Selection of CT

1)    Indoors or Out Door:

• Determine where CT needs to be used. 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 judgment when selecting current transformers.

2)    What do We need:

• The first thing we need to know that what degree of accuracy is required. Example, if you simply want to know if a motor is lightly or overloaded, a panel meter with 2 to 3% accuracy will likely suit for needs. In that case the current transformer needs to be only 0.6 to 1.2% accurate. On the other hand, if we are going to drive a switchboard type instrument with 1% accuracy, we will want a current transformer with 0.3 to 0.6 accuracy. We 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. We 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 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 we are going to drive a relay you must know what relay accuracy the relay will require.

3)    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.

4)    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.

5)    Application:

• The variety of applications of current transformers seems 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

6)    Safety:

• For personnel and equipment safety and measurement accuracy, current measurements on conductors at high voltage should be made only with a conducting shield cylinder placed inside the CT aperture. There should be a low electrical impedance connection from one end only to a reliable local ground. An inner insulating cylinder of adequate voltage isolation should be between the shield cylinder and the conductor at high voltage. Any leakage, induced or breakdown current between the high voltage conductor and the ground shield will substantially pass to local ground rather than through the signal cable to signal ground. Do not create a “current loop” by connecting the shield cylinder to ground from both ends. Current flowing in this loop will also be measured by the CT.

7)     CT output signal termination:

• The CT output coaxial cable should preferably be terminated in 50 ohms. CT characteristics are guaranteed only when CT is terminated in 50 ohms.  The termination should present sufficient power dissipation capability.  When CT output is terminated in 50 ohms, its sensitivity is half that when terminated in a high-impedance load.