Guide for selection of insulator creepage distances to suit polluted conditions

Pollution level	Minimum nominal specific creepage distance(a) (mm/kV)(b)
I–Light	16
II–Medium	20
III–Heavy	25
IV–Very heavy	31

Notes:

1. For actual creepage distances the specified manufacturing tolerances are applicable (IEC 60273, 60305, 60433 and 60720).
2. rms value corresponding to the highest voltage for equipment (phase to phase).
3. In very light polluted areas creepage distances less than 16 mm/kV can be used depending upon service experience. 12 mm/kV seems to be a lower limit.
4. In cases of exceptional pollution a specific nominal creepage distance of 31 mm/kV may not be adequate. Depending upon service experience and/ or laboratory test results a higher value of specific creepage distance can be used.

electrolyte with moisture from rain or condensation. When the grease becomes saturated with dirt it must be removed and a new coat applied.

Greasing is not recommended for use with anti-fog insulator shed profiles.

The grease tends to get trapped in the grooves on the underside of the shed and bridged by conductive dirt forming on the grease surface thereby reducing the overall insulator creepage distance.

Calculation of specific creepage path

Typical creepage values for either cap and pin insulator strings or post insulators at 132 kV under very heavy pollution, classification IV, would be determined as follows:

Nominal voltage                                           132 kV rms

Rated voltage                                              145 kV rms

Maximum rated voltage phase to phase

Creepage (from Table)                                 31mm/kV

Test creepage distance                                145kV * 31mm/kV = 4495mm

From manufacturer’s details:

Typical substation insulator                          80 kN minimum failing load

Creepage                                                   330mm

Spacing distance                                        127mm

Number of insulators required to provide 4495 mm creepage using cap and pin porcelain string insulators =4495/330 = 13.6. Therefore the minimum total number of insulators per string would be 14. In this case failure of one unit would reduce the creepage distance to 4165 mm, equivalent to an effective value of 28.7 mm/kV @ 145kV-rated voltage. However, at the nominal system voltage of 132 kV the design value is maintained at 31.5mm/kV. Minimum length of suspension insulator string =14 * 127 = 1778mm. Alternatively, 15 insulators per string could be specified in order to maintain the recommended creepage distance at 145kV-rated voltage under a one insulator shed failure condition. This allows full overhead line operation between normal line maintenance outages at the expense of increased overall string length of 1905mm.