Lightning Arrestors: EPDM vs Silicon Rubber

[nonsinusoidal]

It is my understanding that ethylene Propylene Diene M-class(EPDM)rubber is injected with some type of oil to provide similar properties as silicon rubber. These two types of rubbers (EPDM and Silicon) are utilized to manufacture lightning arrestors which are used to protect medium voltage (5kV to 35 kV) power electric distribution systems. Are there any differences on how long will these two type of materials maintain its insulation properties? Overall, what are the pros and cons with respect to each of the materials, mentioned above, used to manufacture the lightning arrestors? Which one is the best type of lightning arrestor to buy in order to get the most for your dollar?

 

[7anoter4]

If you ask a manufacturer –Siemens for instance –the answer is:
"The Silicone Rubber (SR) insulation for arrester housing demonstrates
excellent pollution layer characteristics Neither water nor dirt can cause fl ashovers,
a signifi cant factor in an arrester‘s reliability.
The hydrophobic properties of the arrester housing made of SR prevent conductive
moisture from forming on its surface, even when contamination is substantial.
Surface currents and discharges are ruled out. Even the most severe ambient conditions,such as salt fog in coastal regions or dust-laden air in an industrial area, cannot impair the hydrophobicity of SR."
See :

https://www.energy.siemens.com/cms/...rdo/Documents/silicone_rubber_ins_1400185.pdf

But as tested by IEEE this is the conclusion:
"Insulation design of polymer housed surge arresters for distribution lines are evaluated by using IEC 1109 based salt fog test. The concept of “rest time” in salt fog tests of housings has been introduced with an effort to give fairness to silicone materials. The method provides a comprehensive evaluation of insulation design of polymer housed surge arresters. However, at this time, the inclusion of a rest time in salt-fog tests of silicone rubber housings is not warranted, as hydrophobicity did not recover completely even after a seven-day rest period"
See: "Evaluation of the insulation design of polymer housed surgearresters using salt-fog test"

http://ieeexplore.ieee.org/Xplore/l...0885306.pdf?arnumber=885306&authDecision=-203

ABB also emphases the Silicon Rubber will be better [see:]
DESIGN AND TESTING OF POLYMER-HOUSED SURGE ARRESTERS
"Both silicon- and EPDM rubber show hydrophobic behaviour when new. The insulator made of EPDM rubber, however, will lose its hydrophobicity quickly and is thus often regarded as a hydrophilic insulator material.
Hydrophobicity results in reduced creepage currents during external pollution, minimising electrical discharges on the surface; thereby reducing the effects
of ageing phenomena. The material can lose its hydrophobicity if the insulator has been subjected to high leakage currents during a long time due to severe pollution, e.g., salt in combination with moisture. The silicon rubber, though, will recover its hydrophobicity
through diffusion of low molecular silicones to the surface restoring the original hydrophobic behaviour.
The EPDM rubber lacks this possibility completely and hence the material is very likely to lose its hydrophobicity completely with time." See [page 5 from:]

http://library.abb.com/global/scot/...fce00241963/$File/1998-10 Abu Dhabi Cigre.pdf

What ever, my experience is only with porcelain and EPDM .The last seems to be better inside as never exploded there and the first will be still good outside –in a heavy polluted area and mainly close to sea shore.