The standard 7735

The DIN 7735 standard played a central role in electrical engineering for decades, particularly in defining requirements and testing procedures for laminated thermoset composites. Although it has been withdrawn and replaced by more current standards, it still serves as a guideline and is often referenced for the maintenance of older equipment.

Background and History

The DIN 7735 standard was developed by the German Institute for Standardization (DIN) in 1975 to create uniform standards for the manufacturing and testing of laminates such as phenolic or epoxy resin-bound paper, fabric, and glass mats. These materials are used primarily in electrical machines and devices to ensure safe and efficient insulation. The standard was revised several times and eventually replaced by the DIN EN 60893 series to keep up with technological advancements and changing requirements. Additionally, the new standard is a result of harmonizing various national standards into a common European standard.

Scope of the Standard

DIN 7735 set specific requirements for insulating materials used in a variety of applications:

• Electrical Machines and Devices: Used for electrical insulation in transformers, motors, and generators.
• High and Low Voltage Switchgear: Applied in electrical switchgear to prevent short circuits and enhance safety.
• Industrial Manufacturing: Used in the production of components requiring high thermal and electrical resistance.

Technical Specifications

The technical specifications of DIN 7735 covered various aspects:

• Materials: Included thermoplastic and thermoset insulators as well as composite materials and laminates.
• Properties: Defined requirements for electrical breakdown strength, thermal resistance, mechanical strength, and chemical resistance.
• Temperature Classes: Categorized materials based on their maximum operating temperature, determining their suitability for different applications.

Testing Procedures and Methods

To ensure materials conformed to the established standards, various testing procedures were applied:

Electrical Tests

• Breakdown Strength: Measures the maximum electrical voltage a material can withstand before failure. Typically, this involves placing the material between two electrodes and gradually increasing the voltage until breakdown occurs, as described in IEC 60243-1.
• Surface and Volume Resistance: These tests determine the material’s resistance to electrical currents, evaluating its insulating properties as per IEC 62631-3-3.

Mechanical Tests

• Tensile Strength Tests: Measure the material’s ability to withstand tensile loads. A standardized sample is stretched in a tensile testing machine until it breaks, determining the maximum tensile strength according to ISO 527.
• Bending Strength Tests: A beam-shaped sample is placed on two supports and loaded in the middle to determine bending strength and modulus according to ISO 178.
• Impact Strength Tests: Measure the material’s ability to absorb energy from sudden impacts. A sample is struck with a pendulum hammer, and the absorbed energy is measured.

Thermal Tests

• Heat Deflection Temperature: A sample is heated under a defined load, and the temperature at which a certain deformation occurs is measured to determine the material’s operating limits in high-temperature applications.
• Thermogravimetric Analysis (TGA): A sample’s weight is continuously measured while it is heated in a controlled environment to determine thermal stability, outgassing of components, and decomposition temperatures.

Chemical Tests

• Chemical Resistance: Materials are exposed to various chemicals and environmental conditions to verify their long-term durability and suitability for specific applications.

Quality Control and Certification

Ensuring the quality and conformity of materials according to DIN 7735 required extensive quality control and certification processes:

Quality Control Requirements

• Raw Material Testing: Before production, raw materials are tested for purity and specific properties to ensure only high-quality materials are used.
• In-Process Controls: Continuous quality checks during manufacturing identify and correct deviations in real-time.
• Final Inspection: Comprehensive tests are conducted after production to ensure the final product meets all specifications.

Certification Processes and Procedures

• Internal Certification: Manufacturers conduct their own certification processes, including regular audits and internal tests.
• External Certification: Independent bodies such as VDE, TÜV, or other accredited testing organizations conduct audits and tests to confirm compliance with the standard and issue certificates.
• Documentation and Traceability: All production processes and quality controls are documented in detail, and each batch receives a unique identification number to ensure complete traceability.

Benefits and Challenges of Compliance

Compliance with the DIN 7735 standard offers numerous benefits but also presents some challenges. The strict requirements of the standard ensure high product quality and defined properties, leading to more reliable and durable products. These high-quality materials provide improved reliability, especially in demanding applications such as electronics, automotive, and aerospace industries.

On the other hand, the challenges of complying with DIN 7735 should not be underestimated. High-performance reinforcement fibers and special resins required to meet the standard can be expensive, increasing production costs. Additionally, the manufacture and processing of these materials require specialized technologies and expertise, adding to the complexity of manufacturing processes. Finally, the regular and comprehensive testing necessary for quality assurance represents an additional effort but is essential to ensure compliance with the standard and maintain high product quality.

Application of DIN 7735 in the Example of HGW (Phenolic Resin-Based Hard Fabric)

The DIN 7735 standard played a significant role in defining requirements and testing procedures for various insulating materials, including HGW (phenolic resin-based hard fabric). HGW is a specific type of laminate made by pressing fabrics and phenolic resin under high pressure and temperature. These materials are particularly resistant and stable, making them ideal for demanding electrical applications.

• Material Classification: HGW consists of phenolic resin-bound fabric arranged in layers and pressed under pressure. DIN 7735 specified the requirements for these types of laminates to ensure they meet the high mechanical and electrical demands of electrical engineering. The standard defined the properties materials must have to be classified as HGW, including their mechanical strength and electrical insulating properties.
• Scope of Application: HGW is used in many electrical applications where high mechanical and electrical insulation properties are required. Typical applications include transformers, motors, and low-voltage switchgear. DIN 7735 specified how HGW materials must be tested and certified for these applications to ensure their suitability and reliability.
• Technical Specifications: The technical specifications for HGW, as defined by DIN 7735, encompass a variety of mechanical and electrical properties. These include electrical breakdown strength, resistance to high temperatures and chemicals, and mechanical strength. The standard specified exact test methods and limits for these properties to ensure HGW materials meet the requirements.

Future and Development

With the introduction of new standards such as DIN EN 60893, requirements and testing procedures were updated to meet technological advances. These new standards cover similar requirements as DIN 7735 but offer improved specifications and applications better suited to modern industrial needs. Additionally, national European standards have been harmonized and incorporated into EN 60893.

Conclusion

For many years, DIN 7735 set the standards for laminated thermoset insulating materials in electrical engineering. Although it has now been withdrawn, it remains an important reference for the industry, providing valuable guidelines for quality assurance. The continuous development of standards ensures the industry meets high demands and finds innovative solutions for future challenges.