends partly very much on the frequency with which alternating voltage (AC) or direct current superimposed alternating voltages are operated. At frequencies above 100 kHz, the effect can be more than 50%.
Relative temperature index RTI
This value indicates, up to which maximum continuous temperature the material can be used as insulation. The standard refers to a service life of 20,000 hours, which is exceeded in many cases. Materials with a higher RTI ensure a longer service life (rule of thumb: 10 ° C more thermal stability doubles life)
Basic procedure for determining the creepage distances and air gabs:
1.) rated voltage range
2.) Overvoltage class
3.) Rated impulse voltage
4.) Degree of contamination
5.) -> minimum air distances
a.) Overvoltage class
b.) Degree of contamination
c.) CTI value
d.) -> minimum creepage distances
Important in device development is the fact, that there is a whole range of device-specific standards that can make stricter specifications (for example, medical device standard according to EN 60601)!
This list of standard requirements from the basic standard IEC 60664 is not final, but gives an overview of how many factors must be taken into account when selecting a suitable material. CMC Klebetechnik offers a solution for almost every combination of RTI, CTI, HAI, HWI and flammability class.