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Page 374

Table A-2b

Polymer Distribution Arrester Leakage Distance Advantage

(Courtesy Hubbell Power Systems)

Standard MCOV

Standard Porcelain Leakage Diatance (in)

Special Porcelain Height (inches)

Special Porcelain Leakage Distance (in)

Standard Porcelain Height (inches)

Standard Polymer Leakage Distance (in)

Polymer Height (inches)

8.4

9.0

9.4

18.3

15.9

15.4

5.5

15.3

18.3

15.9

22.0

20.0

26.0

8.5

22.0

22.0

20.0

29.0

28.9

52.0

17.2

| insulation dries is critical. The relationship between the outer petticoat diameter and the core is known as the form factor. The leakage current generates heat (I2R) on the surface of the insulator (eddy currents). In addition to the effects of the leakage current, the rate at which the petticoat insulation will dry depends on a number of factors. Starting with its contamination before becoming wet, the temperature and humidity of the atmosphere and wind velocity following the cessation of the inclement weather. In areas where extreme contamination may occur (such as some industrial areas or proximity to ocean salt spray), the polymer petticoats may be alternated in different sizes, Figure A-4, to obtain greater distance between the outer edges of the petticoats across which flashover might occur. When dry, the leakage current (approximately) ceases and the line voltage is supported across dry petticoats, preventing flashover of the insulator. It is obviously impractical to design and manufacture comparable porcelain insulators as thin as polymers and having the same form factor. Table A-3. |

| In addition, in porcelain insulators, the active insulating segment is usually small and, when subjected to lightning or surge voltage stresses, may be punctured. In subsequent similar circumstances, it may breakdown completely, not only causing flashover between the energized element and the supporting structure, but may |

Figure A-4
Polymer insulator arrangement
areas of high contamination
where flashover between petticoat
edges is possible
(Courtesy Hubbel Power Systems)

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