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Doble Test: Another name is Insulation Power Factor Test or Dissipation Factor Test.

Since Doble Test refers to a Power Factor/Dissipation Factor test done on mostly transformers, but also motors and MV cables, and Doble makes the most commonly used test equipment for these tests; hence the name Doble test. [Just like the Megger Test, which is really an Insulation Resistance test, but Megger is the most common brand name equipment, hence the name association and common usage.] Doble equipment puts a 2.5 kV or 10 kV AC signal on the transformer or other test specimen, then carefully measures signals proportional to the watts and VA. From that you can calculate, or read directly on the fancier instruments, the power factor of the current flowing into the insulation. A perfect insulation only has capacitive current flowing, power factor zero. Wet, dirty or degraded insulation will have resistive leakage current, raising the power factor. A power factor of 1% or lower is good, but like megger readings, a lot is relative.

A true Doble test sequence makes several measurements, similar to the megger readings on a transformer: Hi & Lo to Ground, Lo to Hi & Ground, Hi to Lo & Ground plus others with guard inputs. A lot of care is needed to get accurate readings and to accurately adjust for temperature and other factors. Doble supplies a data base of readings for all the transformers tested with their equipment. This gives the customer something to compare his readings too on a unit of a similar size, type and manufacture.