IEC 61251 pdf download.Electrical insulating materials and systems – AC voltage endurance evaluation
1 Scope
This International Standard describes many of the factors involved in voltage endurance tests on electrical insulating materials and systems. It describes the voltage endurance graph, lists test methods illustrating their limitations and gives guidance for evaluating the sinusoidal a.c. voltage endurance of insulating materials and systems from the results of the tests. This International Standard is applicable over the voltage frequency range 20 Hz to 1 000 Hz. The general principles can also be applicable to other voltage shapes, including impulse voltages. The terminology to be used in voltage endurance is defined and explained.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. IEC 62539, Guide for the statistical analysis of electrical insulation dielectric breakdown data
3 Terms, definitions and symbols
3.1 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1.1 voltage endurance VE measures of the capability of a solid insulating material to endure voltage Note 1 to entry: In this International Standard, only a.c. voltage is considered. Note 2 to entry: This note only applies to the French language. 3.1.2 life time to dielectric breakdown 3.1.3 voltage endurance coefficient VEC numerical value of the reciprocal of the slope of a straight line log-log VE plot Note 1 to entry: This note only applies to the French language. 3.1.4 specimen representative test object for assessing the value of one or more physical properties
4 Voltage endurance
4.1 Voltage endurance testing To evaluate the voltage endurance of insulating materials or systems, a number of specimens are subjected to a.c. voltage and their times to dielectric breakdown are measured. In practice, several samples of many specimens are tested at different voltages to reveal the effect of the applied voltage on the time to dielectric breakdown. The arithmetic mean time to dielectric breakdown of each sample is the average time to dielectric breakdown of all specimens tested at that voltage. The time at which a certain percentage of specimens break down is the estimated time to dielectric breakdown with a probability equal to this percentage. The statistical treatment of the data (either by analytical or graphical methods) allows the extraction of additional data such as other failure percentiles or confidence bounds and, possibly, determination of the distribution (Gaussian, Weibull, lognormal, etc.). 4.2 Electrical stress In general, reference to electrical stress (voltage per unit thickness) instead of voltage is required. For a uniform field, electrical stress is given by the voltage (effective value) divided by the thickness of specimens. If the electric field is not uniform, the maximum value shall be considered by the relevant equipment committees. 4.3 Voltage endurance (VE) graph The VE graph represents the time to dielectric breakdown (life) versus the corresponding value of electrical stress. In the VE graph, the electrical stress is plotted as the ordinate with either a linear or logarithmic scale. The times to dielectric breakdown are plotted on the abscissa with a logarithmic scale. The voltage endurance line on this graph gives the final result of the VE tests as it allows clear and complete evaluation of voltage endurance of the specimens under the specified test conditions. For maximum significance, materials or systems shall be compared at equal thickness and using the same type of electrodes, temperature, humidity and ambient gas, or as agreed by the relevant equipment committees. An accurate plotting of the line requires more than three tests at different voltages and one or more tests are required at voltages which result in times to failure longer than 1 000 h. In any case, a minimum number of three tests is required to draw the VE graph. The voltage endurance line is straight or curved. In the latter case, its trend can often be approximated by a few straight regions: sometimes a first part for short times with a low slope, a middle region (which can extend to long times) with a steeper slope and finally a further trend of the line showing a tendency to become horizontal (see Figure 1 , where a general VE line is shown). It is likely that the shape of the VE graph changes significantly from one material or system to another. With a curve as shown in Figure 1 , the VEC is not constant, and the VEC will be different at different times (see n d in Figure 2).
IEC 61251 pdf download
