ASTM D6786: Particle Count in Transformer Insulating Oil

In a transformer, the oil is part of the insulation. Particles in it can lower its dielectric strength, depending on their size, number and what they are made of. ASTM D6786 is the test method for counting them in mineral insulating oil.

What the standard covers

D6786 measures the concentration and size distribution of particles in mineral insulating oil with a viscosity of 6 to 20 mm²/s at 40 °C. It uses automatic particle analysers that work on the light extinction principle: the oil passes through a light beam, and each particle that blocks the light is counted and sized. Samples are taken as set out in ASTM D923, the practice for sampling electrical insulating liquids.

Where the particles come from

• Contamination from outside, during filling, handling or maintenance.

• By-products of the oil breaking down.

• Materials from inside the equipment, such as metal, carbon and cellulose fibres from paper insulation.

What the counts are used for

The standard names three uses: a general measure of how contaminated the oil is, help in judging the condition of some types of electrical equipment, and a check on how well filtering works when oil is processed. If the nature of the particles matters, other tests follow, such as metals analysis or counting of fibres.

For filters, the third use is the important one. Counts before and after a filtration or oil processing run show whether the filter elements are removing what they should.

Particles and water are separate problems

A filter element removes particles. Water in transformer oil, dissolved or free, is removed by other means, usually vacuum dehydration in an oil processing unit. Both are checked: particles by D6786, water by its own tests.

Vens elements and D6786

Vens filter elements and cartridges for transformer oil filtration can meet particle count requirements measured by ASTM D6786, depending on the element and rating. Housings include simplex and duplex filters, and replacement elements for oil processing units can be found in FilterDex. Tell us the oil, the flow and the count or code your specification sets.

More: ASTM standards for filters and the Vens filter test laboratory.

Find a replacement element in FilterDex, send your specification through the enquiry form, or message us on WhatsApp (+91 73974 98659).

ASTM is a trademark of ASTM International. Vens Hydroluft is not affiliated with ASTM International. The standards are described in our own words from the scopes ASTM publishes; work to the edition named in your specification. Whether a filter meets a requirement depends on the product, its rating and the conditions of use, so we confirm it for each enquiry. See also our product information disclaimer.