4.3 CT Theory: Ratio, Polarity, Burden, Accuracy & Knee Point

CT Ratio and the 1 A vs. 5 A Secondary

Quick Answer: A current transformer (CT) has a nameplate ratio such as 600:5 — at 600 A primary, 5 A flows in the secondary. The standardized secondary ratings are 5 A (most common in North American IEEE practice) and 1 A (common in IEC practice).

A 600:5 CT has a turns ratio of 120:1. At any primary current Ip, the ideal secondary current Is = Ip / 120. Real CTs have ratio error (the measured secondary is slightly less than ideal) and phase angle error, both of which increase as the CT approaches saturation.

The choice of 1 A or 5 A secondary is a burden decision. The CT must develop terminal voltage V = Is x Z_burden. For the same C-rating, a 1 A secondary drives 25x the impedance of a 5 A secondary at the same terminal voltage, because the current is 5x lower and voltage = I x Z. That is why long cable runs from outdoor switchyard CTs to the relay room often use 1 A secondaries — the lead resistance is a smaller fraction of total burden.

CT Polarity (X1/H1) and Why It Matters for 87

Every CT has polarity marks, typically H1 on the primary and X1 on the secondary. When primary current enters H1, secondary current leaves X1 (and returns through X2). In schematics, the polarity mark is the dot.

Polarity matters because protective relays compare current direction. The two places it is most critical:

• Differential (87) schemes. Currents entering and leaving a protected zone are summed. If both zone-boundary CTs have correct polarity, a through-fault produces near-zero differential current. If one CT is reversed, the secondary currents add instead of subtract, and the 87 trips on a perfectly healthy through-fault — a commissioning failure that an injection test catches before energization.
• Directional (67) relays. A polarizing quantity (usually voltage or a polarizing CT) is compared with the operate current. Reversed polarity makes the relay declare forward faults as reverse (and vice versa), so it never trips for the fault it is supposed to clear.

Field polarity is verified with a DC kick test (battery, momentary contact, and a DC analog ammeter deflecting in the expected direction) or a commercial polarity tester. Always record the polarity result on the acceptance test sheet.

CT Burden and How Over-Burden Drives Saturation

Burden is the total impedance connected to the CT secondary, expressed in ohms (or VA at rated current). It is the sum of:

• Relay/meter input impedance (often 0.1 to 1.0 ohm per device)
• Lead resistance (round-trip cable from CT to relay and back — a common culprit on long runs)
• CT secondary winding resistance (Rct, internal)

The CT must develop terminal voltage V = Is x Z_burden. If the burden is too high, the CT cannot develop that voltage without saturating its core, the secondary waveform distorts, and both ratio error and phase error grow. A saturated CT is the most common cause of relay misoperation during heavy through-faults.

Rule of thumb: keep actual burden under the rated burden for the accuracy class. For an IEEE C400 CT (rated at B-4.0, which is 4.0 ohm), the secondary circuit should measure well under 4.0 ohm, leaving margin for lead aging and added devices.

CT Accuracy Class: C200, C400, and 10P20

IEEE C57.13 relaying accuracy classes use a letter plus a voltage number:

The number after the letter is the secondary terminal voltage the CT can deliver to a standard burden at 20x rated secondary current with ratio error under 10%.

So a C400 CT can deliver 400 V to a 4.0 ohm burden at 100 A secondary (20 x 5 A) while keeping ratio error under 10%. All C-class CTs are defined at 20x rated current — that 20 is the IEEE standard accuracy limit factor.

The IEC equivalent is the 10P20 class: 10 = maximum composite error (10%), P = protection class, 20 = accuracy limit factor (the CT holds 10% error up to 20x rated primary current at rated burden). A 5P20 is the same but with 5% composite error. IEC and IEEE ratings are not directly interchangeable, but both describe how the CT behaves under heavy fault current.

CT Knee Point

The knee point is the bend in the CT excitation (saturation) curve where the core begins to saturate. Definitions:

• IEC 60044 / IEC 61869: the point where a 10% increase in excitation voltage produces a 50% increase in excitation current.
• IEEE C57.13: the point where a 10% increase in voltage produces a 50% increase in current — historically the same definition; IEEE also uses a 45-degree tangent line on the log-log excitation curve.

The saturation curve is developed by applying variable AC voltage to the secondary with the primary open-circuited, plotting excitation current versus excitation voltage. The knee voltage must meet or exceed the manufacturer's data, and for a protective CT it must be at least high enough that the CT does not saturate at the maximum fault current and burden.

The Open-CT Hazard (Critical Safety Rule)

Never open the secondary of an energized CT. With the secondary shorted or connected to its low-impedance burden, the CT operates happily — primary ampere-turns are balanced by secondary ampere-turns, and the core flux stays low. If the secondary is opened, the secondary current cannot flow, so there is nothing to oppose the primary ampere-turns. The core is driven hard into saturation every half-cycle, and the open secondary voltage spikes to thousands of volts — enough to break down CT insulation, arc across terminal boards, and cause severe injury.

The field rule is absolute: before disconnecting any CT secondary lead, either de-energize the primary or short the CT secondary at a terminal block with a CT shorting block. Modern CT test switches are designed so the CT is automatically shorted before the test plug removes the relay from the circuit.