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OTS80PB and OTS60PB OTS100AF, OTS80AF and OTS60AF
18
Preparations for use
Excluding air from circulating oil when using an impeller to stir the insulating fluid sample.
Fig 6: Baffle plate must come in contact with the oil
If you are using an impeller stirrer that utilises a baffle plate to exclude air from the oil sample, ensure that oil does
not pass over the upper surface of the baffle plate.
It is also important that the oil sample is in full contact with the underside of the baffle plate.
This will prevent moisture being absorbed from the
contact of circulating oil with air as intended by the
test standard.
Fill the test vessel running the oil down the
side wall so as not to cause splashing that will
introduce unwanted air bubbles into the test
sample
Fill the test vessel to the 400 ml fill line, DO NOT
OVERFILL (see figure 7)
Gently place the stirrer lid onto the test vessel so
that the baffle plate contacts the oil slowly, again
so as not to cause splashing that will introduce
unwanted air bubbles into the test sample
Periodically remove baffle plate from the stirrer lid
and clean thoroughly.
NOTE: If the oil is poured too fast, turbulence may
generate bubbles which should be avoided.
NOTE: If the oil is poured too slowly the large surface
area of the oil exposed to the air will absorb moisture.
This should be avoided.
Fill to 400ml line DO NOT OVERFILL
Fit stirrer lid carefully DO NOT SPLASH
400ml Fill to Line
400ml Fill to Line
400ml Fill to Line
400ml Fill to Line
✓
✓
✖
✖
Empty trapped oil - occurs if overfilled
Fig 7: Filling the test vessel
Selecting the optimum stirring option for the insulating oil sample to minimise air/moisture contamination
When testing to IEC 60156:
IEC 60156 allows the optional use of a stirring impeller, the use of a magnetic bead stirrer or even no stirring at all.
The standard states that differences between tests with or without stirring have not been found to be statistically
significant. The use of a magnetic stirrer is only permitted when there is no risk of removing magnetic particles from
the oil sample under test. When oil is used as a coolant as well as an insulator it may be useful to stir the sample
during testing. For example oil from a transformer is normally circulated in use to help cool the transformer, but in
doing so might also pick up contaminants and debris with it; so to achieve the most realistic breakdown results an
oil sample taken from such a device would normally be stirred during test to ensure the best chance of detecting any
detrimental effects of particle contamination. Oil from a circuit breaker is normally static in use, so particles would
naturally fall to the bottom where they are unlikely to cause a problem. So in static use applications, an oil sample
would not usually be stirred during testing.
The use of a magnetic bead for IEC60156 is recommended where possible as oil will circulate in the lower portion
of the test vessel, whereas the impeller will circulate all of the oil in the test vessel. The magnetic bead therefore has
the advantage that any moisture absorbed by oil in contact with air is not stirred into the sample, avoiding unwanted
contamination. If the impeller is used it is vital that air is prevented from coming into contact with the surface of the
oil by fitting the baffle plate and ensuring that the oil level is sufficient to make contact with the bottom surface of
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