ASTM D6786-15(2023)

Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters

SIGNIFICANCE AND USE
5.1 Particles in insulating oil can have a detrimental effect on the dielectric properties of the fluid, depending on the size, concentration, and nature of the particles. The source of these particles can be external contaminants, oil degradation by-products, or internal materials such as metals, carbon, or cellulose fibers.  
5.2 Particle counts provide a general degree of contamination level and may be useful in assessing the condition of specific types of electrical equipment. Particle counts can also be used to determine filtering effectiveness when processing oil.  
5.3 If more specific knowledge of the nature of the particles is needed, other tests such as metals analysis or fiber identification and counting must be performed.
SCOPE
1.1 This test method covers the determination of particle concentration and particle size distribution in mineral insulating oil. It is suitable for testing oils having a viscosity of 6 mm2/s to 20 mm2/s at 40 °C. The test method is specific to liquid automatic particle analyzers that use the light extinction principle.  
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.  
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.4 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

General Information

• Status: Published
• Publication Date: 30-Nov-2023
• ICS: 91.100.15 - Mineral materials and products
• Technical Committee: D27 - Electrical Insulating Liquids and Gases
• Drafting Committee: D27.07 - Physical Test

Relations

• Replaces: ASTM D6786-15 - Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters
• Effective Date: 01-Dec-2023
• Referred By: ASTM D7720-21 - Standard Guide for Statistically Evaluating Measurand Alarm Limits when Using Oil Analysis to Monitor Equipment and Oil for Fitness and Contamination
• Effective Date: 01-Dec-2023
• Referred By: ASTM D7647-10(2018) - Standard Test Method for Automatic Particle Counting of Lubricating and Hydraulic Fluids Using Dilution Techniques to Eliminate the Contribution of Water and Interfering Soft Particles by Light Extinction
• Effective Date: 01-Dec-2023
• Referred By: ASTM D923-15(2023) - Standard Practices for Sampling Electrical Insulating Liquids
• Effective Date: 01-Dec-2023
• Referred By: ASTM D117-22 - Standard Guide for Sampling, Test Methods, and Specifications for Electrical Insulating Liquids
• Effective Date: 01-Dec-2023

Overview

ASTM D6786-15(2023): Standard Test Method for Particle Count in Mineral Insulating Oil Using Automatic Optical Particle Counters outlines the test method for determining particle concentration and particle size distribution in mineral insulating oil. Developed by ASTM International, this standard is essential for ensuring the quality and reliability of insulating oils used in electrical equipment. The test method applies specifically to automatic particle counters that operate on the light extinction principle, enabling rapid and accurate assessment of contamination levels.

Mineral insulating oils play a crucial role in electrical and power systems, and monitoring particulate contamination is key to maintaining the dielectric properties and longevity of such equipment. This standard supports manufacturers, maintenance personnel, and laboratories in assessing oil cleanliness and determining when further purification or analysis is required.

Key Topics

• Significance of Particle Contamination
  Particles in insulating oil, originating from contaminants, degradation by-products, or internal materials like metals and cellulose, can degrade dielectric properties and compromise equipment performance.

• Test Method Scope
  The method is tailored to mineral insulating oils with a viscosity range of 6 mm²/s to 20 mm²/s at 40°C. It specifies the use of automatic optical particle counters-devices that count and size particles as they disrupt a light beam.

• Sampling and Analysis
  Proper sampling in particle-clean bottles and careful handling are emphasized to avoid introducing errors during testing. The sample is agitated, then analyzed by the instrument, which reports particle counts and size distributions.

• Interferences and Limitations
  Potential sources of error include air bubbles, suspended water, excessive contamination, and odd-shaped particles. Calibration with NIST-traceable fluids in accordance with ISO 11171 is required to maintain accuracy.

• Reporting Requirements
  Results are reported as cumulative particle counts per mL for several size thresholds, with an option to include an ISO solid contamination code (per ISO 4406:1999).

Applications

• Condition Monitoring of Electrical Equipment
  Regular particle counting according to ASTM D6786 helps utilities and maintenance teams evaluate the cleanliness and suitability of mineral insulating oils in transformers, circuit breakers, and similar apparatus.

• Quality Control for Oil Processing
  Oil producers and processors use this standard to assess the effectiveness of filtration and purification systems, ensuring that insulating oils meet stringent purity requirements before use or shipment.

• Diagnostic and Predictive Maintenance
  Maintenance professionals use particle count results to detect early signs of wear, internal contamination, or oil degradation, allowing for timely interventions and preventing expensive equipment failures.

• Supporting Laboratory and Field Testing
  Accredited laboratories and field service providers follow ASTM D6786 for consistent, reliable results that conform to international best practices for oil analysis.

Related Standards

• ASTM D923 - Practices for Sampling Electrical Insulating Liquids, referenced for obtaining representative oil samples.
• ISO 4406:1999 - Hydraulic Fluid Power-Method for Coding the Level of Contamination by Solid Particles, used for reporting contaminant codes.
• ISO 11171:2010 - Hydraulic Fluid Power-Calibration of Automatic Particle Counters for Liquids, detailing calibration requirements for optical particle counters.

