he necks in wax, and then stored for future use. These bottles should be
opened only when the bottle temperature and the ambient temperature are
the same or nearly so.
4.4.1 Sampling Oil from Transformers
General sampling instructions are as follows:
At least 2 qt of oil should be taken as a sample for dielectric, acidity, and
IFT tests. Allow space at the top of the container for expansion. If two
1 qt bottles are used for a sample, label the bottles as 1 of 2 and 2 of 2.
Samples from outdoor apparatus should be taken on clear days when the
humidity is near normal and the oil is at least as warm, or warmer than
the surrounding air. Cold oil may condense enough moisture from a
humid atmosphere to seriously affect its insulating properties. Therefore,
this precaution must be observed in sampling spare transformers.
Samples should never be drawn in rain or when the relative humidity
of the atmosphere exceeds 70%.
Guard against wind and dust.
When taking samples from an opening, such as a valve, clean the valve
thoroughly and allow enough liquid to run out (about 1 qt) to remove
any moisture or foreign material.
In a sealed transformer, which has a vacuum, be sure to vent the transformer
before drawing the sample.
Place the sample in the freezing compartment of a refrigerator overnight.
If the sample is cloudy when viewed the next day, it contains free
water. Since free water is undesirable, take another sample to determine
whether water is in the oil or was in the sample container.
4.4.2 Sampling Oil from Drums or Shipping Containers
The oil drum should remain undisturbed for several hours before drawing
the sample.
A glass or Pyrex thief is recommended for sampling because it can be
easily inspected for cleanliness. A glass tube approximately 36 in.
long, 1 in. in diameter, and tapered at both ends is recommended for
the sampling thief.
The thief should be cleaned before and after sampling in the same
manner as for cleaning sample containers. When not being used, the
thief should be corked at both ends.
Discard the fi rst full thief of oil.
Draw the sample in the following manner:
With the top end covered with the thumb, lower the tube to within
approximately 1/8 in. from the bottom of the drum.
212 Electrical Power Equipment Maintenance and Testing
Remove the thumb from the top opening until the thief is fi lled with oil.
Replace thumb over top of thief and remove thief full of oil to the sample
container. Release thumb to permit oil to run into the container.
4.4.3 Taking Oil Samples for Gas-in-Oil Analysis
This procedure has been developed to maintain uniformity of all oil samples
taken in the fi eld for a laboratory gas-in-oil analysis. Special stainless-steel
containers are used for collecting samples of oil for gas-in-oil analysis using
gas chromatograph. These stainless-steel containers are not to be used for
any other purpose and should be kept clean to eliminate all contaminants
and purged with dry air for shipment to the fi eld.
Use a can to catch overfl ow oil from the stainless-steel container.
Obtain two lengths of Tygon clear plastic tubing and attach one to each
end of the stainless-steel container. Make certain that the tubing
between transformer and container is as short as possible.
Attach the tubing from one end of the stainless-steel container to the
sample valve cock on the transformer.
Hold the stainless-steel container in a vertical position with the length
of tubing on the outlet end in the can to catch the overfl ow oil.
Open the sampling valve on the transformer.
Open valve on the inlet side of container.
Open valve on the outlet side of container and allow the stainless-steel
container to fi ll and overfl ow into can. At least 1 pint should overfl
ow to assure removal of all bubbles in the sampling system.
Close top valve (outlet side) fi rst to ensure a contamination-free sample.
Close bottom valve (inlet side) and then close sampling valve on the
transformer.
Do not wrap any kind of tape around valves or nozzles of the stainlesssteel
container.
Forward the sample to the laboratory.
4.5 Maintenance and Reconditioning
of Insulating Oil and Fluids
This section covers the maintenance and reconditioning of the oils and less
fl ammable fl uids such as silicone, RTemp, and Wecosol. As discussed earlier,
moisture and oxygen are the most prevalent contaminants present in
transformer oil and fl uids. As a result of these contaminants and other
catalysts and accelerators, oxidation of these liquids takes place. Overtime
oxidation results in deterioration of the transformer insulating system.
If this degradation is not corrected in time, it eventually leads to terminal