Current Transformer Acceptance Test

What it is

A current transformer acceptance test is a field protocol that proves a newly installed current transformer reproduces primary current faithfully in its secondary circuit, with the right polarity and the right ratio, before any relay or meter is trusted to act on it. It records the unit's identification (manufacturer, model and catalog number, serial number, construction type as bar, window, bushing, or wound, ratio, accuracy class, rated burden, basic impulse level, nominal system voltage, frequency, and equipment designation), then runs a visual and mechanical inspection and a set of electrical tests. The inspection covers nameplate data against the drawings, physical and mechanical condition, correct connection against the system requirements, clearance between primary and secondary wiring, cleanliness, bolted-connection tightness, positive contact on every grounding and shorting connection, lubrication, and a thermographic survey. The electrical work covers insulation resistance from the transformer and its secondary wiring to ground, a polarity test, ratio verification, an excitation test for relaying applications, a current-circuit burden measurement at the shorting terminal block, and, on bar-type units, primary-winding insulation resistance, a primary dielectric withstand test, and insulation power factor on the higher-voltage units, plus a check that the secondary circuit is grounded at exactly one point. NETA ATS-2025 section 7.10.1 lists this as the standard acceptance sequence.

Why it is performed

Every protective relay and every meter downstream of a current transformer sees only what that transformer tells it, so an error here is invisible and systematic. A reversed polarity makes a differential scheme see fault current where there is none, or miss it where there is. A ratio that does not match the drawings quietly biases every setting derived from it. An excitation curve that falls short of the manufacturer's means the transformer saturates during a fault, exactly when the relay needs a faithful signal. Excess burden in the secondary circuit produces the same saturation by a different route. And a secondary circuit with more than one ground, or with none, either circulates current through the ground path or leaves the circuit floating at a hazardous potential when the primary is energized, which is why the single-point grounding check is part of the electrical sequence rather than the inspection.

When it is performed

Acceptance: after installation and before the metering or protection circuits the transformer feeds are placed in service, as part of the standard acceptance sequence, and normally before or alongside the relay commissioning that depends on it. The same form carries a maintenance service type, since insulation resistance, ratio, and the grounding check are repeated on a periodic basis and after any work that opens the secondary circuit. Ratio and polarity are re-verified whenever a transformer is replaced or a tap changed, because those are the two errors most often introduced by a repair rather than by age.

How it is typically performed

With the circuit de-energized and isolated per site safety procedures and the secondary shorted where the procedure requires it, the identification data is recorded and the inspection is worked through, including the clearance between primary and secondary wiring and positive contact on every grounding and shorting connection. Insulation resistance is measured for one minute from the transformer and its secondary wiring to ground at the voltage the standard prescribes, with the manufacturer's recommendation followed instead where solid-state devices in the circuit cannot tolerate it. A polarity test is then performed on each phase and the result compared with the transformer markings. Ratio is verified by the voltage or the current method and the ratio error calculated per phase against the nameplate. For relaying applications an excitation test establishes the knee-point voltage per phase and the curve is compared with the manufacturer's. Burden is measured at the current transformer shorting terminal block and compared with the rated burden. On bar-type units the primary winding insulation resistance is measured with the secondary grounded, a dielectric withstand voltage is applied to the primary for the specified time, and on units at the higher voltage classes insulation power factor is measured per phase. Finally the secondary circuit is confirmed grounded at one point only, at the location the engineer specified. Ambient temperature, relative humidity, and the test equipment with its calibration due date are recorded with the results.

What gets recorded

The unit identification (manufacturer, model and catalog number, serial number, construction type, ratio, accuracy class, rated burden, basic impulse level, nominal system voltage, frequency, and equipment designation); the result of each visual and mechanical item, including the grounding and shorting connections and the thermographic survey; the insulation-resistance test voltage, duration, and the per-phase readings to ground; the per-phase polarity finding; the ratio method, the nameplate ratio, the measured ratio, and the ratio error per phase; the knee-point voltage per phase and whether the excitation results match the manufacturer's curve; the rated and measured burden per phase; the primary-winding insulation-resistance test voltage and readings on bar-type units; the dielectric withstand nominal system voltage, basic impulse level, field test voltage, application time, and result; the insulation power factor per phase with the manufacturer's limit; the single-point secondary grounding result; and the ambient temperature, relative humidity, test equipment with calibration due date, comments, and deficiencies.

How results are evaluated

Insulation resistance, both secondary and primary, must not fall below the value the standard's instrument transformer table gives for the unit's coil rating and construction, and the manufacturer's published data governs where it differs; readings below either are investigated, and the table's own notes make clear that its values are representative recommendations with temperature correction applied from the standard's correction table. Polarity results must agree with the transformer markings. Ratio errors are judged against the instrument transformer standard rather than against a NETA table, which is why the accuracy class on the nameplate matters. Excitation results must match the manufacturer's supplied curve or conform to the instrument transformer test standard. Measured burden is compared with the transformer's own rating. The dielectric withstand test passes if no distress or insulation failure is evident through the full voltage-application time, at the field test voltage the standard's table assigns to the unit's nominal system voltage and basic impulse level. Power-factor limits come from the manufacturer's published data or, failing that, the test equipment manufacturer's. The secondary circuit must show exactly one grounding point, at the specified location, per the instrument transformer grounding standard. Bolt torque follows the manufacturer's data with the standard's torque table used in its absence, and thermography is judged under its own section. See the purchased NETA standard for the table values.