IEC 60544-5-2003 pdf – Electrical insulating materials – Determination of the effects of ionizing radiation – Part 5: Procedures for assessment of ageing in service.
1 Scope and object This part of IEC 60544 covers ageing assessment methods which can be applied to components based on polymeric materials (for example, cable insulation and jackets, elastomeric seals, polymeric coatings, gaiters) which are used in environments where they are exposed to radiation. The object of this part of IEC 60544 is to provide guidelines on the assessment of ageing in service. The approaches discussed cover ageing assessment programmes based on condition monitoring (CM), the use of equipment deposits in severe environments and sampling of real- time aged components. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 60544-1:1994, Electrical insulating materials – Determination of the effects of ionizing radiation – Part 1: Radiation interaction and dissymmetry IEC 60544-2:1991, Electrical insulating materials – Determination of the effects of ionizing radiation – Part 2: Procedures for irradiation and test IEC/TR2 61244-1:1993, Determination of long-term ageing in polymers – Part 1: Techniques for monitoring diffusion-limited oxidation IEC/TR2 61244-2:1996, Determination of long-term ageing in polymers – Part 2: Procedures for predicting ageing at low dose rates IEC/TR2 61244-3:1998, Long-term radiation ageing in polymers – Part 3: Procedures for in- service monitoring of low-voltage cable materials
4 Background There are a number of alternative methods offered for ageing assessment as described in their respective subclauses. Each of these methods has its own advantages and limitations. Selection of the appropriate method will be dependent on the requirements of the individual users. There are a number of factors that need to be considered when assessing ageing of polymeric components in radiation environments. In the following subclauses, some of these factors are briefly discussed and reference made to more detailed information. To accelerate radiation- ageing environments, the normal approach is to increase the radiation dose rate, often combined with an increase in temperature. The two most important potential complications arising from such increases involve diffusion limited oxidation (DLO), which is described in 4.1, and chemical dose-rate effects (DRE), which are described in 4.2. The implications of these factors on the use and interpretation of CM techniques are also discussed. Accelerated ageing programmes briefly discussed in 4.3 and 4.4 introduce the approaches available for ageing assessment in-service. 4.1 Diffusion limited oxidation (DLO) When polymers are exposed to an oxygen-containing environment (for example, air), a certain amount of oxygen will be dissolved in the material. In the absence of oxygen-consuming reactions (oxidation), the amount of dissolved oxygen will be proportional to the oxygen partial pressure surrounding the polymer (well known from Henry’s law). Ageing will lead to oxidation reactions in the polymer, whose rate will increase significantly as the dose rate and temperature of ageing are increased. If the rate of consumption of dissolved oxygen in the polymer is faster than the rate at which oxygen can be replenished by diffusion from the surrounding air atmosphere, the concentration of dissolved oxygen in the interior regions will decrease with time (the oxygen concentration at the sample surface will remain at its equilibrium value).
The importance of this effect is dependent on the sample thickness (thinner samples giving smaller DLO effects) and the ratio of the oxygen consumption rate to the oxygen permeability coefficient P, which equals the product of the oxygen diffusion and solubility parameters. Accelerated radiation environments involve increases in dose rates, which increase the oxygen consumption rate. If the temperature remains constant as the dose rate is increased, the oxygen permeability coefficient will be unchanged. This means that DLO effects will become more important as the dose rate is raised. These effects are described in more detail in IEC 61244-1. The effects of diffusion limited oxidation also need to be considered when carrying out condition monitoring tests. This is not an issue for the many CM techniques which measure properties at room temperature, such as those based on density and modulus measurements. On the other hand, several CM techniques such as oxidation induction time (OIT) and thermogravimetric analysis (TGA) use quite elevated temperatures during the measurements. For these techniques, it is quite possible to have DLO effects present during measurement of the CM parameter. DLO also needs to be addressed when developing correlation curves for condition monitoring methods, to ensure that representative data are obtained for both radiation and thermal ageing. 4.2 Dose-rate effects (DRE) The existence of radiation dose-rate effects and methods for dealing with these effects are described in IEC 61244-2. Generally, DRE are separated into two types. The first type, which is commonly observed in accelerated radiation ageing experiments, is due to the DLO effects described above.