VT Secondary Schematics: Fuses, MCBs, Broken Delta and Metering Segregation

A voltage-transformer secondary looks like a familiar low-voltage source, but a badly designed scheme can back-energise an isolated VT, lose protection voltage silently, sustain ferroresonance or invalidate metering accuracy. Safe MV switchgear design requires coordinated primary/secondary isolation, one defined secondary earth, correctly rated fuses or MCBs, burden control and explicit treatment of residual-voltage and source-selection circuits.

This guide covers inductive VT secondary schematics, not capacitive voltage transformers, low-power voltage sensors or merging-unit sampled values. Exact ratings, earthing and switching depend on the network, VT construction, switchgear and applicable national/project rules.

Executive rules

Never assume an isolated primary means a dead secondary: eliminate every secondary backfeed and test source.
Show the VT winding, ratio, polarity, phase, star point, residual winding and all earths on the schematic.
Use one intentional earth per galvanically connected secondary circuit; coordinate neutral switching/fusing with that earthing philosophy.
Protect secondary conductors close to the VT with devices having suitable AC voltage, fault duty, pole arrangement and selectivity.
Do not treat an MCB and fuse as interchangeable without checking accuracy, discrimination, auxiliary indication and failure modes.
Keep protection and metering circuits segregated when accuracy, availability, ownership or safe testing demands it.
Calculate VT burden and lead voltage drop at actual connected loads; unused nameplate VA is not a design margin for arbitrary additions.
Engineer broken-delta damping and residual-voltage scaling for the actual system earthing and VT data.

1. Standards and data hierarchy

Reference	Purpose
IEC 61869-1:2023	Current general family requirements for instrument transformers
IEC 61869-3:2011	Additional requirements for inductive voltage transformers; check contractual compatibility with the referenced general-part edition
IEC 62271-200:2021+AMD1:2024	MV metal-enclosed switchgear assembly, interlocks and compartment context
IEC 60255-27:2023	Safety of protection/measuring-relay products and associated auxiliaries, not the complete system performance
IEC 61082-1:2014 / IEC 81346-1:2022	Document presentation and unambiguous object identification
Manufacturer/project study	VT ratios/classes/burdens, fuse coordination, ferroresonance/damping, relay input and switchgear arrangement

Because specific and general IEC parts may have different publication dates, the purchase specification must state the complete applicable edition set. Do not silently mix acceptance criteria.

2. Required VT schedule

bay/bus and physical VT location;
inductive construction, primary connection and system earthing;
rated primary and every secondary voltage/ratio;
winding identification, terminal polarity and connection;
metering/protection accuracy class and rated burden per winding;
voltage factor and permitted duration;
residual-voltage winding rating and thermal burden;
primary fuse/disconnector/withdrawable arrangement;
secondary protective devices, earthing point and source-selection interfaces;
connected loads and calculated burden/lead drop;
ferroresonance assessment and damping device;
drawing, terminal, cable, settings and test references.

3. Draw the complete source-to-load architecture

A typical phase path is VT secondary terminal → local fuse/MCB → disconnect/test terminal → cable/marshalling → load distribution → protection/metering input → neutral/star return → one intentional earth. Show every winding separately. Identify phase-to-neutral versus phase-to-phase values; never label a ratio “11 kV/110 V” without connection context.

Show which protective device protects which cable section.
Show the normally earthed conductor and whether any device can interrupt it.
Show VT service/test/isolated positions and all position contacts used by logic.
Show inter-panel ties and alternative VT sources on both source and destination drawings.
Show auxiliary/residual windings even if not initially loaded; define safe terminal condition.
Label voltage-test sockets and injection boundaries so an external source cannot energise the VT.

4. Primary isolation and secondary backfeed

Opening primary fuses, a disconnector or withdrawing a VT does not make it safe if its secondary remains tied to another VT, an auxiliary transformer or a test set. A secondary backfeed can be transformed to dangerous primary voltage. The isolation plan must establish a visible/verified break in every possible source path and discharge/earth where required.

Use mechanically suitable multipole disconnection for all source conductors that can backfeed.
Interlock source selectors so two VTs cannot be paralleled unless explicitly designed for it.
Provide break-before-make transfer unless a validated synchronised/paralleling scheme requires otherwise.
Control test plugs so injection connects to the IED side only after the VT side is isolated.
Use position auxiliary contacts for indication/supervision, but do not make an electrical indication the sole safety isolation.
Apply lockout/tagout, prove dead with a rated device and follow the VT/switchgear manufacturer’s procedure.

