8.1 CT Ratio, Polarity & Saturation Testing

8.1.1 CT Ratio Test

The CT ratio test verifies that the measured secondary current at a known primary current matches the nameplate ratio, and that the polarity markings (H1 / X1) are correct. The standard field method is primary injection: a test set (Omicron CPC 100, Megger Biddle, Doble, or similar) drives a known current through the primary bushing while the secondary current is read at the shorted secondary terminals or at the relay/meter. The measured ratio is I_primary / I_secondary and is compared to the nameplate (e.g., 600:5, 1200:5, 3000:5).

Worked example — 600:5 CT, 120 A primary:

• Nameplate ratio = 600 / 5 = 120:1
• Expected secondary current = 120 A / 120 = 1.0 A
• Measured secondary = 1.0 A → ratio error is approximately 0%

A 1200:5 CT at the same 120 A primary would read 0.5 A; a 600:5 CT at 300 A primary would read 2.5 A; a 600:5 CT at its full 600 A rating reads 5.0 A. NETA and ANSI C57.13 acceptance for metering-class CTs is typically ratio error within +/- 0.5%; relaying-class CTs are held to a few percent. For multi-ratio CTs, test the specific tap in use and document it — the ratio error is different at every tap.

8.1.2 CT Polarity Test

Polarity marks (H1 on the primary, X1 on the secondary) define the instantaneous direction of current. The two field methods are:

• DC flicker (kick) test. A 6 to 9 V battery is momentarily connected across the primary (H1 positive to H2 negative); an analog zero-center galvanometer across the secondary flicks positive on make and negative on break when X1 is the same-polarity end as H1. This is the classic commissioning check.
• AC / test-set method. A modern CT test set injects a known primary current and compares the secondary phase angle to a reference, reporting polarity directly.

Why polarity is critical:

• Differential (87) schemes sum currents at zone boundaries. A reversed polarity on one CT makes the relay see an apparent internal fault for every through-current, so it trips on load.
• Directional (67) and distance (21) elements lose directional security and may operate for faults in the wrong direction.
• Metering: reversed polarity under-reports energy and can flip the sign of kW / kVAR.

8.1.3 CT Saturation (Excitation) Curve Test

The saturation test characterizes the core. With the primary open, variable AC voltage is applied to the secondary and the excitation current is plotted against the applied voltage. The curve is linear at low voltage (the unsaturated region) and bends sharply upward when the core saturates.

Two definitions of the knee point:

• IEC 60044-1 / BS 3938: the point where a 10% increase in voltage produces a 50% increase in excitation current.
• IEEE C57.13 (ANSI 45-degree): the point where a 45-degree tangent touches the curve on log-log axes (nongapped Class C / Class K cores). Gapped cores use a 30-degree tangent.

The measured knee-point voltage must meet or exceed the manufacturer's data. For protection CTs it must also exceed the maximum secondary voltage the CT will develop during the worst symmetrical fault. ANSI relay accuracy class (e.g., C400) means the CT delivers 400 V across its standard 4 ohm burden at 20x rated secondary current (100 A on a 5 A CT) without exceeding 10% ratio error. Standard ANSI classes are C100, C200, C400, and C800 with standard burdens of 1, 2, 4, and 8 ohms respectively.

8.1.4 Why Saturation Causes Relay Misoperation

Once the core saturates, the secondary current is no longer a scaled replica of the primary — it collapses, distorts, and loses the DC-offset information. Field consequences:

• 50/51 overcurrent under-reaches and slows (the relay sees less current than is actually flowing).
• 87 differential may falsely trip on through-faults: one side CT saturates, the other does not, and the relay sees the difference as an internal fault.
• 21 distance over-reaches and loses directional security.
• Asymmetrical faults with DC offset drive the core into saturation much faster than symmetrical faults of the same magnitude.

Rule of thumb: select a CT whose ANSI C-rating voltage is at least twice the secondary terminal voltage developed by the maximum symmetrical fault current — this keeps operation near the knee point.

8.1.5 The Open-CT Secondary Hazard

A CT is a constant-current source. With the primary energized and the secondary open, V = I x Z and Z tends toward infinity, so the open secondary terminal voltage climbs into the thousands of volts. Documented consequences (USBR FIST 3-8, SEL, Phoenix Contact):

• Insulation failure of the CT and of the secondary wiring.
• Dielectric breakdown of relay input circuitry — IEEE C37.90 requires at least 1.5 kV dielectric, but open-CT overvoltages have failed relays at approximately 3.4 kV.
• False trips and inadvertent breaker operations.
• Serious shock or arc-flash injury to personnel.

Mitigations: never open a CT secondary with the primary energized; use shorting blocks and test switches with automatic CT shorting; use touch-safe test switches (e.g., FAME 3); always short the CT before lifting any relay lead. Confirm shorting-block operation as part of commissioning.