A current transformer (CT) is an instrument type transformer that takes a large current and steps it down to a very low value, providing a simple and safe method of monitoring a circuit without disconnecting the line. Measurements are usually made using standard digital or analog meters. Current transformers can be single-phase or multi-phase designs. Current transformers have a variety of applications - from utility power management to precision measurements in the medical, automotive, avionics, telecommunications industries, and military fields.
Current transformers come in three basic configurations:
Toroidal core: Measures currents from 50 to 5,000 amperes, with a core opening diameter of 1 to 8 inches. This type does not contain a primary winding. However, the line carrying the current passes through the center hole of the transformer.
Split core: Measures currents from 100 to 5,000 amperes, with a core opening diameter of 1 to 13 inches. One end of the split core is removable, so the current transformer can be installed without disconnecting the load conductors.
Winding Primary: Measures current from 1 amp to 100 amps as the load current flows through the primary winding in the CT.
Current transformers are the same as power transformers, except that the primary is in series with the conductor carrying the high-current AC current. The primary of this type of transformer has only a very small number of turns. The primary can be a single turn of heavy-duty wire wrapped around a magnetic core. The secondary of a current transformer is usually proportional to the primary. The secondary can consist of a large number of turns wrapped around a low-loss magnetic core, depending on how much current is being reduced, and is usually rated for 1 to 5 amps.
Current transformers can reduce current levels from thousands of amps to a known ratio. The primary and secondary currents are expressed in a ratio of 100:5. This means that if 100 amps are flowing through the primary conductor, the secondary conductor will indicate (flow) 5 amps. Or, if the rating is 500:1, 500 amps flow in the primary conductor and 1 amp flows in the secondary conductor.
