Window-type current transformers are also common, which can have circuit cables run through the middle of an opening in the core to provide a single-turn primary winding. Technical standards and design practices are used to ensure the safety of installations using current transformers. For certain current transformers, this voltage may reach several kilovolts and may cause arcing. This type of CT is used for revenue meters and in energy meters. The C57.13 standard has different accuracy classes: Class 1.2, Class 0.6, and Class 0.3. Since the major insulation of a live-tank current transformer is not exposed to the heat of the primary conductors, insulation life and thermal stability is improved.[1]. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices. Let us now have a look at errors specified for these accuracy classes: The limiting error shall not exceed the value tabulated below at rated frequency and burden from 25% to 100% of the rated burden. The accuracy class of protection class and metering class CTs are defined by Composite Error and Current Error & Phase Angle Error respectively. The secondary of a current transformer should not be disconnected from its burden while current is in the primary, as the secondary will attempt to continue driving current into an effective infinite impedance up to its insulation break-down voltage and thus compromise operator safety. The primary circuit is largely unaffected by the insertion of the CT. A current transformer (CT) is a type of transformer that is used to reduce or multiply an alternating current (AC). a heavy copper bar to carry current through the core. |, LonWorks Plug-Ins, Network Software, and XIF Files. That is something you must know. The knee-point voltage is less applicable for metering current transformers as their accuracy is generally much higher but constrained within a very small range of the current transformer rating, typically 1.2 to 1.5 times rated current.

All rights reserved. Where a metal shield is present in the cable jacket, it must be terminated so no net sheath current passes through the ring, to ensure accuracy.

Also meets or exceeds IEC 60044-1 class 0.5S. Current transformer shapes and sizes vary depending on the end user or switch gear manufacturer. The current error (or ratio error), phase displacement error and composite error, at rated frequency and rated burden, shall not exceed the values tabulated below: On the name plate of Current Transformer, rated accuracy limit factor is indicated following the corresponding output and accuracy class. Each class has an allowable maximum phase error for a specified load impedance. However, the concept of knee point voltage is very pertinent to protection current transformers, since they are necessarily exposed to fault currents of 20 to 30 times rated current.

However, in wattmeters, energy meters, and power factor meters, phase shift produces errors.

[1], The knee-point voltage of a current transformer is the magnitude of the secondary voltage above which the output current ceases to linearly follow the input current within declared accuracy. CTs are specified by their current ratio from primary to secondary.

It produces a current in its secondary which is proportional to the current in its primary. Current transformers typically consist of a silicon steel ring core wound with many turns of copper wire as shown in the illustration to the right.


Dead tank current transformers isolate the measured circuit from the enclosure. The ratio (primary to secondary current) error of a Class 1 CT is 1% at rated current; the ratio error of a Class 0.5 CT is 0.5% or less. For power and energy measurement, the errors are considered to be negligible at unity power factor but become more significant as the power factor approaches zero. A current transformer (CT) is an instrument transformer in which the secondary current is substantially proportional to primary current and differs in phase from it by ideally zero degree.. CT Accuracy Class or Current Transformer Class. In the first case the secondary tries to produce an infinite voltage and in the second case the secondary tries to produce an infinite current. – Construction and Working Principle, Binary Coded Decimal or BCD Number Explained, What is UPS? A current transformer provides a secondary current that is accurately proportional to the current flowing in its primary. High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. Split-core current transformers either have a two-part core or a core with a removable section. Instrument transformers scale the large values of voltage or current to small, standardized values that are easy to handle for measuring instruments and protective relays. The “Opt C0.6” grade CT also meets or exceeds our more stringent limits, which exceed the class 0.5 and 0.5 S requirements.

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