5. Secondary fuses versus MCBs

Criterion	Fuse	MCB
Fault interruption	Often high limiting capability; verify class/rating	Verify AC voltage, prospective current and trip curve
Indication	Needs striker/auxiliary or fuse-failure logic	Auxiliary/trip contact can distinguish position if correctly wired
Reset/replacement	Requires correct spare and controlled replacement	Resettable, creating risk of repeated closing onto fault
Pole operation	Individual elements may create single-phasing	Multipole common trip can remove all phases; assess protection availability
Discrimination	Use time-current/energy data	Use manufacturer selectivity/cascading data
Accuracy impact	Resistance/contact ageing and holder drop	Contact/thermal element drop; verify at burden current

Locate the first secondary protection as close to the VT terminals as practicable so unprotected wiring is short and mechanically protected. Select device rating above maximum credible burden/inrush yet low enough to clear a remote wiring fault without damaging the smallest conductor. Check minimum fault current, not only maximum interrupting duty. Coordinate downstream branch devices so one meter fault does not unnecessarily remove all protection voltage.

6. Neutral earthing, fusing and switching

The secondary earthing point controls which conductors are live to earth and how earth faults behave. A commonly used star-connected secondary has one intentional neutral earth and phase protection, with the earthed neutral not casually fused. However, no universal diagram applies: alternative windings, open-delta circuits, national rules, secondary transfer and insulation-monitoring designs change the answer.

Document one earth point for each connected secondary network.
Prevent source-transfer ties from joining two separately earthed neutrals unless engineered.
If a neutral is switched, control the sequence so it cannot produce an unsafe floating or double-earthed state.
Do not install a single fuse in the only protective-earth reference without a specific approved design.
Distinguish winding neutral earth, cable-screen bonds and equipment protective earth on drawings.
Test for unintended second earths during commissioning with sensitive equipment safely isolated.

7. Metering and protection segregation

Use separate windings where distinct accuracy classes, burdens, ownership or availability are required.
Give critical protection its own branch protection; a meter short circuit should not blind undervoltage, directional or synchronism functions.
Prevent maintenance test plugs in one system from backfeeding or opening the other.
Keep revenue-metering seals/terminal ownership independent of protection maintenance.
Where one winding must serve multiple loads, allocate branches and calculate combined burden/voltage drop explicitly.
Do not connect a new PQ meter or transducer merely because spare terminals exist.

8. Burden and secondary voltage-drop calculation

Total winding burden is the vector sum of all simultaneously connected device burdens and lead losses at the relevant voltage/frequency. For a mainly resistive branch, cable loss is approximately:

VA_lead = I_load² × R_loop,hot, and receiving-end drop is ΔV ≈ I × Z_loop.

Use actual relay/meter burden, not a generic “negligible” value.
Include fuses/holders, MCBs, selectors, terminals, test blocks and long inter-panel ties.
Correct conductor resistance for maximum credible temperature and minimum area.
Check the VT’s declared accuracy at the actual total burden and power factor.
Check voltage-drop error at the metering/protection terminals separately from VT transformation error.
Check maximum load and minimum-load conditions if the accuracy specification imposes a burden range.
Include damping resistor burden only in the correct operating condition and winding rating.

9. Broken-delta/open-delta residual-voltage circuit

Three auxiliary secondary windings connected in series with an open corner form a broken delta. Their phase voltages sum; in an ideal balanced healthy system the output is near zero, while an earth fault or unbalance produces residual voltage commonly represented as 3U₀. The actual secondary value depends on primary earthing, VT ratio/winding connection and selected rating—never assume a universal 100 V or 110 V output.

Show winding polarity and the exact delta series order.
Show the one earthing point, protective device and relay/damping connections.
Check residual-winding voltage factor, thermal burden and earth-fault duration.
Calculate relay pickup in primary residual-voltage terms using the declared ratio.
Distinguish true neutral displacement from fuse failure, wiring error and ferroresonance.
Measure healthy standing voltage and document alarm/trip margins after commissioning.

10. Ferroresonance and damping

Inductive VTs connected to isolated or resonant-earthed systems can interact with network capacitance and nonlinear core magnetisation, especially during single-phase switching, fuse operation or intermittent earth faults. Consequences include distorted overvoltage, overheating and false residual-voltage operation.

Require a system/manufacturer ferroresonance assessment for the actual bus, cables and switching states.
Use the manufacturer-approved damping resistor or damping device on the specified winding/connection.
Check continuous and transient resistor power/energy, insulation, enclosure temperature and open-circuit failure.
Do not select resistance from an internet rule of thumb; too little/too much damping can overload the VT or be ineffective.
Verify operation in abnormal switching cases and monitor residual waveform where justified.
Treat a hot or failed damping resistor as a protection defect, not a cosmetic panel issue.

