Accuracy Classes of a Metering CT: 0.1 to 5 (IEC 61869-2)

Short answer

The accuracy class of a metering current transformer (CT) says how much it can err when reflecting the primary current on the secondary. IEC 61869-2 defines it as the largest percentage ratio error permitted at rated primary current and with rated output. A class 0.5 CT does not exceed 0.5 % error at that current. At lower currents, the standard allows higher errors.

What is measured: ratio error and phase displacement

The ratio error is the difference, in percent, between the actual primary current and the one the rated ratio would indicate from the measured secondary current. The phase displacement is the angle between the primary and secondary currents, expressed in minutes or in centiradians. The first affects the magnitude measured and the second affects power and energy, because these depend on the angle.

For classes 0.1, 0.2, 0.5 and 1, the standard limits both. For classes 3 and 5, only the ratio error.

Standard classes

Clause 5.6.201.2 lists the metering classes: 0.1 - 0.2 - 0.2S - 0.5 - 0.5S - 1 - 3 - 5. The classes 0.2S and 0.5S have limits of their own in Table 202 and start at 1 % of rated current. This guide focuses on classes 0.1 to 5.

Limits of classes 0.1 to 1

Table 201 sets the ratio error and phase displacement limits at four current values, as a percentage of rated current. They are met at rated frequency, with a burden that can take any value between 25 % and 100 % of the rated output.

Ratio error, in ± %

Phase displacement, in ± minutes

The standard gives the same phase displacement limits in centiradians. For example, in class 0.5 at 100 % of current: ±0.5 % ratio error and ±30 minutes of phase displacement, equivalent to ±0.9 centiradians.

Limits of classes 3 and 5

Table 203 defines classes 3 and 5 with the ratio error at 50 % and 120 % of rated current, with a burden between 50 % and 100 % of the rated output. Class 3 allows ±3 % and class 5, ±5 %. There are no phase displacement limits.

Burden and power factor

For all classes, the burden must have a power factor of 0.8 lagging, with one exception (5.6.201.3). When the burden is below 5 VA, the power factor is 1.0, with a minimum value of 1 VA.

In addition, the limits are valid for any position of an external conductor located at an air distance not less than that required for the insulation at the highest voltage for equipment.

Class extensions

• Extended burden range (5.6.201.4). For any metering class, a burden range from 1 VA up to the rated output can be specified, at power factor 1.0. The maximum rated output in that case is 15 VA. It is written, for example, “1-10 VA class 0.2”.
• Extended primary current (5.6.201.5). CTs of classes 0.1 to 1 can be marked with an extended current if their continuous thermal current is that current and the limits that Table 201 gives for 120 % are maintained up to it. It is written, for example, “15 VA cl. 0.5 ext.150 %”.
• Instrument security factor, FS (5.6.201.6). It is optional. The standard values are FS 5 and FS 10. It is the ratio between the instrument limiting primary current and the rated primary current. According to the standard, the safety of the connected instruments is greater the lower the FS.

How it is tested

Clause 7.2.6.201 requires, as a type test, measuring the accuracy at each current value in Tables 201, 202 or 203, with the highest and the lowest value of the specified burden range. A CT with extended current is tested at that current instead of 120 %.

In the routine test (7.3.5.201), accuracy is in principle checked as in the type test. The standard allows it to be done with fewer currents or fewer burdens if a type test on a similar transformer showed that the reduced number is sufficient.

What to ask for and what to review

• The class, rated output and burden range of each metering core.
• That the report shows the errors at each current in the table for your class, with minimum and maximum burden.
• That the actual secondary burden, cable included, falls within the range of the class.
• The class your meter or metering system requires.

What not to do

• Do not use a protection core as if it were a metering core: the metering class holds only at the currents and burdens in its table.
• Do not expect class 0.5 to give 0.5 % at 5 % of the current: there the limit is 1.5 %.
• Do not connect a burden below 25 % of the rated one on classes 0.1 to 1 if the CT does not have an extended burden range.
• Do not compare the class of a metering CT with that of a protection CT: they are defined differently.

Related topics

For more accurate measurement at low currents, see the classes 0.2S and 0.5S. The security factor protects the instrument during a fault, and the terminal marking identifies the terminals.

When to call us

At Tiza Group we verify the ratio, polarity, actual burden and error of a metering current transformer in the field, and we deliver the report with traceability for your audits. Learn about our service of testing and commissioning of current and voltage transformers, available in Colombia, or elsewhere on request. If you need to define classes and burdens in a project, we do it in our engineering design and studies service, which we provide worldwide, remotely. Write to us and we will send you a quote.

Related guides

• Classes 0.2S and 0.5S and extended range of a CT
• Terminal marking and rating plate of a current transformer
• Security factor FS of a metering CT
• Current transformer (CT): parts, functions and tests
• Medium-voltage current transformer (CT): classes

References

• IEC 61869-2, Instrument transformers - Part 2: Additional requirements for current transformers, 2012 edition, clauses 5.5.201, 5.6.201.1 to 5.6.201.6, 7.2.6.201 and 7.3.5.201, with Tables 201, 202 and 203.

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