Shorting every terminal of a tapped current transformer creates two coupled short-circuited winding sections, not a simple passive current divider. With uniform volts per turn, equal resistance per turn, identical coupling, and negligible jumper resistance, both sections carry approximately the same current even when the tap is off-center; the tap-lead current is then approximately zero because it is the algebraic difference between the section currents. Appreciable tap current indicates unequal loop impedance, unequal coupling, a connection problem, a winding defect, or a measurement error.
How Current Flows in a Tapped, Shorted CT
Consider a secondary winding with terminals A and C at its ends and terminal B at the tap. Connecting A, B, and C together creates two closed loops: winding section A-B and winding section B-C. Each section links the common core flux and develops an induced voltage proportional to its number of turns.
Each loop current is limited by winding resistance, leakage reactance, connection resistance, and mutual coupling with the other section. The zero external terminal voltage does not mean that no voltage exists inside the winding. The induced voltage is balanced by the voltage drops within each shorted loop.
At the common shorting point, Kirchhoff's current law requires the instantaneous terminal currents to satisfy when consistent reference directions are used. The tap-wire current is therefore the phasor difference between the two section currents. Equal section currents cancel at the tap; unequal magnitude or phase produces measurable tap current.
Expected Distribution for Center and Off-Center Taps
| Condition | Expected behavior | Interpretation |
|---|---|---|
| Ideal center tap | Equal section currents and approximately zero tap-lead current | Both sections have equal turns, induced voltage, impedance, and coupling. |
| Ideal off-center tap | Section currents can still be approximately equal | Induced voltage and winding resistance both increase with turns, so their ratio remains approximately constant. |
| Real tapped winding | Unequal outer-lead currents and nonzero tap current | Resistance per turn, leakage impedance, coupling, or external connection resistance differs between loops. |
| Faulted or misconnected winding | Large, unstable, or nonlinear imbalance | Possible terminal error, poor joint, damaged conductor, interturn fault, or core-related problem. |
For a first-order resistive model, section current follows I = E/R. If induced voltage is E = N·e_turn and winding resistance is R = N·r_turn, then:
I = (N·e_turn)/(N·r_turn) = e_turn/r_turn
The number of turns cancels. This explains why moving the tap away from the center does not automatically make the shorter section carry more current. The approximation fails when end leads, tap leads, joints, jumpers, leakage reactance, and unequal flux linkage become significant relative to the winding impedance.
The marking 1200/800/1 normally identifies selectable ratios associated with different secondary terminals, but it does not by itself establish terminal order or the exact internal connection. Use the CT nameplate diagram to identify which terminal pair forms the full winding and which pair provides the tapped ratio. When every terminal is shorted, neither section operates as an independent ratio output.
Diagnostic Sequence for Tap-Current Imbalance
- Confirm the terminal diagram. Match every secondary lead to the CT nameplate or approved drawing. Verify that the apparent center or intermediate terminal is actually the intended tap.
- Record current as phasors. Measure all three terminal currents using consistent probe orientation. Do not add RMS magnitudes arithmetically; compare phase as well as magnitude and verify the algebraic current sum at the shorting junction.
- Eliminate instrument error. Move the same current probe between conductors, repeat the test, and then swap channels if multiple instruments are used. Check zero offset, range, resolution, bandwidth, and susceptibility to nearby primary conductors.
- Inspect the shorting network. Look for different conductor lengths or sizes, loose fasteners, oxidized contacts, damaged strands, extra test-switch contacts, and unintended parallel paths. Small resistance differences matter because CT secondary winding resistance is low.
- Measure loop resistance with the primary de-energized. Isolate the CT from external circuits and use a low-resistance method that compensates for test-lead resistance. Compare the two winding sections and include the actual shorting links in a separate measurement.
- Repeat injection at multiple current levels. A nearly constant percentage imbalance points toward impedance asymmetry. An imbalance that changes sharply with current can indicate connection heating, magnetic nonlinearity, residual magnetism, core trouble, or a winding defect.
- Compare equivalent CTs. Apply the same wiring, instrument, conductor routing, and injection conditions to another CT of the same construction. Moving the anomaly with the test leads identifies a test-system problem; leaving it with one CT identifies a CT or local connection problem.
Correcting the Shorted-Secondary Arrangement
De-energize the primary before changing secondary wiring. A CT secondary must not be opened while primary current flows because the core can generate hazardous secondary voltage and retain magnetization.
- Remove unintended parallel paths through meters, test switches, relays, shields, or grounded conductors.
- Clean and secure the shorting contacts. Use conductors and terminations with comparable resistance where two section loops must be tested under equivalent conditions.
- For an unused tapped CT, install the short on the terminal pair specified by the CT drawing or protection design. Do not tie every tap together merely because the winding is unused; doing so creates circulating section currents and can complicate later testing.
- If imbalance remains after the external network is corrected, test each ratio independently using its designated terminal pair. Compare measured current ratio, polarity, winding resistance, and excitation behavior with approved acceptance criteria.
- Remove the CT from service if the abnormality remains attached to the unit and the measurements indicate a winding, joint, insulation, or core defect.
Verification After Correction
Repeat primary injection using the same primary conductor position and the same measurement points used for the baseline. Verify that the outer-lead currents have the expected magnitude and phase relationship, that the tap current has fallen to the measurement noise or the accepted project limit, and that the phasor sum at the junction closes.
Then test the intended ratio connections individually. For a 1200/1 connection, the ratio designation corresponds to 1 A secondary at 1200 A primary under nominal ratio conditions. For an 800/1 connection, it corresponds to 1 A secondary at 800 A primary. Apply the actual project test current and calculate expected secondary current proportionally rather than requiring full rated primary current.
Document terminal identification, shorting-link resistance, winding-section resistance, primary injection current, each secondary current magnitude and phase, instrument model, and probe orientation. These records distinguish a real CT change from a later difference in test setup.
Recurring Pitfalls
- Treating the tap as a resistive current-divider node: Both winding sections generate voltage and interact magnetically through the common core.
- Assuming an off-center tap must carry current: With equal resistance per turn and equal coupling, unequal turn counts can still produce equal section currents.
- Comparing only RMS magnitudes: Tap current depends on the phasor difference, so phase error can create residual current even when magnitudes appear equal.
- Ignoring milliohm-level external resistance: Test switches, jumpers, and terminal joints can dominate the difference between two low-impedance loops.
- Moving probes between crowded conductors without controlling orientation: Reversed reference direction or magnetic pickup from the primary can imitate imbalance.
- Opening a secondary during injection: Always stop primary current before lifting a short, changing taps, or inserting an instrument.
FAQ
Why is current flowing in the tap wire of a shorted CT?
The tap wire carries the algebraic difference between the currents in the two shorted winding sections. Nonzero current indicates unequal loop magnitude or phase caused by impedance, coupling, connection, winding, or measurement differences.
Will an off-center CT tap divide current according to turns?
Not necessarily. If induced voltage and resistance are both proportional to turns, the turns cancel in I = E/R, and both sections carry approximately equal current.
Should all terminals of an unused multi-ratio CT be shorted together?
Use only the terminal pair specified by the CT drawing or protection design. Tying every tap together creates multiple coupled short-circuited loops and circulating currents.
How do I determine whether CT current imbalance is real?
Measure all leads with consistent orientation, swap probes or channels, verify the phasor current sum, inspect the shorting network, and repeat injection at several current levels. If the anomaly remains with one CT after the test setup is exchanged, test that CT's winding resistance, ratio, polarity, and excitation behavior.
