CT Secondary Schematics with Test Switches, Shorting Links and Safe Isolation

An energised current-transformer secondary is a current source: opening it can create hazardous voltage, damage insulation, magnetise the core and corrupt protection performance. A correct MV switchgear CT scheme therefore has to be readable, polarity-correct, burden-controlled and deliberately arranged so that testing can short the CT before any relay or meter path is disconnected.

This practical guide develops the drawing from CT core to relay terminal, explains test-switch and shorting-link sequences, treats differential/REF and CBCT details, and gives design, FAT, SAT and maintenance checks. It applies to conventional analogue CT secondaries; low-power sensors and sampled values require a different safety and test philosophy.

Executive safety rules

Never open an energised CT secondary. De-energise the primary or apply an approved secondary short before opening any conductor.
Use purpose-designed CT shorting/test hardware with a mechanically assured short-before-open sequence.
Earth each galvanically connected CT secondary circuit at one intentional point only, unless a documented special design requires otherwise.
Trace core, ratio/tap, P1/P2 and S1/S2 polarity through every terminal, test switch and IED input.
Keep protection and metering cores independent; never parallel unequal CT secondaries.
Calculate total connected burden at the selected secondary current and worst lead resistance.
Treat differential, restricted-earth-fault and residual connections as zone-and-polarity systems, not merely three current wires.
Prove safe isolation by an approved switching sheet, test-before-touch and independent check.

1. Standards and engineering hierarchy

Reference	Application
IEC 61869-1:2023	General requirements for newly manufactured instrument transformers, including analogue/digital signals and commissioning-test framework
IEC 61869-2:2012 + ISH1:2022	Additional requirements for inductive current transformers
IEC 60255-27:2023	Product safety for measuring relays/protection equipment and associated auxiliaries; it does not define the complete CT-system performance
IEC 61082-1:2014	Rules for electrotechnical diagrams, drawings and tables
IEC 81346-1:2022	Unambiguous object structures and reference designations across documents
Project/manufacturer documents	Exact CT ratings, terminal/test-block sequence, relay input burden, wiring and approved work method

Freeze editions in the project specification. Product-standard compliance does not prove that an assembled scheme has the correct ratio, burden, polarity, earthing or test sequence. Those remain system-engineering and commissioning responsibilities.

2. Start with a CT data and allocation schedule

equipment/bay and phase location;
CT tag and physical orientation P1–P2;
core number, purpose and ownership;
rated primary/secondary current and selected tap;
accuracy class, rated burden and instrument-security/accuracy-limit or transient data as applicable;
short-time thermal/dynamic rating and system fault duty;
secondary terminal labels and accessible test points;
connected IEDs/meters, calculated lead/device burden and spare capacity;
single earthing-point location;
drawing, cable and terminal references.

One approved schedule must reconcile the CT nameplate, single-line diagram, protection philosophy, schematic, terminal plan, cable schedule, relay settings and test report. A “Core 1” that changes purpose between drawings is a latent protection failure.

3. Polarity, phase and current direction

IEC terminal markings and the manufacturer’s plate define polarity; do not infer it from physical appearance. Draw P1/P2, S1/S2, phase and primary power-flow reference. For a chosen instantaneous primary direction into P1, the secondary reference direction follows the adopted terminal convention. What matters operationally is consistency from CT through the relay’s current arrow and protection-zone definition.

Mark A/B/C (or L1/L2/L3) at both CT and terminal ends.
Show star-point position explicitly; never hide it in a cable multicore note.
For multi-ratio CTs, show the used tap and isolate unused taps exactly as the manufacturer requires.
Do not change polarity in software to conceal incorrect physical wiring without a reviewed engineering decision and as-built correction.
For differential zones, show current arrows entering/leaving the protected object and verify every end independently.

4. Canonical secondary path

A maintainable phase path is normally represented as:

CT S1 → CT marshalling terminal → multicore cable → shorting/test block → relay or meter current input → return/test block → star point → intentional earth (one point) → CT S2.