11. VT source selection and transfer

Requirement	Engineering control
No unintended paralleling	Break-before-make selector/interlocked contactors and common-mode analysis
Correct source	Breaker/disconnector/bus-section topology plus VT service position
Loss detection	Fuse/MCB auxiliary, phase/sequence voltage and current plausibility
Safe failure state	Defined behaviour on DC loss, selector failure or both sources healthy/dead
Protection security	Block only affected voltage-dependent functions; preserve current-only protection
Testability	Separate VT and IED injection boundaries with source indication

Do not use voltage magnitude alone to select a source: a dead bus can be the correct source for dead-bus closing, while a backfed or induced voltage can be misleading. Use the primary topology and operating philosophy. Avoid transferring directional/distance/synchronism quantities during an undefined transient unless the protection logic is designed for it.

12. Fuse-failure and VT-supervision logic

Use MCB/fuse auxiliary contacts where dependable, supervised and correctly timed.
Compare negative/zero-sequence voltage with current sequence quantities to distinguish a network fault from lost voltage.
Detect individual phase loss, all-phase loss, broken neutral and wrong source selection.
Coordinate delays with genuine fault transients and VT transfer.
Define which functions block: directional overcurrent, impedance, undervoltage, synchronism and power calculations may be affected differently.
Alarm with source, phase and protective-device identity; avoid one vague “VT fail” bit.
Test fuse-failure logic with the breaker open/closed, load/no-load and earth-fault simulations.

13. Withdrawable VT states

Service: primary and secondary connected, protection enabled as designed.
Test: primary isolated; secondary/test arrangement explicitly defined—never infer it from truck position.
Isolated: visible/assured primary and secondary separation as the switchgear design provides.
Removed: shutters, stored energy, earthing and access controlled.
Between positions: logic must not report a valid source; auxiliary contact overlap/gap and bounce assessed.

IEC 62271-200 assembly interlocks provide a product framework, but the project must verify the actual position sequence, secondary plug behaviour and protection consequences.

14. FAT/SAT test programme

Reconcile VT plate, winding schedule, ratio, class, burden, voltage factor and terminal polarity.
Inspect primary/secondary protective devices, wire size, labels, earth and physical segregation.
Prove point-to-point continuity, phase rotation/polarity and absence of unintended earths/cross-ties.
Verify fuse/MCB ratings and auxiliary contacts; inject each failure combination.
Operate VT truck/disconnector through service, test, isolated and intermediate states.
Prove source selector break-before-make and no backfeed in every bus-coupler topology.
Secondary-inject nominal phase quantities, phase loss, wrong phase, residual voltage and frequency variation as applicable.
Verify broken-delta polarity/scaling, healthy residual and approved damping hardware.
Measure burden/receiving voltage where required and compare with calculation.
Test protection blocking/alarming and restoration without unintended trip/close.
Record as-left links, source selection, measurements, settings, instrument calibration and as-built changes.

15. Frequent mistakes

Mistake	Risk	Correction
Primary isolated, secondary assumed safe	Dangerous transformed backfeed	Isolate/prove every secondary source
Two VT neutrals earthed after transfer tie	Circulating current/double earth	Engineer neutral/earth transfer state
One fuse feeds all loads	Meter fault blinds protection	Selective segregated branches
Broken-delta ratio assumed	Wrong earth-fault pickup	Use declared winding ratio and primary study
Damping resistor copied from another project	Overheating or ineffective damping	Manufacturer/system-specific calculation
Only device VA checked	Accuracy lost in long leads	Calculate combined burden and terminal drop
VT fail blocks all protection	Unnecessary loss of fault clearing	Function-specific fail-safe matrix

16. Design-review checklist

Winding connection, ratio, class, burden and voltage factor approved?
Phase, polarity and grounded conductor unambiguous?
Primary and all secondary isolation/backfeed paths defined?
Fuse/MCB AC duty, cable protection and selectivity demonstrated?
One-point earthing preserved through every source-transfer state?
Metering/protection branches and ownership safely segregated?
Combined burden and receiving-end error within requirement?
Broken-delta ratio, protection and damping validated?
VT supervision blocks only affected functions?
Withdrawable/intermediate states included in logic and tests?
FAT/SAT proves phase, residual, selector and negative cases?
Maintenance drawing shows safe injection and restoration sequence?

References and further reading

Engineering note: This is a design/verification framework, not permission for live testing. Follow the actual VT, fuse/MCB, test-block and switchgear instructions plus site isolation rules.