Clearance, Creepage Distance and Voltage Rating

Sometimes it can be confusing trying to find information about electrical operating voltages for personal or system protection. In general, the minimum insulation distances for equipment can offer the best information possible for protection, especially malfunction of equipment. An adequate dimension of clearance and creepage distance can help to determine the voltage rating.

Clearance
The clearance is defined as the shortest distance through the air between two conductive elements.
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We’ll use HARTING’s part number 09031966921 as an example and step through figuring this out.

To identify the clearance distance

  • Define the installation category
  • Define the degree of pollution expected
  • Select the rated impulse withstand voltage from Table 1
  • Select the minimum required clearance from Table 2

Creepage Distance
Creepage is defined as the shortest distance on the surface of an insulating material between two conductive elements.

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To identify the creepage distance

  • Define the installation category
  • Define the degree of pollution expected
  • From the nominal voltage and the type of supply system select the rated voltage from below Table 3

For the dimension of the creepage distance, the tracking formation of the insulating material has to be considered. If not indicated contrary, the CTI value of the insulating material is <400 and the isolation group is III a/b.

CTI (Comparative Tracking Index)
This index is an indication of the conductivity of insulating materials and affects the specified creepage distances. The influence of CTU value on the creepage distance is as follows:

  • the higher index value, shorter the creepage distance.
    CTI is used to divide the plastics into insulation groups.
Insulation Group CTI
I 600≤CTI
II 4000≤CTI<600
IIIa 175≤CTI≤400I
IIIb 100≤CTI≤175I

Exemplary calculation
What voltage can be used, if the creepage, the insulation category, the degree of pollution are known:
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How to detemine the Voltage Rating

  1. Define the installation category
  2. Define the degree of pollution expected
  3. Selected the rated impulse withstand voltage in kV from Table 2
  4. Select the voltage phase to earth derived from rated system voltages from Table 1
  5. Select the voltage rating from Table 4
  6. Define the number of phases and whether Table 3 is relevant for the application
  7. Select the nominal voltage of supply system from Table 3
  8. Select the lower voltage from above step 4 to 7
Table 1

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Table 2

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Table 3
  • Single phase, three or two wires AC or DC systems
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  • Three phase, four or three wires AC systems

Table 4

e.g. What is the voltage rating of 09031966921 connector Harting?
09031966921 would fall under normal usage circumstance as 50V rated voltage, because

  1. 1.2mm creepage
  2. Installation category II
  3. Degree of pollution 2
  4. Isolation group III a/b
  5. CTI Value <400

To learn more creepage and clearance, you can refer to HARTING’s application note Voltage - Creepage and Clearance (002).pdf (274.2 KB)

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