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The concise guide to oil sampling

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Two things are particularly important when taking oil samples. The first is to ensure that the proper sampling procedure is followed, and the second is to ensure that all of the essential information is properly recorded. If the sample is to be sent to a test house for testing, the test house should advise on the information needed, as well as the volume of the sample and the type of container to use.

For oil samples from transformers, the information generally required is:

  • Description of the sample
  • List of tests to be performed
  • Transformer nameplate information
  • Type of transformer
  • Type of insulating fluid
  • Any leaks noted
  • Insulating fluid service history (has it been dried, etc)
  • Transformer service history (has it been rewound, etc)
  • Type of breather
  • Type of insulation, including temperature rise rating
  • Details of cooling equipment (fans, radiators, etc)
  • Temperature of top of fluid, read from gauge
  • Actual fluid temperature measured
  • Fluid level
  • Vacuum and pressure gauge readings

For load tap changers, it is also advisable to record the counter reading, the selector range and the sweep range. 

Sampling should be performed in accordance with the appropriate standard. In the USA, there are two relevant standards:

  • D923 – Standard Practices for Sampling Electrical Insulating Liquids
  • D3613 – Standard Practice for Sampling Electrical Insulating Oils for Gas Analysis and Determination of Water Content

Internationally, there are two further sampling standards:

  • IEC 60475 Ed. 2.0 – Method of Sampling Insulating Liquids
  • IEC 60567 Ed. 3.0  – Oil-filled electrical equipment – Sampling of gases and of oil for analysis of free and dissolved gases – Guidance

These IEC standards should be consulted together, especially as part of IEC 60567 has been transferred to IEC 60475.

Hints for taking oil samples

The sample must be representative of the oil in the equipment, which means cleanliness extremely important. 

  • Samples are normally drawn from a drain valve or sampling cock, and this must be cleaned both inside and out before the sample is taken to ensure dirt does not fall into the sampling container.
  • 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, or more if the equipment has been out of service for some time.
  • Don’t be tempted to use old engine oil bottles, as even a few parts per million of engine oil will cause the sample to fail a breakdown test.
  • Do let the oil flow down the side of the sample bottle, or use a clean tube run to the bottom of the bottle. This will prevent air mixing with the oil.
  • Do store oil samples in the dark if they are in glass or clear plastic bottles, as mineral oil deteriorates if exposed to UV light.


  • Before taking samples, ensure that all of the required permissions and permits have been obtained.
  • Ensure that everything needed for locking out and/or tagging out is to hand.
  • 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.
  • Use the correct personal protective equipment (PPE) and correctly rated tools.
  • Check the area for electrical and tripping hazards.
  • Check for wildlife – snakes, bees and other creatures like transformers!
  • 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

  • Take extra sample bottles and syringes – they’re often needed.
  • Ensure that the sample bottle seals are airtight.
  • Use only ground glass syringes.
  • 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 putting the thermometer into the actual sample.

Taking the sample

  • 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 is 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).

  • 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.
  • Do not take samples when it is windy, as dust blown by the wind may contaminate the sample.
  • Try not to take samples when the ambient temperature is high, as perspiration is a common source of contamination problems.

This article is an extract from a new publication The Megger Guide to Insulating Oil Dielectric Breakdown Testing. Copies of this publication, which provides comprehensive guidance on oil test techniques and equipment, can be obtained free of charge from Megger, or can be downloaded in Adobe Acrobat (pdf) format from the Megger website (

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