UL 840 Standard for Insulation Coordination Including Clearances and Creepage Distances for Electrical Equipment
First Edition – December, 1984 / Second Edition – May, 1993 / Third Edition / January 6, 2005

9 Creepage Distances

9.1 The requirements of this section may be used to evaluate creepage distances. Creepage distances
shall be at least the value in Table 9.1 or 9.2, based on the operating voltage across the distance, the
comparative tracking index (CTI) of the insulating material, and the level of pollution expected or controlled
at the creepage distance. For printed wiring boards using Table 9.2, the existence of recurring voltages is
to be evaluated in accordance with 9.6.
table9_1

table9_2

9.2 The material groups of Tables 9.1 and 9.2 are related to the CTI performance level category values
of insulating materials that are specified in the Standard for Polymeric Materials – Short Term Property
Evaluations, UL 746A, to be included in the group, as follows:
Material Group:
I         – CTI ≥ 600 (PLC = 0)
II        – 400 ≤ CTI < 600 (PLC = 1)
IIIa     – 175 ≤ CTI < 400 (PLC = 2 or 3)
IIIb     – 100 ≤ CTI < 175 (PLC = 4)
Note: PLC stands for Performance Level Category, and CTI stands for Comparative Tracking Index.


9.3 Pollution degrees based on the presence of contaminants and possibility of condensation or moisture
at the creepage distance are as follows:

Pollution Degree enviroment
1 No pollution or only dry, nonconductive pollution. The pollution has no influence.
無汙染或僅乾燥,非導電物汙染,無汙染環境
2 Normally, only nonconductive pollution. However, a temporary conductivity caused by condensation may be expected.
正常,只有非導電物之汙染環境 ,僅可能有暫時性之導電集結物
3 Conductive pollution, or dry, nonconductive pollution that becomes conductive due to condensation that is expected.
導電物汙染環境或乾燥,非導電物可能汙染集結
4 Pollution that generates persistent conductivity through conductive dust or rain and snow.
經由灰塵/雨水/雪之持續性導電物集結汙染

9.4 Steps can be taken to control the pollution degree at the creepage distance by design features or the
consideration of the operating characteristics of the product. See following examples:

Pollution Degree Steps can be taken to control the pollution degree at the creepage distance by design
1 can be achieved by the encapsulation or hermetic sealing of the product.
For printed circuit boards, coatings may be used that comply with the performance criteria of Section 15.
2 can be achieved by reducing possibilities of condensation or high
humidity at the creepage distance, through the provision of ventilation or the continuous
application of heat, through the use of heaters or continuous energizing of the equipment when
it is in use. Continuous energizing is considered to exist when the equipment is operated
without interruption every day and 24 hours per day or when the equipment is operated with
interruptions of a duration which do not permit cooling to the point of condensation to occur.
3 can be achieved by the use of appropriate enclosures which act to exclude or reduce environmental influences, particularly moisture in the form of water droplets.
4 ---

 

9.5 It is also necessary to consider some conditions where equipment may make the pollution degree at
creepage distances more severe than the general environment. Examples of this concern would be
operation which generates contaminates such as carbon brush particles or the arcing of switch parts.

9.6 The value of recurring peak voltages appearing across creepage distances based on Table 9.2 on
printed wiring boards shall be limited to a value not greater than the maximum allowable recurring peak
voltage given in Table 9.3. The measurement of recurring peak voltages is to be in accordance with
Section 13, Recurring Peak Voltage Determination.
Exception No. 1: Measurement in accordance with Section 13 need not be done if circuit analysis can be
employed to determine the maximum recurring peak voltage due to regular operating characteristics, and
due to adjustment of device controls.
Exception No. 2: The value of the recurring peak voltage need not be limited to the value in Table 9.3 if
no air or gases are in contact with the related creepage point. Coating or encapsulation does not
necessarily ensure that no air or gases are present.

table9_3