Selecting Current Transformer Accuracy Class and Output

Current transformer accuracy class and rated secondary current are separate selection decisions. A requirement that the measuring instrument be Class 1 does not, by itself, assign Class 1 to the CT. Before anything else, identify whether the governing specification separately states a CT accuracy class, then match the CT output, instrument input, connected burden, and primary-current range.

Instrument and CT requirement check

1. Read the Class 1 clause and identify the device it names. If it names the measuring instrument, apply Class 1 to that instrument. Do not transfer the requirement to the CT without a separate CT clause, project specification, or accuracy calculation.
2. Check the CT nameplate or datasheet for its metering accuracy class. If no class is marked, stop the selection process and obtain the manufacturer’s rating data.
3. Check the instrument input specification. Record whether it accepts 1 A, 5 A, or another stated value.
4. Locate the applicable edition of VDE 0410 and verify its scope, definitions, and permitted CT characteristics. The cited requirement addresses Class 1 measuring instruments; the nominal CT secondary must be resolved from the CT and instrument specifications.

The deciding question is therefore not “Is every CT feeding a Class 1 instrument also Class 1?” It is “What CT class, ratio, burden, and secondary rating keep the complete measuring chain within the required performance?”

Secondary-current decision

Metering CTs can have either a 1 A or 5 A rated secondary. Accuracy Class 1 does not restrict the secondary to 1 A; a Class 1 metering CT may have a 5 A secondary when specified and manufactured that way. The instrument must have the corresponding current input.

The secondary rating also changes circuit burden. For the same apparent-power burden, secondary-circuit voltage follows V = VA / I, and allowable loop impedance follows Z = VA / I². Because cable loss follows I²R, a 5 A circuit develops 25 times the cable loss of a 1 A circuit through the same resistance. This makes lead resistance a more restrictive part of the burden calculation on long 5 A runs.

Accuracy-class interpretation

The class designation describes permitted transformation error under defined test conditions; it is not the nominal secondary current. For the Class 1 CT description in the supplied technical data, the limits from 100% through 120% of rated current are:

Use these values only with the applicable CT specification and its defined burden and test conditions. Read the governing class table for limits below rated current rather than extending the stated 100%–120% values to the entire measuring range.

Ratio error directly affects current indication. Phase displacement also affects measurements derived from the phase relationship between current and voltage, including power and energy. A higher numerical accuracy class cannot correct an incorrect CT ratio, excessive burden, reversed polarity, or a mismatched instrument input.

Ratio and burden checks

1. Record the maximum expected primary current and the CT ratio. Confirm that rated primary current covers the operating requirement without placing normal measurements in an unsuitable part of the CT range.
2. Calculate the instrument’s burden from its datasheet. Add the burdens of every series-connected device in that CT secondary.
3. Calculate cable resistance for the complete outgoing-and-returning secondary path. Convert it to burden at rated secondary current with VA = I²R.
4. Add instrument, cable, terminal, and other series burdens. Compare the result with the burden at which the CT accuracy class is specified.
5. Do not move on until the connected burden and secondary current fall within the CT manufacturer’s class-rating conditions.

Excessive burden requires the CT to develop more secondary voltage. Core flux and excitation increase, which raises ratio and phase errors and can drive the CT toward saturation. The meter may still display a plausible value while the measurement chain no longer meets its intended class. If burden is excessive, shorten or enlarge the conductors, reduce series-connected burdens, select 1 A equipment for a long run, or select a CT with a suitable rated burden.

Polarity and wiring checks

1. Isolate the circuit under the site’s electrical safety procedure. Treat a CT secondary as an energy source whenever primary current can flow.
2. Trace the CT polarity markings through the terminal blocks to the instrument. Reversed polarity changes the sign or phase relationship used by directional power and energy measurements.
3. Confirm that the CT secondary is not open-circuited while the primary is energized. Use the approved shorting and test arrangement before disconnecting an instrument.
4. Inspect terminal tightness and compare conductor resistance with the burden calculation. Unexpected resistance indicates loose joints, undersized conductors, excessive length, or an incorrect route.
5. Confirm that no unintended parallel path bypasses part of the secondary current around the instrument.

A current indication alone may not expose reversed polarity in a simple ammeter. Verify polarity with a measurement that preserves the intended current direction and phase relationship. An open CT secondary is not a valid diagnostic condition and can produce hazardous voltage.

Commissioning procedure and acceptance

1. Document the CT ratio, rated secondary current, accuracy class, rated burden, and polarity from the nameplate or datasheet.
2. Document the instrument’s Class 1 designation, nominal current input, input burden, scaling, and channel assignment.
3. Reject any 1 A/5 A mismatch before energization. Configure the instrument scaling to the installed CT ratio.
4. Calculate total secondary burden and compare it with the CT class-rating conditions. Record conductor length, resistance, and all series devices.
5. Energize under a controlled load and compare the instrument reading with a suitable reference measurement. Use more than one operating point when the required range extends below rated current.
6. For power or energy channels, confirm current polarity and phase association as well as current magnitude.
7. Record the applied primary current, expected secondary current, displayed value, reference value, and resulting deviation. Accept the installation only against the governing project and standard limits.

Frequently asked questions