Under pollution, the response of external insulation to power-frequency voltage can govern its design. IEC 60071-2 defines four levels (I light, II medium, III heavy, IV very heavy) with examples of environment and a minimum specific creepage distance of 16.0, 20.0, 25.0 and 31.0 mm/kV for glass or porcelain insulators. The insulator must continuously withstand the highest system voltage when polluted.
Short answer
At a polluted site, an insulator can fail through surface flashover at normal service voltage, with no overvoltage needed. IEC 60071-2 defines four pollution levels: I (light), II (medium), III (heavy) and IV (very heavy). Table 1 of the 1996 edition associates with each one examples of environment and a minimum specific creepage distance of 16.0, 20.0, 25.0 and 31.0 mm/kV. The insulator must continuously withstand the highest system voltage, polluted and wet, with an acceptable risk of flashover.
Why pollution can govern the design
Surface flashover generally occurs when the insulator surface is polluted and becomes wet with drizzle, snow, dew or fog, without a heavy rain to wash it. Under those conditions the pollutant layer becomes conductive and leakage current flows along the surface. That is why, where there is pollution, the response of external insulation to power-frequency voltage can define its design ahead of lightning or switching overvoltages.
Coordination handles it this way: the coordination voltages are taken equal to the representative overvoltages, and the performance criterion is met by choosing a withstand pollution severity suited to the severity of the site. The long-duration power-frequency coordination voltage corresponds to the highest system voltage for phase-to-phase insulators, and to that voltage divided by the square root of three for phase-to-earth insulators.
The four levels
Table 1 of IEC 60071-2 describes them with typical environments:
| Level | Typical environment | Minimum specific creepage distance |
|---|---|---|
| I, light | Areas without industry and with a low density of heated houses; areas with little industry but frequent winds or rain; agricultural areas; mountainous areas. All at least 10 to 20 km from the sea and not exposed to winds directly from the sea | 16.0 mm/kV |
| II, medium | Areas with industries that do not produce particularly polluting smoke, or with a medium density of heated houses; areas with a high density of industry or housing but frequent winds or rain; areas exposed to winds from the sea, but not too close to the coast (at least several kilometres) | 20.0 mm/kV |
| III, heavy | Areas with a high density of industry and suburbs of large cities with polluting heating; areas close to the sea or exposed to relatively strong winds from the sea | 25.0 mm/kV |
| IV, very heavy | Areas of moderate extent with conductive dust and industrial smoke producing thick conductive deposits; areas very close to the coast, exposed to sea spray or very strong, polluting sea winds; desert areas with no rain for long periods, with strong winds carrying sand and salt, and regular condensation | 31.0 mm/kV |
The notes to the table are part of the information. The nominal minimum creepage distance refers, according to IEC 815, to that of phase-to-earth insulators, related to the highest system voltage between phases. The use of fertilizers by spraying or the burning of crop residues can raise the level through wind dispersion. And distances to the coast depend on topography and extreme winds.
A reading example
An insulator at a 72.5 kV substation in a level III coastal area needs a minimum creepage distance of 25.0 mm/kV times 72.5 kV, that is, 1,812.5 mm. If the site moves to level IV, the figure rises to 31.0 mm/kV, or 2,247.5 mm. This is an illustration of how to use the table: the final distance is defined by the insulator design and its product committee.
Limits of the table
The table applies only to glass or porcelain insulation and does not cover some environmental situations: snow and ice with heavy pollution, heavy rain, arid areas and similar. The standard clarifies that this distance is more closely related to insulator design than to insulation coordination, and that its inclusion is informative.
Two further clarifications help in using the concept correctly:
- Each type of insulator needs its own measure of severity. Different insulators, and even different orientations of the same type, accumulate pollution at different rates in the same environment. With the same degree of pollution, their flashover characteristics also change. The nature of the pollutant can favour some insulator shapes over others.
- Site severity can be measured. To assess the pollution level of a site quantitatively by measurement, the standard refers to IEC 815. Verifying pollution withstand through artificial pollution tests is left to the product committees, which rely on IEC 507.
What to do at heavily polluted sites
For sites with a high degree of pollution, the standard mentions two measures on insulating surfaces: greasing and washing. For altitude correction on polluted insulators, the standard gives a provisional exponent m, between 0.5 for standard insulators and 0.8 for anti-fog design.
Relation to service conditions
IEC 60071-1:2006 takes as a normal service condition that the air is not significantly polluted by dust, smoke, corrosive gases, vapours or salt, and that pollution does not exceed level II, medium. The standard withstand voltage values in tables 2 and 3 of that standard apply to those conditions. If the site is level III or IV, the equipment is outside normal conditions and a specification of its own is required.
What not to do
- Do not choose the creepage distance from the voltage level alone: it depends on the pollution level of the site and the type of insulator.
- Do not use table 1 for polymeric insulators or for snow and ice environments with heavy pollution: it does not cover them.
- Do not classify a site only by its distance to the coast: topography and extreme winds change the effective distance.
- Do not treat the minimum creepage as part of the coordination design: it is an insulator design datum.
To go further, see Insulation coordination of overhead lines, Special service conditions for switchgear, Substation busbars and insulators: what to define and Selection of primary substation equipment.
When to call us
Classifying the pollution of a site and sizing the outdoor insulation of a substation is part of its engineering. At Tiza Group we include it in engineering design and studies, remotely and worldwide. For installed equipment, we run tests within our testing and commissioning service, in Colombia or elsewhere on request. Write to us and we will send you a quote.
Related guides
- Insulation coordination under IEC 60071: the steps
- Creepage distance on instrument transformer insulators
- Altitude correction of external insulation: Ka and m
- Insulation coordination of overhead lines
- Special service conditions for switchgear
References
- IEC 60071-2:1996, Insulation co-ordination, Part 2: Application guide, clauses 3.3.1 and 3.3.1.1, table 1 and clause 4.2.2.
- IEC 60071-1:2006, Insulation co-ordination, Part 1: Definitions, principles and rules, clause 5.9.1.
- IEC 815, guide for the selection of insulators in respect of polluted conditions, cited by IEC 60071-2.
Frequently asked questions
What are the pollution levels in IEC 60071-2?
Four: I (light), II (medium), III (heavy) and IV (very heavy). Table 1 of the 1996 edition gives, for each one, examples of environment and the nominal minimum specific creepage distance: 16.0, 20.0, 25.0 and 31.0 mm/kV, measured on the highest system voltage between phases.
Does the creepage distance table apply to any insulator?
No. The table itself states that it applies only to glass or porcelain insulation and does not cover situations such as snow and ice with heavy pollution, heavy rain or arid areas. In addition, creepage distance belongs more to insulator design than to coordination.
Up to what pollution level are normal service conditions considered to apply?
IEC 60071-1:2006 (clause 5.9.1) takes as normal conditions a pollution not exceeding level II (medium) of table 1 of IEC 60071-2. Above that, the equipment is outside the normal insulation conditions.