1.“INDOOR OR OUTDOOR”
Determine if the transformer is going to be subjected to the elements or not. Indoor transformers are usually less costly than outdoor transformers. Obviously, if the current transformer is going to be enclosed in an outdoor enclosure, it need not be rated for outdoor use. This is a common costly error in judgement when selecting current transformers.

2.“WHAT DO YOU WANT TO DO”
If you want an indication, the first thing you need to know is what degree of accuracy is required. For example, if you simply want to know if a motor is lightly or overloaded, a panel meter with a 2 to 3% accuracy will likely suit your needs. In that case the current transformer need to be only 0.6 to 1.2% accurate. On the other hand, if you are going to drive a switchboard type instrument with a 1% accuracy, you will want a current transformer with a 0.3 to 0.6 accuracy. You must keep in mind that the accuracy ratings are based on rated primary current flowing and per ANSI standards may be doubled (0.3 becomes 0.6%) when 10% primary current flows. As mentioned earlier, the rated accuracies are at stated burdens. You must take into consideration not only the burden of the load (instrument) but you must consider the total burden. The total burden includes the burden of the current transformers secondary winding, the burden of the leads connecting the secondary to the load, and of course, the burden of the load itself. The current transformer must be able to support the total burden and to provide the accuracy required at that burden.

If you are going to drive a relay you must know what relay accuracy the relay will require.

“VOLTAGE CLASS”
You must know what the voltage is in the circuit to be monitored. This will determine what the voltage class of the current transformer must be as explained earlier.

“PRIMARY CONDUCTOR”
If you have selected a current transformer with a window you must know the number, type and size of the primary conductor(s) in order to select a window size which will accommodate the primary conductors.

3.Applications of Current Transformers

The variety of applications of current transformers seem to be limited only by ones imagination. As new electronic equipment evolves and plays a greater role in the generation, control and application of electrical energy, new demands will be placed upon current transformer manufacturers and designers to provide new products to meet these needs.