Overview of Multi-Point CT Ratio Testing
Performing a current transformer (CT) ratio test at multiple primary current points (e.g., 100 A, 200 A, 300 A, and 400 A on a 400:5 A CT) is not a redundant exercise. When executed alongside a magnetization (V-I) curve and an RCT (CT secondary winding resistance) measurement, the multi-point injection constitutes a simplified CT acceptance test per IEEE C57.13 and IEC 61869-2. The objective is to confirm turns ratio, polarity, saturation knee-point, and protection-class accuracy under the burden the CT will actually see in service.
Why Test at 10%, 50%, and 100% Primary Current
Field practice and the discussion referenced in the source confirm three injection levels are typical:
- 10% rated primary – proves the core is not saturated at low flux density and that remanence from prior fault current has not displaced the operating point.
- 50% rated primary – confirms the core is operating on the linear portion of the B-H curve at normal load.
- 100% rated primary (or 1000 A, whichever is smaller) – validates ratio at full nameplate current and verifies the continuous thermal rating factor (RF).
If the engineer is asked to inject at four points (100/200/300/400 A on a 400:5 CT), the practice likely originated from the accuracy test portion of IEEE C57.13.13, where ratio error and phase error are measured at 10% and 100% of rated, with optional 50% and rating-factor points. The expected secondary currents on a 400:5 A CT (standard ratio 80:1) are:
| Injected Primary (A) | Expected Secondary (A) | % of Rated |
|---|---|---|
| 100 | 1.25 | 25% |
| 200 | 2.50 | 50% |
| 300 | 3.75 | 75% |
| 400 | 5.00 | 100% |
Prerequisites and Test Equipment
- Primary injection source: 0–500 A variable, ≥2 kVA (e.g., Megger MRCT or Omicron CT Analyzer for secondary-side testing). For 400 A primary injection on a multi-tap CT, a current booster with 6–8 m cable set is standard.
- Reference ammeter: 0.2% accuracy class or better on the secondary side, or use a high-accuracy primary clamp (±0.1%).
- Digital low-resistance ohmmeter (DLRO) or Kelvin bridge for RCT measurement (10 µΩ to 20 Ω range).
- Variac + AC voltmeter (0–250 V) for the secondary magnetization (V-I) curve.
- CT nameplate data: ratio, taps, accuracy class (e.g., C400, 0.3 B-0.9, MR), rated burden (VA), and voltage rating (e.g., 0.6 kV class).
Step-by-Step Multi-Point Ratio Test Procedure
- Isolate the CT: open the secondary circuit, short the secondary to ground at the test block, and de-energize the primary conductor. Per OSHA 1910.333 and NFPA 70E, follow lockout/tagout procedures.
- Demagnetize the core by gradually applying AC voltage to the secondary from a variac and slowly reducing to zero, or perform an opposing DC step decay.
- Measure RCT with the DLRO on each unused tap. Compare to factory test report (typically 0.1–2 Ω for relaying CTs).
- Verify polarity with a DC kick test (battery + analog ammeter on secondary; kick direction confirms H1–X1 polarity per IEEE C57.13 markings).
- Inject primary current at the four specified levels (100/200/300/400 A). For each point, record primary current (IP), secondary current (IS), and applied burden.
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Calculate ratio error:
Ratio Error (%) = ((NR × IS) − IP) / IP × 100, where NR is the nameplate turns ratio (80 for 400:5). - Compare to accuracy class: A C400 protection CT allows ≤10% ratio error at 20× rated secondary voltage; a metering 0.3 class allows ±0.3% at 100% rated current.
- Record magnetization curve: with the primary open, apply 0–250 V AC to the secondary in 10 V steps, recording current at each step. Plot VEX vs IEX; the knee-point voltage (VK) should meet or exceed the nameplate C-class voltage.
Acceptance Criteria Summary
| Test | Acceptance Limit | Reference |
|---|---|---|
| Ratio error (metering class) | ±0.15% to ±0.3% per IEEE C57.13.13 | IEEE C57.13.13 |
| Ratio error (relaying class) | ≤10% at 20 × rated VS (C400 = 400 V at 20 × In) | IEEE C57.13 §6.4 |
| Polarity | H1, X1 markings match test kick | IEEE C57.13 §5.2 |
| Magnetization knee voltage | ≥ nameplate VK (e.g., C400 → VK ≥ 400 V) | IEEE C57.13 §6.3 |
| RCT deviation | ≤ ±5% of factory value | Manufacturer test report |
Interpreting Four-Point Test Results
If the source was not specified as a full accuracy test (no defined burden, no phase-error measurement, no C-class voltage rating), then the four points are arguably redundant for a simple ratio confirmation. The source discussion captures this practical reality:
- For protection CTs, confirming non-saturation under fault is more important than ratio linearity. A single-point ratio check at ≥10% rated, combined with the V-I curve, is generally sufficient.
- For metering CTs, the four points may be requested to verify the 0.3 or 0.15 accuracy class over the operating range, especially if revenue billing depends on it.
- A 100/200/300/400 A sweep on a 400:5 CT may also be a check against the CTs being different cores with mismatched ratios (e.g., 300:5 mistakenly wired where 400:5 is expected). The 80:1 ratio at all four points will expose that.
Verification and Documentation
- Plot the four measured points on a ratio-vs-IP graph; deviation from the ideal 80:1 line should be linear and within the class limit.
- Overlay the magnetization curve on the manufacturer's published V-I plot; the field curve should reach or exceed the factory knee-point voltage.
- Stamp the test sheet with date, ambient temperature, test equipment serial numbers, calibration dates, and CT serial numbers.
- Issue a Pass/Fail certificate per NETA MTS-2023 (InterNational Electrical Testing Association) Section 7.6 for instrument transformers.
Frequently Asked Questions
What is the purpose of testing a CT ratio at four different primary currents?
Testing at multiple points (e.g., 100/200/300/400 A on a 400:5 A CT) verifies the turns ratio across the operating range, confirms the core is not in saturation at any point, and validates the accuracy class. A single-point test only confirms ratio at that one current and may miss errors caused by core damage, tap mis-connection, or incorrect CT model.
What is the standard CT accuracy class for 400:5 A relaying CTs?
Common relaying accuracy designations per IEEE C57.13 are C100, C200, C400, and C800, where the number indicates the secondary voltage the CT will deliver at 20 times rated secondary current without exceeding 10% ratio error. A 400:5 A C400 CT will maintain ratio within 10% up to 400 V across the secondary at 100 A secondary (20 × 5 A).
How do I perform a CT magnetization (V-I) curve test?
With the primary open-circuited, apply variable AC voltage (typically 0–250 V) to the secondary through a variac, and record the exciting current at 10 V increments. Plot VEX versus IEX; the knee-point voltage (VK) is the intersection of the linear air-core line and the saturated portion of the curve. VK must meet or exceed the nameplate C-class voltage.
Is a single-point ratio test acceptable for protection CTs?
Yes, for protection CTs a single ratio check at 10%–100% of rated primary, combined with a magnetization curve and polarity test, is generally sufficient to certify the CT. The key acceptance criterion is that the CT will not saturate excessively at the maximum symmetrical fault current expected at the bus.
What equipment is used for field CT ratio testing?
Primary injection test sets (0–500 A, 2–5 kVA) are used for 400:5 class CTs. Secondary-side CT analyzers such as the Omicron CT Analyzer or Megger MRCT perform ratio, polarity, RCT, and magnetization curve tests in a single setup without needing to inject high primary current.