The actual order can differ, but every conductor and bridge must be visible. Show removable links, fixed bridges, disconnect terminals, earthing bars, shield drains and spare cores. Terminal plans must state which side remains connected to the CT when a knife is opened; “top” and “bottom” are unsafe descriptions unless the physical orientation is controlled.

5. Shorting links and test switches

Function	Required behaviour	Failure to prevent
CT short	Connect each active CT lead to the common return/star on the CT side	Open-circuit secondary
IED isolation	Open the protected-device side only after CT short is established	Backfeed or parallel test current
Test injection	Provide touch-safe isolated access to relay-side inputs	Injection into CT or adjacent equipment
Restoration	Reconnect IED before removing CT short	Momentary open circuit
Keying/coding	Prevent wrong plug/block or phase application	Cross-bay/phase errors

A generic safe functional order is short CT → verify short → isolate relay → test; restoration is remove test source → reconnect relay → verify path → remove CT short. The hardware vendor’s documented sequence and the site switching procedure are authoritative. Do not improvise with loose jumpers, and never rely only on a software “test mode” to make an analogue CT circuit safe.

6. Terminal-block design

Use CT-rated disconnect/shorting terminals whose current, insulation and short-time capability suit the circuit.
Fit captive, clearly visible bridges; prevent a bridge from falling onto live terminals.
Separate CT circuits from voltage, trip/close and communications terminals.
Provide phase barriers, durable labels and test access without disturbing adjacent wiring.
Limit ferrules to one wire unless terminal approval explicitly permits more.
Control torque, conductor preparation and bridge position by inspection records.
Place shorting facilities where technicians can operate them without entering an unsafe HV compartment.
Show normal-service link positions on the drawing and, where useful, on a local schedule.

7. Single-point secondary earthing

An intentional earth establishes secondary potential and provides a defined path for primary-to-secondary insulation failure. Multiple earths create circulating current, measurement error and hazardous unintended loops; no earth can leave the circuit floating at an uncontrolled potential.

Select one accessible, documented point for each galvanically connected circuit—commonly at a central protection panel or designated CT terminal location.
Do not automatically earth both ends of a long circuit or every panel star point.
Distinguish the CT secondary earth from cable-screen bonding and equipment protective earth.
For multi-ended differential circuits, define the earthing point for the complete connected network.
Test earth continuity and absence of unintended second earths with the CT safely shorted/isolated and sensitive equipment protected.

8. Burden and lead calculation

For a conventional current circuit, total secondary burden is the vector/impedance sum of connected relay/meter inputs, both lead conductors, terminals/test devices and interposing equipment. A useful resistive approximation is:

VA_total = I_s² × (R_out + R_return + ΣR_devices) + device reactive burden.

At the same impedance, a 5 A secondary produces 25 times the VA of a 1 A secondary. Use maximum hot-conductor resistance, actual route length and manufacturer input data at relevant frequency/current. Check CT class/application performance—not only that calculated VA is below a nameplate number. High-impedance differential/REF circuits require their dedicated CT knee-point, excitation-current, secondary-resistance and stabilising-resistor calculations.

9. Multiple devices and core segregation

Conventional current inputs on one core are connected in series, subject to burden and maintenance requirements.
Do not series-route a critical protection trip through a revenue-meter panel that another party may open.
Use dedicated protection/metering cores when accuracy, security, ownership or isolation requires it.
Never parallel CT secondary windings unless a specifically engineered arrangement and manufacturer data permit it.
Do not connect two independently earthed circuits by an unnoticed common terminal or test link.
For spare cores, follow the CT manufacturer/project rule; provide a safe documented terminal condition.

10. Differential and restricted-earth-fault circuits

Define the protected zone by physical CT locations and breaker/transformer connections.
Match ratio, polarity and phase compensation to the relay algorithm and transformer vector group.
For low-impedance differential, verify CT class/transient behaviour, through-fault saturation and restraint mapping.
For high-impedance differential/REF, keep secondary loops symmetrical where required and calculate stability voltage, CT knee point, stabilising resistor and nonlinear resistor duty.
In REF, reconcile phase-CT residual with neutral CT polarity, ratio and earthing-transformer/neutral arrangement.
Put test facilities so each branch can be safely shorted, isolated and injected without destabilising an in-service parallel zone.
Record normal test-link positions; one omitted phase can create false differential current.

