Power factor testing of high-voltage SF6 circuit breakers

Author: Stephen Bosak




Should I perform power factor testing on my high-voltage SF6 gas circuit breaker?




Siemens does not require or advocate power factor testing of SF6 high-voltage circuit breakers, therefore you will not find it in your instruction manual.


Power factor testing is used for checking the capacitance level and any current or dielectric losses, designed principally around such devices that use organic insulating materials. Secondly, the test voltage level is insufficient and does not fully prove or disprove the insulation integrity of a SF6 gas circuit breaker. At the time of production, SF6 gas circuit breakers are subject to a high-voltage withstand test. This incorporates an AC High-Potential (Hi-Pot) test transformer capable of achieving the required high-voltage levels (per industry standards). Therefore, field Hi-Pot testing is also not necessary or required.


The insulating material used in the manufacture of modern SF6 gas circuit breakers is synthetic, which have no capacitor characteristics and have no ability to absorb moisture. The main insulating materials are Synthetic filament wound epoxy insulating tubes, Hollow core insulators (porcelain or composite), and the SF6 gas itself. All SF6 breakers are sealed and incorporate a molecular sieve type desiccant to maintain moisture to extremely low levels.


Although power factor testing of a SF6 gas circuit breaker will not cause any harm or detrimental affects to the device, misleading results may be witnessed. Any values measured will be extremely low and many times are influenced by site specific conditions or test practices such as:

  • Contamination on the exterior of the entrance bushings

  • Energized over head lines or equipment adjacent to the breaker

  • Placement of the test leads and cables.

Since Siemens does not perform power factor testing during manufacturing, there is no data to substantiate or differentiate any values measured in the field - as to whether they are acceptable, questionable, or indifferent. Any pertinent questions would need to be deferred to the manufacture of such test equipment. Keep under advisement “Questionable” or “Bad” test results may not be a true indicator that the insulation or components are/or may be suspect, nor should that data alone warrant the need for their replacement or inspection.