Current transformers

The current through an AC wire can be monitored using a Current Transformer (CT). There are several types of CT’s to choose from.


Solid Core vs Split Core

Split core transformers can be opened. That way they can be clamped around the monitored wire without interfering with the circuit – sometimes even without switching the power off. While convenient, split core CT’s usually costs a little more than solid core CT’s, and sometimes suffer from lower accuracy.


Output types

The simplest form of CT outputs a current that is relative to the current flowing through the eye of the CT. An unconnected CT output may carry a dangerous voltage, so safety may be an issue with this type of CT. The most common type is rated 5A at full scale, so for example a 5/400A CT would output 5A when 400A flows through the eye, and 2.5A when 200A flows through it.

Some CT’s have a calibrated resistor built in that makes the output from the CT a voltage rather than a current. These CT’s are safe to work with even when they are not connected to the meter. A common rating is 1/3V (0.333V) at full scale.

Some CT’s have a DC output. This type of CT cannot be used with an energy meter, but may be convenient if only a current reading is required, for example to monitor run time. Note that monitoring only current will not be satisfactory to calculate energy/power in an AC circuit. Many of the “home energy monitors” only monitor current, and thus will not be very accurate.


Rogowski coil (a.k.a Rope CT)

Rogowski Coils are very convenient for monitoring large diameter, or oddly shaped conductors, and since they are flexible they offer many mounting options. Rogowski coils are suitable for very high loads (1000A ++), but are usually less accurate than regular CT’s at lower currents. They require a separately powered integrator circuit to work with most meters.


Selecting the right CT

These questions must be answered in order to select an appropriate CT:

  • What is the diameter of the cables? How large must the eye of the CT be?

  • Are there any space constraints where the CT’s are to be mounted?

  • What is the max current that will flow through the circuit? If this can’t be determined, what is the fuse rating on the measured circuit?

  • What types of CTs does the meter support (5A/0.333V/other?)

  • What accuracy is required for the application?


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