Specifications

183
ii) The test should be conducted when the oil is cold and not when hot. The dielectric
strength varies with temperature as shown below :
Temperature
0
C 30 40 50 60 70 80
BDV, kV 33 35 36 37 38 39
iii) Rubber is affected by oil. Therefore use plastic tubes for drawing out the sample.
Sampling bottles should have glass stoppers; cork absorbs moisture and may
contaminate the oil.
iv) The sample of oil should preferably be drawn from the bottom of the transformer tank.
As water is heavier than oil, it settles down at the bottom. The first sample or two may
be thrown away if it contains sludge or droplets of water; the next sample drawn will
surely not fail to reveal high moisture content if the oil has been exposed to drops of
water. The main difficulty in drawing samples from the bottom of the tank is when it is
not fitted with a drain cock but only with a drain plug. The sample may then be drawn by
siphoning off the conservator tank. In circuit breakers of good make special oil sampling
cocks are often fitted to facilitate drawing out a small quantity of oil for test purposes
without any spilling.
v) The glass bottle into which oil is drawn should be perfectly clean, clear, transparent
and dry. It should then be thoroughly rinsed with oil known to be good. Collecting the oil
directly into the oil test cup may appear more convenient but is not recommended, to
avoid the possibility of damaging the cup by handling it unnecessarily, it is also
impracticable if a number of oil samples have to be tested. The sampling bottles should
have sufficient capacity to allow at least two tests. After collecting the oil, check up the
oil level in the transformer and make good if deficient. Every bottle should be clearly
labeled and dated.
vi) The oil testing set is best kept at the central maintenance depot and operated by trained
and intelligent staff. The testing equipment comprises:
a) A standard test cup, which can be readily removed for cleaning, fitted with two spheres
each 13 mm dia. with some means of adjusting the gap. This should be set at 4 mm
using the calibrated gauge, usually supplied with the equipment. The gap should be
checked every time the test set is used.
b) A fixed ratio transformer is used to step up the voltage. A small Variac connected to the
AC 230 volt supply permits application of variable voltage to the primary so that the HT
voltage applied to the test gaps may be raised gradually from zero to 40 kV or more. A
voltmeter is provided on the primary side but calibrated to show the secondary voltages
directly.