A spare CT secondary appears harmless, yet an open secondary can develop hazardous voltage whenever primary current flows. The correct arrangement is a continuous short across the complete secondary winding, with one intentional ground connection where the approved drawing or terminal-block design requires it. Confirm each electrical path by measurement; screw position alone is not proof.
What happens when a CT secondary is left open?
A current transformer belongs to a measurement chain: primary current produces secondary current, the secondary wiring carries that current, and a relay or meter converts it into a protection or indication function. A connected burden lets secondary current oppose the primary magnetizing force. Opening the secondary while primary current continues removes that balancing current.
The resulting core flux can rise sharply. The secondary terminal voltage may then become hazardous, insulation can be stressed, arcing can occur, and the core can retain magnetization that degrades later measurement accuracy. Treat an unused CT as energized whenever its primary conductor can carry current. A disabled relay input does not make the CT electrically inactive.
Shorting provides a low-impedance path for secondary current. Grounding serves a different purpose: it fixes the secondary circuit reference and limits its floating potential relative to the enclosure. A ground bond does not replace the winding short, and a short does not prove that the intended ground exists.
Check 1: Is the CT truly unused?
Trace the conductors from the CT terminals through the terminal block before changing any screws or jumpers. A truly unused winding has no active relay, meter, transducer, or test circuit that depends on its current. If the winding supplies protection, closing a short across it diverts current away from the relay and can prevent a trip.
| Signal or path | Source or reading to check | Wrong-value symptom |
|---|---|---|
| Primary current | Feeder state, one-line drawing, and an approved current indication | Assuming zero current when the primary can be energized exposes an open secondary during work |
| CT secondary current | Secondary drawing, relay metering, or a suitable current measurement made under the approved procedure | Zero relay current with primary load may indicate an open circuit or an unintended short around the relay |
| Complete winding path | Terminal identification and continuity between the winding ends, such as X1 and X5 where those are the documented endpoints |
Shorting only part of a tapped winding can leave another section open |
| Shorting path | Low-resistance continuity across the selected terminals with the equipment in a safe state | An open or unstable reading points to a missing jumper, loose connection, wrong terminals, or ineffective shorting screw |
| Ground path | Continuity from the designated secondary point or grounded shorting bar to the intended ground | No continuity leaves the secondary floating; multiple separate bonds create an unintended multi-point ground |
| Relay operating path | Secondary-injection and, where required, primary-injection results after a spare winding is returned to service | A relay can pass a bench test while a field-installed short still bypasses its input |
If drawings, wire markers, and field routing disagree, stop at this branch. Resolve the circuit identity before treating the winding as spare. Tuning or relay configuration cannot correct a wiring path that never delivers current to the input.
Check 2: Which terminals span the complete winding?
Use the CT nameplate and approved wiring diagram to identify the two ends of the unused secondary. The labels X1 and X5 are valid only when the installation drawing identifies them as the required endpoints. A multi-ratio CT may expose taps between its end terminals, so a jumper across two convenient terminals might short only one section.
With the primary de-energized, isolated, and controlled under the site electrical work procedure, trace every secondary lead and compare it with the terminal schedule. Account for factory links, rear-mounted jumpers, test-switch contacts, and grounded bars that are not visible from the front. Use a continuity or low-resistance measurement appropriate to the circuit to verify the selected shorting path.
Do not disconnect a secondary lead merely to identify it while primary current may flow. Establish the short first with hardware designed for make-before-break CT service, or remove primary current under the approved isolation procedure. If terminal identity remains uncertain, the next check is the manufacturer drawing rather than an experimental opening of the circuit.
Check 3: Does the shorting block also provide the ground?
A shorting screw can bridge adjacent CT terminals, connect them to a grounded shorting bar, or perform both functions, depending on the terminal-block construction. Inspect the actual block drawing and measure the installed path. The presence of a metal bar or a screw in the closed position does not by itself identify which nodes it connects.
If the documented shorting bar both connects the winding ends and bonds that short to ground, closed shorting screws can provide the complete arrangement. If the bar shorts the winding but is not grounded, add one deliberate ground bond at the designated point. If a wire jumper shorts X1 to X5, ground one end of that short only where the drawing calls for the secondary bond.
A second independent ground jumper is not automatically an improvement. Two ground points can carry circulating current, conceal wiring errors, or complicate testing. Use one identifiable ground reference for the secondary circuit unless the equipment manufacturer specifies another topology. Verify continuity from that point to ground after all hardware is installed.
Check 4: Should the spare CT use screws, a jumper, or both?
