Transformer oil immersion compression set testing evaluates the ability of elastomeric sealing materials to recover their original thickness after prolonged compression in hot insulating oil, a critical performance indicator for gaskets and O-rings used in oil-filled electrical equipment. This test combines thermal aging and fluid exposure to simulate the harshest service conditions inside power transformers, switchgear, and bushings. In this guide, we explain the testing principles, applicable international standards, specimen requirements, test procedures, calculation methods, and typical acceptance criteria.
Technical Overview
Compression set is the permanent deformation that remains in an elastomer after it has been compressed between two surfaces for a defined period and then allowed to recover. In transformers and other oil-immersed electrical apparatus, sealing components such as O-rings, gaskets, and diaphragms are permanently under compressive load while being continuously exposed to hot mineral insulating oil. Over time, the combined action of heat and oil can break down the polymer network, cause plasticizer extraction or swelling, and permanently reduce the sealing force. When the compression set becomes excessive, leakage paths form, oil loss or moisture ingress follows, and the dielectric integrity of the entire equipment is compromised.
Transformer oil immersion compression set testing therefore measures how much of the original deflection is retained as permanent set after the compressed specimen has been aged in transformer oil at elevated temperature. The test is typically performed in accordance with ASTM D395 (Method B) for compression set, combined with the immersion principles of ASTM D471, or with ISO 815-1 together with ISO 1817. In the Chinese national standard system, GB/T 7759.1 and GB/T 1690 are the corresponding methods, and IEC 60296 defines the properties of the unused mineral insulating oil used as the immersion medium. Manufacturers of transformer seals, corrugated expansion joints, and rubber membranes rely on this test to qualify materials for long-term service, while accredited third-party laboratories use it for independent verification and type testing.
Testing Samples
Standard Molded Cylindrical Specimens
The preferred test piece is a molded cylindrical button of Type A (29 mm diameter and 12.5 mm thickness) or Type B (13 mm diameter and 6.3 mm thickness), as specified in ASTM D395 and ISO 815-1. Molded specimens are preferred over cut samples because molding eliminates surface defects that could initiate oil attack or uneven stress distribution. The specimens must be free from bubbles, porosity, and flash, and they must be conditioned for at least 16 hours after vulcanization before testing.
O-Rings and Radial Seals
When testing actual O-rings, the compression set is measured in a groove-and-piston fixture that reproduces the squeeze geometry of real sealing assemblies. The cross-section diameter is measured in several positions before and after aging, and the permanent set is expressed relative to the applied compression. This approach is common for qualification of shaft seals, flange seals, and valve seals used in transformer radiators, valves, and tap changers.
Gaskets and Flat Seals
Flat rubber gaskets used on transformer tank flanges, bushing flanges, and inspection covers can be tested by cutting representative buttons from the sheet or by compressing strip specimens between rigid plates. For fabric-reinforced or cork-rubber composite gaskets, the test determines whether the oil plasticizes the binder and accelerates stress relaxation.
Diaphragms and Corrugated Expansion Components
Rubber diaphragms for conservator breathers and corrugated expansion sheets that compensate oil volume changes are frequently qualified by compression set in oil, because these components operate in direct, long-term contact with hot transformer oil and cannot be replaced without opening the tank.
Testing Items
- Compression set in transformer oil (constant deflection, Method B type) at specified temperature and duration
- Compression set in air under identical conditions, as a reference for isolating the oil effect
- Volume change (swelling or shrinkage) after immersion
- Mass change after immersion
- Hardness change (Shore A) before and after immersion
- Change in tensile strength and elongation at break after immersion
- Visual assessment of surface condition: cracking, stickiness, blistering, or deposits
Testing Methods
Test Conditions
Common immersion temperatures range from 100 °C to 150 °C, selected according to the service class of the elastomer and the hotspot temperature of the equipment. Typical test durations are 24 h, 70 h, 168 h (7 days), and 1000 h, with 168 h at 125 °C being a widely used qualification point for nitrile rubber (NBR) seals in transformer applications. The immersion medium is normally unused transformer oil meeting IEC 60296, and the oil-to-rubber volume ratio must be sufficient, generally at least 15:1, with the oil replaced for long-duration tests to prevent saturation with extractables.
