What is Creepage Distance?

The Creepage distance of out door mounted equipments varies with the atmospheric conditions.

It is 16mm/Kv for Normal Atmosphere condition.

It is 25mm/KV for Heavily polluted Condition

It is 31mm/Kv for heavily polluted condition.

(say P mm/KV)

For Normal atmospher condition it is 16mm/Kv.If the equipments is of 11oKV/145KV/400Kv(N-kv) , the the creepage distance(CD) required is N x P= CD mm

P=16,25 or31 and N=110,145 or 400

Dear Friend,

Creepage Distance. Creepage is the shortest path between two conductive parts (or between a conductive part and the bounding 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. Used for electrical insulating materials, the CTI provides a numerical value of the voltage that will cause failure by tracking during standard testing. IEC 112 provides a fuller explanation of tracking and CTI.1 Tracking that damages the insulating material normally occurs because of one or more of the following reasons:
Humidity in the atmosphere.
Presence of contamination.
Corrosive chemicals.
Altitude at which equipment is to be operated.
Clearance Distance. Clearance is the shortest distance between two conductive parts (or between a conductive part and the bounding surface of the equipment) measured through air. Clearance distance helps prevent dielectric breakdown between electrodes caused by the ionization of air. The dielectric breakdown level is further influenced by relative humidity, temperature, and degree of pollution in the environment.
When designing a switch-mode power supply for use in information technology (IT) equipment, a typical rule of thumb is to allow an 8-mm creepage distance between primary and secondary circuits, and a 4-mm distance between primary and ground. If these dimensions are allowed for during the design stage, there is a high probability (95%) that no failure will occur with respect to creepage or clearance when the final product is submitted for test.
Working Voltages. A working voltage is the highest voltage to which the insulation under consideration is (or can be) subjected when the equipment is operating at its rated voltage under normal use conditions. The appropriate creepage and clearance values can be determined from the figures provided in the relevant tables in EN 60950.2 These values must sometimes be calculated. To use Tables I-IV (2H, 2J, 2K, and 2L of the standard), the following factors must be considered: determination of working voltages, pollution degree of the environment, and the overvoltage category of the equipment's power source.

When measuring working voltages, it is important to measure both peak and root-mean-square (rms) voltages. The peak value is used to determine the clearance, and the rms value is used to calculate creepage. For example, if one measures a peak voltage of 670 V between two pins of a switching transformer in a switch-mode power supply, the clearance distance between primary and secondary circuits must be calculated using Table I. If the unit is powered via 240 V mains and has a pollution degree of 2, the figures in the center row (marked 300 V rms sinusoidal) and center column (since the mains voltage is >150 V and < 300 V) are used to establish the required clearance distance. In this case, the value for reinforced insulation is 4 mm. One then turns to Table II (Table 2J of EN 60950), which provides additional clearance based on the working voltages and pollution degree. (The middle column was used for calculating this example.) The appropriate row in that column covers the actual repetitive peak insulation working voltage. In this example, the value would be 0.8 mm for reinforced insulation. Adding the two figures together gives a total of 4.8 mm clearance distance. Similarly, if a voltage of 337 V rms was measured between the two pins of the switching transformer, Table IV (2L)

REFERENCES

1. IEC 112:1979, "Method for Determining the Comparative and the Proof Tracking Indices of Solid Insulating Material under Moist Conditions," International Electrotechnical Commission, Brussels.
2. BS EN 60950:2000, "Safety of Information Technology Equipment," British Standards Institute (BSI), United Kingdom.
3. IEC 60664:1980 "Insulation Co-ordination within Low-Voltage Systems Including Clearances and Creepage Distances for Equipment," International Electrotechnical Commission, Brussels.

4. BS EN 61010-1:1990, "Safety Requirements for Electrical Equipment for Measurement, Control, and Laboratory Use, Part 1: General Requirements," BSI, United Kingdom.

DHAYANANDHAN.S