Practical Value

Complying with ASTM D6786-15(2023) allows organizations to systematically assess particle contamination in insulating oils, promoting enhanced equipment reliability, extended service life, and adherence to global standards. By following procedures for sampling, handling, and reporting as specified in this standard, users can make informed decisions regarding oil treatment, maintenance schedules, and asset management in the electrical power industry.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6786 − 15 (Reapproved 2023)
Standard Test Method for
Particle Count in Mineral Insulating Oil Using Automatic
Optical Particle Counters
This standard is issued under the fixed designation D6786; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Terminology
1.1 This test method covers the determination of particle 3.1 Definitions:
concentration and particle size distribution in mineral insulat- 3.1.1 coincidence, n—the presence of more than one particle
ing oil. It is suitable for testing oils having a viscosity of in the sensing zone of a particle analyzer at the same time,
2 2
6 mm /s to 20 mm /s at 40 °C. The test method is specific to causing mis-sizing and mis-counting of the particle present; the
liquid automatic particle analyzers that use the light extinction coincidence limit of the counter is determined by the maximum
principle. acceptable concentration of particles in the sensing zone and is
supplied by the instrument manufacturer.
1.2 The values stated in SI units are to be regarded as
3.1.2 concentration limit, n—a direct function of coinci-
standard. No other units of measurement are included in this
dence and electronic saturation; the concentration limit of the
standard.
system is determined by the maximum acceptable concentra-
1.3 This standard does not purport to address all of the
tion of particles in the given sample and is supplied by the
safety concerns, if any, associated with its use. It is the
instrument manufacturer.
responsibility of the user of this standard to establish appro-
3.1.3 electronic saturation level, n—particle concentration
priate safety, health, and environmental practices and deter-
at which the electronic circuitry of the analyzer ceases to
mine the applicability of regulatory limitations prior to use.
function properly due to excessive counting rates.
1.4 This international standard was developed in accor-
dance with internationally recognized principles on standard- 3.1.4 light extinction, n—the reduction in intensity of a light
ization established in the Decision on Principles for the
beam passing through the sensing zone of a particle analyzer,
Development of International Standards, Guides and Recom- caused by the absorption and/or scattering of the light by
mendations issued by the World Trade Organization Technical
particles; synonyms: light obscuration, light interruption, light
Barriers to Trade (TBT) Committee.
blockage.
4. Summary of Test Method
2. Referenced Documents
4.1 Samples are taken in particle-clean bottles that are
2.1 ASTM Standards:
suitable for particle analysis. The sample bottle is agitated to
D923 Practices for Sampling Electrical Insulating Liquids
redistribute particles in the oil, then the oil is placed in an
2.2 ISO Standards:
automatic particle counter, where the number of particles and
4406:1999 Hydraulic Fluid Power—Fluids—Method for
their size distribution are determined by the light extinction
Coding the Level of Contamination by Solid Particles
principle.
11171:2010 Hydraulic Fluid Power—Calibration of Auto-
4.2 As particles pass through the sensing zone of the
matic Particle Counters for Liquids
instrument, the quantity of light reaching the detector is
obscured. This signal is translated to an equivalent projected
1 area diameter based on calibration with a NIST-traceable fluid
This test method is under the jurisdiction of ASTM Committee D27 on
Electrical Insulating Liquids and Gases and is the direct responsibility of Subcom-
(ISO Medium Test Dust suspension).
mittee D27.07 on Physical Test.
Current edition approved Dec. 1, 2023. Published January 2024. Originally 5. Significance and Use
approved in 2002. Last previous edition approved in 2015 as D6786 – 15. DOI:
5.1 Particles in insulating oil can have a detrimental effect
10.1520/D6786-15R23.
on the dielectric properties of the fluid, depending on the size,
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
concentration, and nature of the particles. The source of these
Standards volume information, refer to the standard’s Document Summary page on
particles can be external contaminants, oil degradation by-
the ASTM website.
products, or internal materials such as metals, carbon, or
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036, http://www.ansi.org. cellulose fibers.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6786 − 15 (2023)
5.2 Particle counts provide a general degree of contamina- directly from NIST (SRM 2806) or a secondary sample
tion level and may be useful in assessing the condition of prepared in accordance to ISO 11171 and traceable to NIST.
specific types of electrical equipment. Particle counts can also
8.4 Dilution Oil, insulating oil that has been filtered to
be used to determine filtering effectiveness when processing
contain fewer than 1 % of the total particles expected in the
oil.
cleanest sample.
5.3 If more specific knowledge of the nature of the particles
9. Sampling
is needed, other tests such as metals analysis or fiber identifi-
cation and counting must be performed.
9.1 Proper sampling is crucial to particulate analysis. Obtain
a sample of the liquid to be tested in accordance with Practice
6. Interferences
D923.
6.1 Dirty environmental conditions and poor handling tech-
niques can easily contaminate the sample and/or test specimen. 10. Calibration
Care must be taken to ensure test results are not biased by
10.1 Calibration of the instrument shall be with a NIST-
introduced particle
...