Shorting screws are purpose-built and can provide a compact, visible short, especially when the same bar supplies the ground connection. Their weakness is procedural: a technician can remove one screw, misunderstand the contact arrangement, or later leave a screw installed when placing the CT into service.
A terminal-to-terminal wire jumper makes the spare status visually explicit and remains effective if a shorting screw is removed. It must have suitable insulation, termination, mechanical security, and current-carrying capability for the CT secondary circuit. Route and label it so later work cannot mistake it for normal relay wiring.
For a truly unused CT, using the shorting screws together with a documented jumper across the full secondary winding provides two shorting paths. This belt-and-suspenders arrangement addresses loss of one shorting method, but it creates two items that must be removed when the winding is commissioned. Record both locations on the terminal-block drawing and in the commissioning checklist.
Do not add redundant hardware without configuration control. An undocumented rear jumper can remain in place for years and bypass a connected protection relay even though the relay passes bench or secondary-injection tests. If maintenance practice requires serviceable gear to contain no installed shorting screws, use the approved jumper arrangement and document that policy consistently.
How should the short and ground be installed?
- Confirm the operating state. Determine whether primary current can flow. If it can, do not open or rearrange the CT secondary unless an approved CT shorting device already provides a verified closed path.
- Identify the winding. Match the CT nameplate, schematic, terminal schedule, wire labels, and physical conductors. Confirm that the chosen endpoints cover the complete unused winding.
- Inspect hidden connections. Check both sides of the terminal block for factory jumpers, rear wiring, shorting bars, and ground links. Mark each discovered path on the working drawing.
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Establish the short. Close the designated shorting screws, fit the approved terminal jumper, or use both when the site design calls for redundancy. For the example endpoint notation, the jumper would connect
X1toX5only after the drawing confirms those are the full-winding terminals. - Establish one ground reference. Use the grounded shorting bar if its bond is documented and measured. Otherwise connect one point of the shorted secondary to the designated ground terminal according to the equipment drawing.
- Measure the finished paths. Verify low-resistance continuity across the winding terminals and continuity from the designated secondary ground point to ground. Also verify that no unintended separate ground bond remains.
- Label and document the state. Identify the CT as spare and shorted. Show every screw, jumper, and ground connection on the terminal drawing, including connections on the rear of the block.
How is the arrangement verified before energization?
Begin with a point-to-point inspection against the marked-up drawing. Tug-check terminated jumpers under the approved workmanship procedure, confirm screw positions, and measure the shorting and grounding paths. Look at the complete signal chain rather than accepting a relay self-test as proof of field wiring.
When an unused CT is later connected to a relay, remove every intentional bypass listed in the commissioning record. Verify the normal secondary path from the CT through the test hardware to the relay input, and confirm the single intended ground remains. A secondary-injection test proves the relay and selected downstream wiring, but it may not expose a short located upstream of the injection point.
Use primary injection when the commissioning scope requires proof of the entire chain. Primary injection exercises the CT, field conductors, terminal blocks, test devices, relay input, and protection response together. Record measured current relationships and relay operation against the approved design criteria; do not substitute a successful relay bench test for this end-to-end check.
What errors defeat an otherwise correct design?
The most dangerous error is opening an energized CT secondary while moving a jumper or screw. The most consequential protection error is the opposite: leaving a short in place after connecting the relay. Both arise when work focuses on an individual terminal rather than the current path from primary conductor to final protection element.
Other recurring failures include shorting the wrong taps, assuming that a shorting bar is grounded, installing multiple ground points, overlooking a rear-mounted jumper, and failing to update the terminal drawing. Measure before adjusting. If the measured path differs from the drawing, correct the documentation and circuit identity before proceeding.
FAQ
Can I rely only on CT shorting screws for an unused secondary?
Yes, when the terminal-block design documents that the screws short the complete winding and the installed path passes a continuity check. A documented wire jumper can add redundancy for a truly unused CT, but both devices must later be removed before the CT supplies a relay.
Does a jumper between X1 and X5 also ground the CT?
No. A jumper between X1 and X5 shorts those terminals; grounding requires a separate verified connection from one point of the shorted circuit, or from its documented shorting bar, to ground.
Can I return a spare CT to service after removing one short?
Only after accounting for every shorting screw, front or rear jumper, test contact, and ground connection, followed by an end-to-end commissioning test appropriate to the protection function. Stop if terminal identities, hidden connections, ground topology, or primary-current status cannot be verified; contact the equipment manufacturer or its official technical support channel before disturbing the secondary circuit.