Test Procedure
- Condition the specimens and measure the original thickness t0 at the center, or the original cross-section for O-rings, to the nearest 0.01 mm.
- Place each specimen between the polished plates of a compression fixture fitted with limit spacers (bars) whose thickness ts equals 75% of t0, producing a 25% compression.
- Tighten the fixture bolts to a defined torque so the plates rest firmly on the spacers, ensuring uniform and reproducible deflection.
- Immerse the complete compressed assembly in transformer oil in a sealed vessel, then place the vessel in a circulating-air oven or oil bath held at the test temperature for the specified duration.
- At the end of the aging period, remove the assembly, disassemble it within a short defined time, and allow the specimens to recover freely at standard laboratory temperature for 30 minutes (or as specified).
- Measure the final thickness t1 at the same point, and record any visual defects.
Calculation of Compression Set
The compression set at constant deflection, denoted CB, is calculated as follows: CB = [(t0 − t1) / (t0 − ts)] × 100%, where t0 is the original thickness of the specimen, t1 is the thickness after recovery, and ts is the thickness of the spacer (the compressed height). A result of 0% indicates perfect recovery, while 100% indicates total permanent set with complete loss of sealing capability. Typical acceptance limits for transformer sealing compounds are 15% to 35% depending on the material class, the temperature, and the duration of exposure.
Reporting
A complete test report includes the material identification and cure data, specimen type and dimensions, the oil specification, the compression percentage, the test temperature and duration, the recovery conditions, the individual and median compression set values, the volume and hardness changes, and a description of any surface degradation observed.
Applications
- Qualification of sealing materials for power transformer tanks, radiators, and conservators
- Quality control for O-rings in transformer valves, drain plugs, and gas relays
- Type testing of gaskets for transformer bushings and cable termination boxes
- Material comparison studies for nitrile rubber (NBR), fluoroelastomer (FKM), and fluorosilicone (FVMQ) in oil-immersed switchgear
- Lifetime assessment and accelerated aging programs for rubber diaphragms and expansion bellows
- Incoming material inspection and supplier certification for manufacturers of electrical equipment and elastomeric components
FAQ
Why is compression set tested in transformer oil rather than only in air?
Seals inside transformers are simultaneously exposed to compressive stress, elevated temperature, and hydrocarbon oil. Mineral oil can extract plasticizers and soluble additives, causing shrinkage and stiffening, or it can swell the polymer and weaken the network. Air-aging data alone therefore overestimates sealing performance, and the oil-immersion test reproduces the true combined degradation mechanism.
What compression percentage and spacer height are used?
A 25% constant deflection is the standard condition defined by ASTM D395 Method B and ISO 815-1. The spacer thickness is set to 75% of the original specimen thickness, and this fixed compressed height is what makes results comparable between laboratories.
Which elastomers give the best results in this test?
For standard transformer oil service up to about 100 °C, medium- or high-acrylonitrile NBR compounds are widely used and show compression set values typically below 30% after 168 h at 125 °C. For higher temperatures or extended service life, FKM and fluorosilicone materials deliver superior oil resistance and lower permanent set, at higher cost. Silicone rubber (VMQ) offers excellent heat resistance but only moderate oil resistance, and EPDM is generally unsuitable for direct oil contact.
How long does the test take?
Short screening tests at 70 h or 168 h can be completed within one to two weeks including conditioning and measurement. Long-duration qualification tests of 1000 h or more, sometimes combined with cyclic temperature profiles, require several months of laboratory time.
Can finished seals be tested instead of molded buttons?
Yes. O-rings, gaskets, and cut parts can be tested in purpose-built fixtures that reproduce the actual squeeze, and this is often preferred for final product qualification. However, button specimens molded from the same compound remain the reference method for material-level comparison, because they remove the influence of part geometry.