How Is Particle Count Analysis Applied to Other Oil-Filled Equipment?

This article is Part 6 of a seven-part series on testing insulating liquids in load tap changers. Part 5 addressed analytical ferrography as a complement to particle count distribution analysis for LTC oil evaluation. Here we will address how particle count distribution analysis is applied to oil-filled circuit breakers, switches, and transformers — including the conditions under which it is used as a routine versus diagnostic tool for each equipment type. In Part 7, we will cover standards activities and real-world case studies.

Particle count distribution analysis was introduced in this series in the context of LTC oil monitoring, where it plays a central role in routine condition assessment. Its application extends to other oil-filled equipment types, though the diagnostic purpose and threshold criteria differ meaningfully depending on equipment function and failure mode.

Oil-Filled Circuit Breakers and Switches

Oil-filled circuit breakers (OCBs) and switches represent the most frequent non-LTC application for particle count distribution analysis. Like LTCs, these devices generate particles as a natural consequence of arc-in-oil operation, and excessive particle accumulation — particularly in larger size ranges — creates mechanical failure risk due to binding.

Applicable Thresholds

The same typical and acceptable particle count distribution values established for LTCs are applied to OCBs and switches. Upward trending is monitored but carries somewhat less interpretive weight than for LTC oil. Closer attention is warranted when >6 µm values exceed 10,000 particles/mL. Timely maintenance is recommended when >6 µm values reach 50,000 particles/mL or when larger size range values reach ten times the typical acceptable values.

Role of DGA and Moisture for OCBs

DGA and moisture content are generally considered optional tests for OCBs and switches; the liquid screen and particle count distribution provide the most diagnostically valuable results for routine annual testing. DGA can serve a specific optional role: helping to determine whether an OCB or switch operates frequently enough to warrant a preventive maintenance program that includes periodic oil service and internal cleaning. Importantly, acceptable oil test results do not guarantee continued mechanical performance — some manufacturers have recommended transitioning oil-filled switch designs to alternative technologies.

Vacuum Interrupter Switches

Switches using vacuum interrupters employ oil as a cooling medium rather than an arc-quenching medium. Arcing occurs within the vacuum bottles, so dissolved fault gases and suspended particles are not generated in the oil under normal conditions. Any presence of particles or dissolved fault gases in a vacuum interrupter switch requires investigation.

Particle Count Analysis for Transformers

Particle count distribution is not a routine test for in-service transformers. Transformers generate substantially lower particle levels than arc-in-oil equipment, and elevated particle counts are generally not a primary operational concern during normal service life.

Diagnostic Use With Low D1816 Results

Particle count distribution serves a valuable diagnostic role for in-service transformers when D1816 dielectric breakdown voltage results (spherical electrode VDE method) fall below the guidelines in IEEE C57.106. Low D1816 values can result from elevated moisture, oil oxidation and aging, particle contamination, and dissolved gases (including those from inert gas blanket systems). When cross-referencing other oil test results — liquid power factor, moisture, acid number, IFT — does not fully explain depressed D1816 values, elevated particle content is a viable hypothesis. Particle count distribution analysis at the next routine sampling interval can confirm or exclude this.

Typical & Acceptable Distribution

SDMyers uses this table as a typical and acceptable distribution based on our database of values and experience (compared to values for other equipment types).

Acceptable Values for Transformer Oil

No established industry standard exists for particle count distribution in in-service transformer oil. Laboratory databases and field-correlated experience provide the basis for typical and acceptable distributions, which are substantially lower than those for arc-in-oil equipment.

Summary:

• OCBs and switches use the same acceptable particle distribution thresholds as LTCs; maintenance action is recommended when >6 µm values reach 50,000 particles/mL or larger size ranges reach 10x acceptable values.
• DGA and moisture are optional for OCBs and switches; the liquid screen and particle count distribution are the most diagnostically valuable routine tests for these devices.
• Acceptable oil test results do not guarantee continued mechanical integrity for OCBs and switches; some manufacturers have recommended technology transitions away from oil-filled switch designs.
• Particle count distribution for transformers is diagnostic, not routine — its primary use is to investigate depressed D1816 dielectric breakdown voltage results when other oil test parameters do not fully explain the condition.
• No industry standard for in-service transformer oil particle counts is currently established; laboratory-specific databases provide the working reference values.

In This Series:

• Part 1: How Does Mineral Oil Age Inside a Load Tap Changer?
• Part 2: What Causes Accelerated Film Formation in LTCs?
• Part 3: How Are Liquid Screen, Moisture, and DGA Used to Monitor LTCs?
• Part 4: What Does Particle Count Distribution Reveal About LTC Condition?
• Part 5: What Can Analytical Ferrography Reveal About LTC Particle Sources?
• Part 6: How Is Particle Count Analysis Applied to Other Oil-Filled Equipment?
• Part 7: What Standards and Case Studies Define LTC Particle Analysis?