By performing an ungrounded-specimen test (UST), what is eliminated from the power factor measurement?

In the context of power factor measurements, an ungrounded-specimen test (UST) is employed specifically to minimize the influence of grounding effects on the measurements being taken. By isolating the specimen from ground references during the test, the grounding effects that typically introduce errors in the measurements can be eliminated.

Grounding can create variations in the potential of the system being tested, which can affect the accuracy of power factor measurements. When a test specimen is grounded, it may allow for stray currents or noise to interfere with the accurate reading of the power factor. The ungrounded condition ensures that these external influences do not cause distortions in the measurement, leading to more accurate results.

While voltage fluctuations and external noise can affect measurements, they are not the primary concerns addressed by UST. The focus of UST is primarily to handle and eliminate grounding effects, allowing for a clear view of the winding effects—such as resistive and reactive components—individual to the specimen itself.