Instrument Transformer High-Accuracy Acceptance Test

What it is

This form records the acceptance test of one metering-class current or voltage transformer whose declared accuracy class is proved by ratio correction factor and phase angle readings against a reference measuring system, graded to IEEE C57.13-2016. A graded class needs the direct comparison method - a reference transformer or a transformer test set at rated frequency - with a measuring system at least 4 times better than the class; a turns ratio check, an in-service clamp-on check or the indirect secondary excitation result of a field analyzer confirms the ratio but does not prove the class, so those readings are recorded against the manufacturer certificate, not graded. For a current transformer, readings are taken at the 100 percent current point and at the low point the class defines (5 or 10 percent by class), and at the rating factor current when the factor is above 1.0. For a voltage transformer, readings between 90 and 110 percent of rated voltage are graded on one band, with no required point set. At each reading the form computes the transformer correction factor from the ratio correction factor and the phase angle, and the class band for the class and test point. One transformer per record; relaying cores on the same unit can be recorded in optional excitation and winding resistance sections, while a full relaying current transformer test belongs on the current transformer form.

Why it is performed

Metering transformers are the interface between the primary power system and the energy meter they serve; if a transformer's ratio or phase angle drifts outside its accuracy class, the meter registers a quantity that does not match what is flowing. An inaccurate metering transformer causes under-billing or over-billing on every reading until it is found. Confirming that the accuracy class is genuine before the transformer goes into service - rather than trusting the nameplate or a factory certificate alone - prevents metering disputes and gives the owner a traceable record of the ratio correction factor and phase angle at each test point. The form also compares the nameplate ratio with the ratio on the utility record and the ratio programmed in the meter, because a transformer that is accurate but programmed wrongly mismeasures all the same, and it records the reference system's traceability so the readings can be defended.

When it is performed

The test is performed at the acceptance stage, before a new metering transformer is first put into service, in a shop or at the installation site; ANSI C12.1-2026 expects each new transformer to be tested for withstand, ratio correction factor and phase angle before it is installed, and the record type (pre-installation, commissioning, as found or as left) is selected on the form. The test is repeated when the transformer is moved, reconditioned or suspected of having drifted, and at an installation inspection the connections and condition are checked and the ratios are compared. The metering code exempts instrument transformers from its in-service performance tests, so a re-test interval comes from the owner metering program or the manufacturer; the basis, the next due date and the program document are recorded. Readings are taken at the declared class burden or at the maximum rated burden, because a class stated at the maximum burden covers the lower standard burdens; classes 0.15 and 0.15S use up to four points, the 100 and 5 percent points each at the E0.04 burden and at the maximum rated burden.

How it is typically performed

Safety and preparation come first: the secondary is shorted or its loop completed before the primary is energized, the secondary ground is kept while the primary is energized, the burden is isolated and the neutral ground lifted for insulation tests, and the core is demagnetized before the accuracy readings and after any dc test, because dc polarity, continuity and resistance checks leave remanence in the core. A visual and mechanical inspection confirms the nameplate against the metering design, the H1 and X1 polarity marks, the secondary tap connection, the shorting and grounding provisions and the condition of the enclosure, insulation and terminals. Insulation resistance is read with connection sets A and B, or C, D and E, and polarity is checked by one of the methods of IEEE C57.13.1-2017 (dc kick, ac oscilloscope, current comparison, phase angle meter, comparison with a known transformer or a transformer test set). The reference measuring system is identified with its traceability and calibration dates, and its ratio and phase angle uncertainties are entered; the form computes how many times better than the class the system is on each axis. The accuracy test method and test frequency are then selected, and up to six readings are entered, each with its test point, the burden at test, the applied quantity in percent of rated, the ratio correction factor and the phase angle in minutes of arc, positive when the secondary leads the primary. For each reading the form computes the transformer correction factor at 0.6 power factor lagging and the class band minimum and maximum. The connected burden is measured at the shorting block with current near rated flowing and compared with the rated burden. The metering installation inspection records the pre-installation test evidence and compares the nameplate ratio with the utility record and the ratio programmed in the meter, with the register constant and any transformer-loss compensation. Optional sections take the excitation knee, secondary winding resistance and admittance of a relaying core on the same unit, in-service circuit checks, and the associated meter and field standard.

What gets recorded

Identification: transformer type, declared metering accuracy class and accuracy rating as marked, nameplate data (manufacturer, model, serial, ratings, marked ratio, tap in use, continuous thermal rating factor, BIL, system voltage, frequency, standard marked, any relaying rating), designation, location and metering point. Checklists for safety and test preparation (including that readings were taken at the declared class burden or the maximum rated burden), visual and mechanical inspection, and the metering installation inspection. Insulation resistance: test voltage, connection set, devices left connected and each set reading. Polarity: method, result and demagnetization after a dc test. Reference system: equipment, traceability, calibration dates, ratio and phase angle uncertainties and the computed margins over the class. Ratio and phase angle: test method, test frequency and, per reading, test point, burden, applied quantity, ratio correction factor, phase angle in minutes of arc, the computed transformer correction factor and class band. Connected burden: designation, rated and measured burden, margin and method. Ratio records: pre-installation report, the ratios on the utility record and in the meter, whether they agree, register constant and loss compensation. Optional sections for a relaying core (excitation, winding resistance, admittance), in-service checks and the associated meter and field standard. Record of test: tester, date, record type, re-test basis and due date, program document, ambient conditions, test equipment, comments and deficiencies.

How results are evaluated

The record passes only when, at every graded reading, both the ratio correction factor and the transformer correction factor lie inside the IEEE C57.13-2016 band for the class and test point, ends included. For a current transformer the class is proved only with a passing reading at the 100 percent point and at the class low point, and at the rating factor current when the factor is above 1.0; a missing point leaves the record indeterminate with a reason naming the point, while a failed reading fails the record. A voltage transformer is graded on one band from 90 to 110 percent of rated voltage. Grading has prerequisites: the method must be a direct comparison against a reference transformer or a transformer test set at rated frequency, and the measuring system must be at least 4 times better than the class on both the ratio and the phase axis, compared on the unrounded figure; otherwise the class is recorded, not graded. The rating factor must be entered (1.0 when none), or a passing record stays indeterminate, and an applied quantity that does not agree with its selected test point leaves that row ungraded. Polarity must be as marked; reversed fails. Nameplate, utility record and meter ratios that do not agree fail the record, an UltraDb criterion since the metering code recommends the comparison without a tolerance. Other readings raise issues for the reviewer without setting the verdict: an insulation resistance below 1 megohm, a connected burden above the rated burden, a field standard error outside 0.05 percent, a relaying core whose voltage at 10 A exciting current falls short of its class voltage, or a winding resistance corrected to 75 C above the stated maximum. The phase angle reaches the grade only through the transformer correction factor, and the burden at each reading is recorded, not checked against the burden the class is declared at; the checklist item that the readings were taken at the declared or maximum rated burden puts a repair issue on the report when unsatisfactory.