IEC TS 61251:2008

FOREWORD
1 Scope and object
2 Normative references
3 Terms and definitions
4 Voltage endurance
  4.1 Voltage endurance testing
  4.2 Electrical stress
  4.3 Voltage endurance (VE) graph
  4.4 Short-time electric strength
  4.5 Voltage endurance coefficient (VEC) - n
  4.6 Differential VEC (n[d])
  4.7 Electrical threshold stress (E[t])
  4.8 Voltage endurance relationship
5 Test methods
  5.1 Introductory remark
  5.2 Tests at constant stress
      5.2.1 Conventional VE test
      5.2.2 Diagnostic measurements
      5.2.3 Detection of an electrical threshold
  5.3 Tests at higher frequency
  5.4 Progressive stress tests
  5.5 Preliminary tests to determine the initial part
      of the VE line
  5.6 Suggested test method
6 Evaluation of voltage endurance
  6.1 Significance of the VEC
  6.2 Significance of the electrical threshold stress
  6.3 Dispersion of data and precision requirements
  6.4 Presentation of the results
Annex A (informative) The Weibull distribution
Bibliography

IEC/TS 61251:2008(E) explains 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 a.c. voltage endurance of insulating materials and systems from the results of the tests. The terminology to be used in voltage endurance is defined and explained. It should be emphasized that where this technical specification is concerned with materials, the results may not be directly applicable to the performance of insulating systems. Voltage endurance tests are used to compare and evaluate insulating materials with regard to their various applications in electrical systems. Determining the ability of electrical insulating materials and systems to endure a.c. voltage stress is complex. The results of voltage endurance tests are influenced by many factors so this technical specification should only be considered as an attempt to present a unified view of voltage endurance for simplified planning and analysis. Some documents for the various practical cases exist and others are being developed. The main changes with respect to the previous edition are listed below:
- extension of the scope to cover electrical insulating material and insulation systems;
- removal of references to short time dielectric breakdown strength measurements as an indicator of voltage endurance.