Hints and tips for taking oil samples for BDV testing
For a sample to be truly useful, it must be representative of the oil in the equipment. This means that cleanliness is extremely important.
(1) Samples are normally drawn from a drain valve or sampling cock. This must be cleaned both inside and out before the sample is taken to ensure that dirt does not fall into the sampling container.
(2) The drain valve is at the bottom of the equipment, where all of the sludge, water and contaminant particles collect. It is important therefore, to flush the system thoroughly to ensure that the sample is drawn from the main bulk of the oil. This may involve removing two litres of oil, and even more if the equipment has been out of service for some time.
(3) Do not be tempted to use old engine oil bottles, even for a few p.p.m.of engine oil will cause the sample to fail a breakdown test.
(4) Do let the oil flow down the side of the sample bottle, or use a clean tube run to the bottom of the bottle; it will prevent air being mixed with the oil.
(5) Do store the oil samples in glass or clear plastic bottles in the dark, mineral oil will deteriorate if exposed to UV light.
Safety
(1) Before taking samples, ensure that you have all of the required permissions and permits
(2) Have everything you need to lock out/tag out to hand
(3) Make sure that the PCB (polychlorinated biphenyl) content of the oil, if any, is known and that the equipment is labelled. PCB is very hazardous and requires special handling
(4) Use all of the correct personal protective equipment (PPE) and correctly rated tools
(5) Check the area for electrical and tripping hazards
(6) Check for wildlife - snakes, bees, etc like transformers!
(7) Check that the transformer is under positive pressure - are the pressure gauges reliable? Could they be blocked or broken? NEVER try to take a sample from a transformer under negative pressure. Air could be drawn into the transformer and cause it to fail.
Sampling equipment
(1) Take extra sample bottles and syringes - they are often needed
(2) Ensure that the sample bottle seals are airtight
(3) Use only ground glass syringes
(4) If rubber hose is used, discard after each sample is taken
Flushing the system
When flushing the system, a spare sample bottle is usually repeatedly filled and emptied into the waste. It is good practice to measure the oil temperature using the last bottle that will be discarded, as this avoids having to put the thermometer into the actual sample.
Taking the sample
(1) Wherever possible, try to take samples during times of relatively steady loads and temperature – in other words, when the equipment is at equilibrium. (This is particularly important with transformers, as if the sample happens to be taken after the transformer has cooled following a long period of running at full load, the breakdown voltage of the oil will be much lower than normal. This is because moisture in the paper insulation will have migrated to the oil during the period of full load, and will not yet have had time to migrate back. This is usually considered to be a normal phenomenon, but it is possible that it may also be a factor in so-called "sudden death" transformer incidents where, for no apparent reason, a seemingly healthy transformer suddenly fails. This is another good reason for recording as much information about the transformer as possible and for trending results to look for unexplained changes).
(2) Do not take samples when it is raining or snowing, or when the relative humidity is above 50%, as there is a high probability that samples taken in these conditions will be contaminated.
(3) Do not take samples when it is windy, as dust blown by the wind may contaminate the sample.
(4) Try not to take samples when the ambient temperature is high, as perspiration is a common source of contamination problems.
