Quantities, units and formulas

Capacitance is the property of two conductors separated by a dielectric that relates the electric charge stored on them to the voltage between them; for a fixed geometry, a larger capacitance stores more charge for the same voltage.

Power factor test sets report the dissipation factor or power factor of an insulation system together with its capacitance and dielectric loss at the test voltage, and correct the reported factor to a reference temperature so that readings taken at different specimen temperatures can be compared.

Frequency is the number of cycles of a periodic quantity completed in unit time; for an alternating voltage or current it is the reciprocal of the period of one cycle.

Impedance is the complex ratio of the voltage across a circuit or circuit element to the alternating current through it; its magnitude is the ratio of the voltage amplitude to the current amplitude, and its phase angle is the angle by which the voltage leads the current.

Resistance is read at the micro-ohm level by low-resistance ohmmeters, which pass a heavy direct current through the specimen and sense the voltage drop with separate potential leads; through the ohm and kilohm range by multimeters; and at the gigaohm and teraohm level by insulation resistance testers, which apply a direct test voltage and measure the small current that flows.

The turns ratio of a transformer is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding; it is dimensionless and is estimated in the field from the ratio of the primary voltage to the secondary voltage with the secondary winding open.