How to Perform a Current Transformer Ratio Test: Field & Bench Methods

The Direct Answer: What is a Good CT Ratio Reading?

A successful current transformer ratio test confirms that the secondary current output matches the nameplate ratio within the device's specified accuracy class. For a standard 100:5A metering-class CT (IEEE C57.13 Class 0.6), injecting exactly 100.0A AC into the primary winding must yield a secondary current between 4.97A and 5.03A. If you inject 50.0A primary, the secondary must read between 2.485A and 2.515A.

Protection-class CTs (e.g., Class 10P or 5P) allow wider tolerances at nominal current but must maintain their ratio up to their Accuracy Limit Factor (ALF) without saturating. A reading that deviates by more than 1% on a metering CT, or shows non-linear scaling when primary current is doubled, indicates degraded core insulation, shorted secondary turns, or an exceeded burden limit.

Meter Setup, Probe Placement, and CAT Safety

Before touching any wiring, verify your test equipment matches the environment. Panel-level measurements require strict adherence to safety categories to protect against transient overvoltages.

Meter Setup Block: Field Clamp Method

For in-service verification without breaking connections, use a True-RMS clamp meter rated for the panel's fault current and voltage.

• Meter Model Reference: Fluke 376 FC or equivalent CAT IV 600V / CAT III 1000V True-RMS clamp.
• Dial Position: A~ (AC Amps). Do not use the mV or V~ settings for direct secondary measurement.
• Lead Jacks: No test leads required for the primary jaw measurement. If using the flexible iFlex probe for tight busbars, plug the iFlex connector into the dedicated iFlex jack.
• Range: Set to Auto-Range, or manually lock to the 600A or 10A range depending on whether you are clamping the primary bus or the secondary wire.

Meter Setup Block: Bench Injection Method (DMM)

For de-energized bench testing using a primary injection test set (e.g., Vanguard EZCT-2000B or a DIY step-down transformer setup), use a digital multimeter to measure the secondary loop.

• Meter Model Reference: Fluke 87V MAX or equivalent CAT III 1000V DMM.
• Dial Position: A~ (AC Amps).
• Lead Jacks: Black lead to COM. Red lead to the 10A MAX fused jack. Never place the red lead in the V/Ω/mA jack for this test.
• Range: Manual 10A range to prevent auto-ranging delays during current ramp-up.

Step-by-Step Primary Injection Field Test

This procedure outlines the safe verification of a CT ratio on a de-energized feeder using a portable primary injection kit and a DMM. For authoritative testing standards, always refer to the InterNational Electrical Testing Association (NETA) Maintenance Testing Specifications.

1. De-energize and Lockout: Shut off the upstream breaker. Apply LOTO (Lockout/Tagout). Verify zero voltage on the primary bus using a CAT IV voltage detector.
2. Short the Secondary: Ensure the CT secondary terminals (X1-X2) are shorted via the designated shorting terminal block. This protects the core if accidental primary current flows.
3. Route Primary Cable: Pass the injection test set's primary lead directly through the center of the CT window. Ensure the cable is centered to avoid flux leakage errors.
4. Connect DMM to Secondary: Open the shorting block only at the test terminals, placing your DMM (set to 10A AC) in series with the secondary loop. The current must flow from X1, through the DMM red lead, out the black lead, and back to X2.
5. Inject and Measure: Ramp the primary injection kit to 25% of the CT's rated primary current (e.g., 25A for a 100:5 CT). Record the DMM secondary reading.
6. Scale and Verify: Ramp to 50% and 100% rated current. Record readings at each step. The ratio must remain linear.
7. Secure and Restore: Ramp primary current to zero. Remove the DMM and immediately re-engage the shorting block. Remove primary leads, restore LOTO, and re-energize.

Expected Readings and Misleading Mistakes

When performing a current transformer ratio test, knowing the exact numeric boundaries of a pass/fail result prevents unnecessary equipment replacement. The table below assumes a 100:5A CT with a 0.6 Metering Class accuracy (0.6% maximum error at rated current).

Mistakes That Give Misleading Readings

If your numbers fail the table above, do not immediately condemn the CT. Field errors frequently mimic internal CT faults:

• Exceeding the Burden Limit: If the secondary wire run is too long or the relay impedance is too high, the total VA burden exceeds the CT's nameplate rating (e.g., 15VA). This causes premature core saturation, resulting in secondary readings that drop off non-linearly as primary current increases.
• Clamp Meter Zero-Offset: When using a clamp meter on the secondary wire, failing to press the "Zero" button after closing the jaw introduces a DC offset error that skews low-current AC readings by up to 10%.
• Measuring Harmonics with an Average-Responding Meter: If the primary load is a VFD or solar inverter, the current contains high-frequency harmonics. An average-responding clamp meter will read 20-30% lower than the True-RMS value. You must use a True-RMS meter (indicated by the True-RMS badge on the tool).
• Wrong Multi-Ratio Tap: Many protection CTs have multiple taps (e.g., 100/200/400:5). If you test across X1-X3 (400:5) but calculate your expected values for X1-X2 (100:5), your secondary reading will be exactly 25% of what you expect.

Current Transformer Ratio Test FAQ

Why does my current transformer ratio test show a lower secondary current than expected?

A consistently low secondary current usually points to one of three issues: shorted turns within the CT secondary winding (which alters the effective turns ratio), an incorrect tap connection on a multi-ratio CT, or excessive burden causing magnetic saturation. If the reading is low at high currents but accurate at low currents, core saturation due to over-burden is the culprit. Calculate your total secondary loop resistance (wire + relay + meter) and multiply by the square of the secondary current (I²R) to verify you are within the CT's VA rating.

Can I use a standard multimeter to test a 5A CT secondary?

Yes, but only if you use the correct jack and understand the risks. You must plug the red test lead into the high-current (usually 10A) jack and set the dial to AC Amps. Never connect a multimeter in parallel across a CT secondary like you would for voltage; this creates a dead short that will blow the meter's internal fuse or destroy the meter. Furthermore, standard DMMs introduce a small voltage drop (burden) into the circuit. For highly precise metering-class verification, a dedicated CT analyzer or a clamp meter is preferred to avoid altering the circuit's burden.

What is the difference between a ratio test and a polarity test?

A ratio test verifies the magnitude of the step-down (e.g., confirming 100A primary yields 5A secondary). A polarity test verifies the phase direction of the current. In protection relaying and power metering, the instantaneous direction of current flow matters. Polarity testing ensures that when primary current enters the H1 mark, secondary current exits the X1 mark. A CT can have a perfect ratio but reversed polarity, which will cause differential relays to trip immediately upon energization or cause a power meter to read negative watts.

How often should I perform a CT ratio test?

According to NETA MTS guidelines, a current transformer ratio test must be performed during initial commissioning and after any major fault event or protective relay misoperation. For routine maintenance, testing every 3 to 5 years is standard for critical infrastructure. Metering CTs in non-critical commercial buildings are rarely re-tested unless a billing discrepancy or power quality audit flags an anomaly.