Sampling Procedure for Transformer Oil in Drum/Barrel For Testing

In this article, you will learn about the sampling procedure for transformer oil & insulating oil in their delivery containers as well as insulating oil in service in transformers and switchgear. Physical contaminants like solid particles and water do not disperse uniformly through the liquid being sampled. Hence it is necessary to draw samples from specific locations in the container according to the specific gravity of the liquid.


We select drums from a lot for this purpose which depend on the size of the lot as per procedure. Since the results of the prescribed tests for liquid dielectrics greatly depend on the possible impurities in the sample, it is essential to observe the following precautions:

  1. Reserve all sampling equipment (sampler and containers) exclusively for this purpose.
  2. The equipment should be perfectly clean and dry.
  3. Glass equipment is better, although stainless steel and aluminium are also suitable. You can use other equipment if it does not introduce contaminants in the samples. Don’t touch sample quantity by hand and does not much exposure in an open atmosphere.

Pipette enables the sample to be taken from any level in the drums.  For this purpose, the pipette shall have a capacity of about 500 ml. Another device suitable for drawing samples from drums containers from the required level is the Siphon pressure thief.
It consists of two tubes; one (internal diameter 13 mm) is for applying air pressure is made of metal. Both tubes are set in a bung whose dimensions correspond to the diameter of the bung hold in the drum/container. The cream dipper may also be useful for taking top samples. Sampling container should be airtight after taking the sample. Don’t expose it in the open air. Avoid an outdoor sampling of Insulating oil in the rain, fog or high wind.

Electric Strength (Break Down Voltage) in New Insulating oil (Transformer oil)

Test for Break Down Voltage (BDV)  is a normal test which intends to find the extent of physical moisture and other suspended matter. To carry out the test, it is advisable to dry out the apparatus.
The BDV Test intends for the acceptance of new insulating oils at the time of ‘their delivery, it is applicable, to all class of insulating oil having less than a viscosity of 50 Cst.
We do the sampling in accordance with the procedure as in IS 6855. The test consists of applying to the electrodes an increasing  AC voltage of frequency 40 to 60 Hz. The rate of increase of the voltage being uniform and equal to approximately 2 kV/s.. After the cell fills, we make the first application of the voltage as quickly as possible. There should be no longer any air bubbles in the oil, or at the latest 10 minutes after filling.
After each breakdown, we stir the oil gently so as to keep away the carbon particles between the electrodes. It also avoids the possible production of air bubbles. For the subsequent five tests, we reapply the voltage one minute after the disappearance of any air bubbles.
We have to perform the test six times on the same cell filling. The electric strength shall be arithmetic mean of the six results which have been obtained.

Dielectric breakdown voltage test

The dielectric breakdown voltage test is an important test to determine the withstanding capacity of an insulating oil or liquid. There is a degradation of transformer oils or ingress of moisture. And it is necessary to test the insulating oils periodically. BDV test is one of the most common tests done on all Insulating fluids, but a very critical one. Breakdown (BDV) test is a test of choice because it takes very less time to conduct and is a precursor to the condition of the insulating liquid, before carrying out an extensive series of tests. Power Electronical offers accurate Automatic Oil BDV Test sets and Motorised Oil Breakdown testers in a variety of maximum test voltages

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