11. Core-balance CTs and earth-lead routing

A CBCT measures the vector sum of all conductors passing through its window. Route all phase conductors—and the neutral where it carries load current—through the aperture in the same direction. Cable screen/armour earthing must be arranged so normal screen current does not bypass or re-enter the window incorrectly; a common solution returns the earth lead back through the CBCT before bonding, according to the approved design. Verify with the actual cable termination geometry, not a generic schematic.

12. Open-circuit detection: useful but not a safety barrier

IED algorithms may detect current unbalance, implausible phase relationships or channel loss, but a balanced load, de-energised circuit or failure location can defeat detection. Supervisory logic does not eliminate hazardous voltage and cannot replace CT-rated shorting hardware, safe work isolation or visual/continuity verification.

13. FAT and SAT test plan

Reconcile nameplate, core allocation, ratio/tap, polarity and terminal drawings.
Inspect wire size, ferrules, labels, terminal type, bridges, torque and physical segregation.
Prove point-to-point continuity and absence of cross-connections with controlled test energy.
Verify the one intentional earth and test for unintended additional earths.
Functionally operate every test switch: demonstrate short-before-open and safe restoration.
Perform ratio, polarity and excitation/winding-resistance tests where specified and safe.
Secondary-inject each phase/residual path and confirm relay measurement, phase, records and trip logic.
Primary-inject where practical to prove the complete CT-to-IED chain, direction and ratio.
For differential/REF, inject internal/external fault patterns and loss/saturation-sensitive scenarios.
Check live load phasors before enabling sensitive protection; resolve rather than mask unexpected spill current.
Archive as-left link positions, readings, instruments, settings and red-lined/as-built documents.

14. Maintenance isolation workflow

Identify all primary sources and every CT core feeding the device.
Review current as-built drawings and approved risk/switching method.
Block or transfer protection only under the system operator’s approved plan.
Apply CT shorts with rated hardware; independently verify their position.
Prove the device side is isolated before touching or injecting.
Prevent test current from operating unintended trips/GOOSE/SCADA outputs.
After work, remove test leads, reconnect device, check continuity/measurements, then remove shorts in the approved order.
Confirm protection healthy, alarms reset, links sealed and records updated.

15. Frequent failure modes

Failure	Consequence	Control
Relay wire removed before CT short	Dangerous voltage/core stress	Interlocked short-before-open block and switching sheet
Wrong star-point/second earth	Circulating current or measurement bias	One-point earth schedule and insulation/earth test
S1/S2 or phases crossed	Directional/differential maloperation	Polarity/phasor/primary-injection proof
Test plug shorts wrong side	CT opened or test source backfeeds CT	Keyed hardware, annotated drawing and functional test
Burden omitted	Saturation/accuracy failure	Hot-loop and device burden calculation
Protection and metering share unsafe isolation	Maintenance disables protection	Dedicated cores/test boundaries
Temporary link left in place	Protection blind or false residual	As-left checklist, link count and seal

16. Design-review checklist

CT type/data and primary fault duty approved?
Core allocation unique across all documents?
P1/P2, S1/S2, phase, ratio tap and current arrows shown?
Complete outgoing and return paths drawn?
CT side of every disconnect unmistakable?
Short-before-open function documented and proven?
Single intentional earth defined and accessible?
Protection/metering/test ownership segregated?
Burden/class and differential/REF application calculations approved?
CBCT screen/earth geometry correct?
FAT/SAT/maintenance sequences include negative tests and restoration evidence?
As-built labels, terminal plans and setting files traceable?

References and further reading

Engineering note: This article is a design and verification framework, not a live-work procedure. Site safety rules, the CT/test-block manufacturer’s instructions and an approved switching programme govern actual isolation and testing.