3.5 Static contact angle measurement
In order to estimate the hydrophobic properties of the housed
insulating materials, the static contact angle was measured on
samples 3 and 4: ethylene-propylene diene monomer (EPDM)
and SiR surge arresters, respectively. The measurements were
performed immediately after finishing a salt-fog cycle on a
surface cleaned by using ethylic alcohol. The evaluation
was done on the top surface of the sheds located on the top
and in the middle of each surge arrester: sheds no. 1 (top)
and no. 12 (middle) for sample 3 and sheds no. 1 (top) and
no. 11 (middle) for sample 4.
Fig. 7 shows the trend of the static contact angle as a
function of ageing test time for EPDM and SiR surge
arresters. The hydrophobicity is similar at the different
shed positions for the same apparatus, but the behaviour
is different for EDPM and SiR material. On the EDPM
surge arrester, the hydrophobicity decreases drastically
after 3000 h of testing. This hydrophobicity reduction is
attributed to polymer degradation caused by the salt fog
conditions and electrical stresses. On the other hand, the
SiR arrester did not lose hydrophobicity during the test.
According to visual observations performed around 2500 h
of testing, the SiR surge arrester showed higher deterioration
than the EPDM surge arrester. After this testing time, its
deterioration almost did not grow, while on the EPDM
surge arrester, it grew significantly. However, the contact
angle on SiR did not change very much during the whole
test as it does on EPDM insulating material. It can be said
that the static contact angle does not give enough
information about the ageing process in SiR as it does in
EPDM surge arrester.