procedures stipulated by IEC 62535. Naphthenic oil, which
contains neither DBDS nor DBPC, was used as the base oil,
labeled oil as shown in Table 1. Two types of tested oil
were prepared as listed in Table 2. 1154ppm of DBDS was
added to oil for oil A. 1154 ppm of DBDS and 4000ppm of
DBPC were added to oil for oil B. Copper strips wrapped
with a single layer of insulating paper (Kraft paper) were
immersed in the test oils. The copper strips were made of
electrolytic tough pitch copper (ASTM C11000) and its
dimensions were 7.5 by 30 by 1.5 mm. The dimensions of
the insulating paper were 14 by 35 by 0.05mm. Appearance
of the test chamber and glass vessel was shown in Figure 1.
The test chamber was made of stainless steel (type 304),
and its volume was 1 liter. The glass vessel, filled with
15ml of the test oil, was placed in the test chamber, which
has a gas-inlet and a gas-outlet on the lid. For the heating
tests, the test chamber was placed in an electric furnace
kept at 150 ºC for 72 h. During the heating tests, an
atmosphere in the test chamber was strictly controlled by
flowing gas through the gas-inlet/outlet. The flowing gases
were pure nitrogen and mixtures of nitrogen and oxygen.
The oxygen concentrations of the mixtured gases were 0,
2.5, 5, 10 and 20 volume %. The flow rate of the mixed gas
was kept at 0.03 liter/min.