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Comments on Rain Testing
Sigbjörn Haraldsen, ASEA, Ludvika
G21.317.333
För att förvissa sig om att elektriska apparater för
utomhusdrift klarar den minskade isolationen vid
härda klimatiska förhållanden, utföres ofta
spänningsprov under artificiellt regn. Sådana prov har
utförts under en följd av år och de flesta länders
normer har regler om hur regnprov skall göras. Det
har visat sig att reproducerbarheten vid regn prov
kan vara ganska dålig.
Några undersökningar tyder på att en av orsakerna
kan ligga i varierande tid för nedvätning av
isolatorerna före påsättningen av spänningen. Den
viktigaste orsaken ligger dock troligen i den statistiska
spridningen i överslagsspänningen pä grund av den
sporadiska vattenöverbryggningcn på delar av
över-slagssträckan.
Electrical equipment for outdoor installation must
be made to withstand all normal atmospheric
conditions such as mist, heavy rain, smog, dirt as well
as hot, sunny weather. Testing of electrical
equipment under conditions corresponding to all these
factors are of course out of the question, as the price
for testing in many cases could be comparable to
the manufacturing cost of the equipment itself. The
tests are therefore usually confined to dry tests and
tests under artificial rain, both of which can be
done by relative simple means in a laboratory.
Rain tests have been carried cut for years, and test
rules are laid down in most specifications. Recently
doubts have arisen whether rain tests should be
considered to be representive for outdoor conditions,
partly because there seem to be conditions much
more serious and more frequently oecuring than
heavy rain, and partly because of the large
dispersion in laboratory rain testing. As the rain tests in
many cases are dimensioning for electrical
equipment, a test with large inherent dispersion is not
desirable, especially not as delivery test.
The artificial rain is obtained from special spray
apparatus. A typical design is shown in fig. 1. A
water pressure of something between 1.5—C
kg/cm-is suitable for most test objects. The rain should fall
as drops at about 45° angle on to the test object,
and the resistivity of the water and the rate of
precipitation is measured (10 000 ohmcm and 3 mm/
minute respectively according to Swedish
standards). The test object shall have a preraining time
of 1—5 min. in order io obtain a stationary value.
Specifications are made in order to facilitate
repro-ducable measurements. Factors which may affect
reproducibility of wet tests are rate of
precipitation, water resistivity, angle of rain, and time of
Fig. 1. Typical rain apparatus
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