b'Partner News Spring 2024 19Enterpriseinstrumentation. Other common types of CTs are Wound type, Bar-type, and Bushing-type transformers, each of which have their own specific uses.Understanding CT terminology is essential when selecting and specifying the appropriate CT for the end application, as various performance factors must be considered. CT Ratio: This rating of the CT is the ratio of the primary current to the secondary current given at full-load. For example, if a CT has a ratio of 500/5, when 500Amps primary current is present, the secondary winding will have 5 Amps flowing through it. This ratio is dictated by the number of windings turns on the primary side vs the secondary side and on some CTs this ratio can be adjusted to suit its purpose.Figure 3: Principles of a current transformerCT Accuracy: This is given as a percentage,Selection of current transformers can typically between 0.25% and representsbe simplified into two main categories: the accuracy of secondary current in relationmetering, and protection.to the actual primary current. For example, a CT with an accuracy rating of 0.5% isMetering CTs are used for the precise certified by the manufacturer to be accuratemeasurement of a range of electrical to within 0.5% of its rated ratio value for aparameters such as amperes, power, and primary current of 100% of the rated ratio.energy. These types of CTs are used for energy meters either for monitoring or CT Class Rating: This is indicatedbilling purposes. For this reason, these by a specific letter depending on thetypes of CTs are designed to have high characteristics of the CT. For example, theaccuracy over a narrow range of values for letter P indicates that a CT is Protectionwhich they are suited. Class while the letter B indicates aFigure 4: Solid-Core CT Metering Class CT. Protection CTs are engineered to measure substantial fault currents without saturating CT Burden: The burden relates to the totalthe CT. This allows protective devices resistance of the secondary circuit of the CTor control relays to activate during fault and is usually given in ohms (). If this limit isor abnormal conditions in the electrical exceeded, CT accuracy is not guaranteed.system. Manufacturers of protective devices often specify the classification of the CT CT Polarity: This is determined by therequired.direction in which the conductor coils are wound around the core of the CT as wellConclusions/ Recommendations as the way that secondary windings areAs with all electrical equipment, accurate connected to the transformer casing.selection is crucial to ensure the CT All CTs have power flow designations tooperates effectively in its intended role. ensure proper installation.While this article covers key considerations,Figure 5: Clamp-on CTthere are additional factors to bear in mind It is important to observe correct polaritywhen selecting CTs. Nonetheless, this brief when installing and connecting CTs forintroduction highlights the core information power metering, and protective relays.which must be considered when specifying Incorrect polarity on any single phase cancurrent transformers.lead to incorrect readings.CT Labelling is the quickest way of identifying the key characteristics discussed above. Typically, you would expect to see a label similar to the one adjacent in figure 6.Figure 6: Typical CT Label example'