Figure; Example Initial Stand Time Screen; Example Voltage Screen; Example Stirring Screen - Megger OTS 60PB Guide De L'utilisateur

Operation
Standard Deviation
The sample standard deviation s (also known as σ
Where s = sample standard deviation.
Data Dispersion
The ratio of standard deviation upon mean ( s / x ) also known as the co-efficient of variation is used to
determine whether the spread of a set of test results is within acceptable limits. This value may be
expressed as a percentage.
5 minute test
Note:– This is a non-standard test, designed to provide a rapid assessment of the oil condition.
1. On pressing the 'START' key the display screen appears similar to that shown in figure 5.
The sub-display area informs the operator that the initial oil sample stand time is being undertaken and
the time remaining is given (time is counted down to zero). In this case a 1 minute initial stand time is
used.
2. After the initial stand time is complete the high voltage test is switched on and applied to the oil
sample. The screen display showing 'TEST 1' appears, similar to that shown in figure 6.
The sub-display shows the voltage applied to the sample oil; this voltage is rising at the rate of 2 kV/s.
It continues until breakdown occurs (or up to maximum). The high voltage warning light is illuminated
during this time. The number of the test in the sequence is shown and the words 'IN PROGRESS'
appear.
3. When breakdown occurs (or if the maximum voltage is reached) the test voltage is cut off, the 'kV'
warning light goes out and the test set begins its oil sample stirring period. The stirrer rotates for a
period of 30 s and the breakdown voltage value of the previous test is retained on the display. The
display screen appears as in figure 7.
n-1
s =
2
x
i
√ n - 1
INITIAL STAND TIME
TIME REMAINING
00 : 48
5 MINUTE TEST

Figure 5

Example Initial stand time screen

46 kV
TEST 1 IN PROGRESS
5 MINUTE TEST
Figure 6

Example Voltage Screen

) is given by:-
2
- n x
STOP
STOP
12