CT Metering

Good question phillip,

and if you followed Wareagle's link to the wiki folks, the first sentence of explanation states that a "CT is like any other transformer . . ", and then gives how it's special design features result in tracking the current values of the primary.

As you are aware, voltage pushes current in a circuit. This is true of the secondary of a CT also, and that voltage comes from the turns ratio with the primary.

If you measure the voltage on the open secondary of a voltage transformer you can do so, and you will see the turns ratio voltage.

If you measure the voltage in the open secondary of a CT, with a turns ratio of 1:2000 or so, you will find that by Ohm's law, when resistance approaches zero, voltage approaches infinity. Some very bad accidents have occurred this way. However, if as recommended (and designed) the secondary has resistance characteristics built into the wire size, core size and material, and the circuit is kept in short-circuit when the primary is energized, then you can measure a very small voltage of around 6 volts in effect.

That voltage measurement will vary due to the design features mentioned in such a way that the secondary current will vary in proportion to the primary current.

(I don't recommend this be done without expert supervision since opening a CT secondary under load can produce very high voltage) Of course, there are several sites that go into greater detail as to the special design features that produce the accuracy classes etc, so this is just a general explanation.

In short, a CT is a specialized version of a voltage transformer.

Regards, CJM