EN IEC 60156 Insulating Fluids - Determination of Breakdown Voltage at Mains Frequency - Test Method

The standard “EN IEC 60156 Insulating fluids - Determination of breakdown voltage at mains frequency - Test method”, developed by the International Electrotechnical Commission (IEC) and later published as a European standard as part of harmonization efforts, describes the method for determining the dielectric breakdown voltage of insulating fluids at power frequency. This test procedure is performed in a specific device where the oil sample is exposed to an increasing AC electric field until fracture occurs.

^{The described method is applicable to all types of insulation fluids with a nominal viscosity of up to 350 mm²} /s at 40 degrees . It is suitable both for acceptance testing of unused fluids upon delivery and for determining the condition of samples taken during equipment monitoring and maintenance.

The EN IEC 60156 standard is required to evaluate the electrical insulation performance of transformer oils and other insulating fluids used in power transformers, switchgear, circuit breakers, reactors, and high-voltage equipment. This standard is widely applied by transformer manufacturers, electricity companies, testing laboratories, and maintenance organizations.

The primary purpose of the EN IEC 60156 standard is to determine the voltage at which an insulating fluid electrically breaks down when exposed to an increasing alternating current (AC) electric field. The measured breakdown voltage indicates the condition of the insulating fluid and helps determine whether it is suitable for:

• Commissioning of new electrical equipment.
• Routine quality control
• Preventative Maintenance
• Transformer status monitoring
• Oil refining verification
• Acceptance test of newly supplied insulation oils

Breakdown voltage is one of the key indicators of insulation quality. An insulating fluid normally prevents the flow of electric current between energized conductors. As the voltage increases,

• The electric field inside the liquid strengthens.
• The pollutants become electrically active.
• Microscopic discharge channels (currents) are formed.
• Eventually a conductive path develops.
• An electric arc forms in the electrode gap.

The voltage at which this occurs is called the dielectric breakdown voltage. A higher breakdown voltage generally indicates better electrical insulation.

The EN IEC 60156 standard includes the following:

• Mineral transformer oils
• Natural ester insulation fluids
• Synthetic ester liquids
• Silicone sealing fluids
• Other insulating fluids used in electrical equipment.

This standard is also applicable to the following:

• New insulation fluids
• Stored fats
• Oils taken from operational transformers
• Switchgear insulation fluids
• Circuit breaker oils

This standard is not intended to provide a complete assessment of the oil condition; instead, it evaluates dielectric strength as a key property. The test measures the voltage required to create an electrical breakdown between two standardized electrodes immersed in an insulating fluid. The test process follows these steps:

• Fill a clean test container with the oil sample.
• Positioning standardized electrodes within a specified range.
• Applying alternating current voltage at the power frequency (typically 50 or 60 Hz).
• Increasing the voltage at a steady rate.
• To record the fault voltage.
• Repeat the test several times.
• Calculating the average breakdown voltage.

These tests require a dielectric breakdown testing device consisting of the following components:

• High voltage transformer
• Automatic voltage regulator
• Voltage measurement system
• Fault detection circuit
• Test container
• Standard electrode arrangement
• Security guard

Modern devices automatically,

• It increases the voltage.
• It detects the fault.
• Stops the test
• It records the value.
• It performs multiple consecutive tests.

Rather than relying on a single failure event, this standard requires multiple consecutive measurements on the same sample. Between each test,

• The arc gases are allowed to disperse.
• The oil can be mixed or left to rest as directed.
• Electrode conditions are checked.

The last reported value is usually the arithmetic mean of the breakdown voltages.

Several factors can significantly reduce dielectric strength:

• Moisture: Water is the most common cause of a decrease in breakdown voltage. Even small amounts can significantly reduce insulation performance.
• Solid pollution: For example,
  • Paper fibers
  • Powder
  • Metallic particles
  • Carbon residues

These particles intensify local electric fields and promote premature failure.

• Air bubbles: Trapped air creates localized concentrations of electric fields that can initiate discharges at lower voltages.
• Fat aging: Aged fats can include the following:
  • Oxidation products
  • Mud
  • acids
  • Degradation by-products

These contaminants reduce dielectric performance.

• Electrode condition: Poorly polished or contaminated electrodes can significantly affect results. Therefore, electrodes need to be carefully cleaned and inspected.

Accurate fat sampling is crucial. Incorrect sampling can lead to:

• Gender
• Powder
• Air bubbles
• Pollution

This results in artificially low breakdown voltages. The IEC 60475 standard is widely referenced for accurate sampling procedures.

The EN IEC 60156 standard is widely used in the following areas:

• Power Transformers
• Distribution transformers
• Measuring transformers
• Oil-filled switchgear
• High voltage bushings
• Circuit Breakers
• Reactors
• Capacitors
• Electricity grid maintenance programs
• Transformer oil production
• Oil regeneration plants

The EN IEC 60156 standard is frequently used, often in conjunction with complementary insulation oil standards, including:

• IEC 60422 Mineral insulation oils used in electrical equipment - Inspection and maintenance guide.
• IEC 60475 Sampling method for insulation fluids.
• IEC 60296 Fluids for electrotechnical applications - Mineral insulating oils for electrical equipment.
• IEC 61125 Insulating fluids - Test methods for oxidation stability - Test method for evaluating the oxidation stability of insulating fluids in their delivered state.
• IEC 62021 Insulating fluids - Acidity determination
• IEC 60814 Insulating fluids - Oil-impregnated paper and pressed cardboard - Water determination by automated coulometric Karl Fischer titration.

Together, these standards provide a comprehensive assessment of insulation fluid quality.

The EN IEC 60156 standard is widely applied in the following areas:

• Electricity production
• Transmission companies
• Distribution networks
• Transformer manufacturing
• Electricity maintenance companies
• High voltage laboratories
• Oil testing laboratories
• Renewable energy facilities
• Industrial power systems

Electrical insulation failure is a leading cause of transformer and high-voltage equipment failures. Measuring dielectric breakdown voltage helps identify moisture, contamination, and oil degradation before they lead to insulation failure, extending equipment life and reducing maintenance costs. Regular testing in accordance with EN IEC 60156 standard supports predictive maintenance and improves the reliability of power systems.

For many years, our organization, with its competent and experienced staff, has been understanding the needs of its clients, providing the advanced testing services they require, and assisting them in the establishment, implementation, and improvement of management systems. Within this framework, we also provide testing services compliant with the EN IEC 60156 standard.