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CREEPAGE DISTANCE - THEORY

In the early phase of electronic equipment design, the engineer should pay close attention when calculating minimum insulation distances.

Creepage distance. Creepage is the shortest path between two conductive parts (or between a conductive part and the binding surface of the equipment) measured along the surface of the insulation. A proper and adequate creepage distance protects against tracking, a process that produces a partially conducting path of localized deterioration on the surface of an insulating material as a result of the electric discharges on or close to an insulation surface. The degree of tracking required depends on two major factors: the comparative tracking index (CTI) of the material and the degree of pollution in the environment.

Creepage distance



Clearance Distance. Clearance is the shortest distance between two conductive parts (or between a conductive part and the binding surface of the equipment) measured through air. The clearance distance helps prevent dielectric breakdown between electrodes caused by ionization in the air. The dielectric breakdown level is further influenced by relative humidity, temperature and degree of pollution in the environment.

Clearance distance



Creepage and clearance distances should be essential from the viewpoint of Engineers designing inductive components, therefore our customers are asked about required insulation distances. Selecting the appropriate distances is necessary in order to meet the requirements of the safety standards obligatory in Europe and many other countries. The first step is to determine the classification of the device in terms of usage of the device, as differing standards require different insulation distances, then to calculate the insulation distances in accordance with the chosen safety standard.

The most common safety standards for electronics devices are:

EN-61558-2-16Safety of power transformers, power supply units and similar. Particular requirements for transformers for switch mode power supplies
EN-60950Safety of information technology equipment (IT Equipment)
EN-60601-1Medical electrical equipment. General requirements for basic safety and essential performance
EN-60079Electrical Apparatus for Explosive Gas Atmospheres
EN-60335Household and similar electrical appliances. Safety. General requirements
EN-60065Audio, video and similar electronic apparatus. Safety requirements

Listed below are the explanation terminology, necessary to calculate insulation distances.

Material Group - this depends on the comparative tracking index (CTI). CTI is an index used for electrical insulating materials defined as the numerical value of the voltage level which will cause failure by tracking during standard testing. Tracking is the process that produces a partially conducting path of localized deterioration on the surface of an insulating material, this as a result of electric discharge action on or close to an insulation surface.

CTI acc. to IEC 60112
Material Group ICTI > 600
Material Group II400 < CTI < 600
Material Group IIIa 175 < CTI < 400
Material Group IIIb100 < CTI < 175
CTI acc. to UL 746
Material Group 0CTI > 600
Material Group 1400 < CTI < 600
Material Group 2 250 < CTI < 399
Material Group 3 175 < CTI < 250
Material Group 4 100 < CTI < 175
Material Group 5 CTI < 100


Pollution Degrees - Pollution degrees (according to IEC-60950) are classified as follows:

Degree 1 - no pollution or only dry, nonconductive pollution occurs. The pollution has no influence: (example: hermetic sealed or potted products),
Degree 2 - applies where there is only nonconductive pollution that might temporarily become conductive due to occasional condensation (example: product used in typical office environment).
Degree 3 - Conductive pollution occurs, or dry nonconductive pollution occurs that becomes conductive due to expected condensation (example: products used in heavy industrial environments that are typically exposed to pollution such as dust).


Over-voltage categories. The over-voltage categories are defined as follows
Category I - equipment of this over-voltage category work mainly within low voltages. Example: protected electronic circuits. Over-voltage up to 1.5kV for AC mains supply voltage 150-300V rms.

Category II -- Energy consuming equipment supplied from a fixed installation. Examples: Appliances, portable tools, and other household gadgets. Over-voltage up to 2.5kV for AC mains supply voltage 150-300V rms.

Category III -- This applies to equipment for fixed installations, i.e. protective devices, relays, switches and plugs. Over-voltage up to 4.0kV for AC mains supply voltage 150-300V rms.

Category IV -- This applies to open air line equipment. Over-voltage up to 6.0kV for AC mains supply voltage 150-300V rms.


The vast majority of electrical devices (mains up to 300vAC) work as category II – 2.5kV.

Insulation Types In any isolation scheme, a certain level of insulation is required to create an isolation barrier. IEC 1010 calls this basic insulation. If a breakdown in the insulation level could possibly result in dangerous current flowing through to a human body, then the basic insulation is not enough to ensure safety. IEC 1010 gives the designer several insulation improvement options. Two of these options are double insulation and/or reinforced insulation. Double insulation is basic insulation plus supplementary insulation (for example; another repeated layer). If the basic insulation breaks down (single fault), then the supplementary insulation keeps the user safe. Reinforced insulation serves the same purpose as double insulation except that the basic and the extra insulation cannot be tested separately, therefore most applications require reinforced insulation.

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