IEC 61869 Instrument Transformers: CT, VT and LPIT Application

Instrument-transformer selection is a system-design task: the CT, VT or LPIT must reproduce the required quantity accurately during the exact load, fault, transient and communication conditions seen by the meter or protection function. Ratio and class alone are never a complete specification.

This guide applies the IEC 61869 family to conventional CTs/VTs, low-power instrument transformers, merging units and digital outputs in MV switchgear. It covers class/burden, saturation, transient performance, residual circuits, ferroresonance, safety, testing, IEC 61850 interfaces and the important 2023 general-standard transition.

Executive conclusions

Use IEC 61869-1:2023 with the relevant specific product part; record how the 2023 general edition is paired with older specific parts.
IEC 61869-2 covers inductive CTs and IEC 61869-3 inductive VTs; low-power passive current/voltage transformers use -10/-11, digital interfaces use -9 and stand-alone merging units use -13.
IEC 61869-6:2016 has been replaced by IEC 61869-1:2023; do not continue citing -6 as the current general LPIT document without transition review.
Separate metering, protection and special/transient cores. Each needs ratio, class, burden and limiting/saturation behaviour suitable for its function.
Calculate actual connected burden, including leads, terminals, test switches and relay inputs; nominal burden is not the actual burden.
For protection CTs, verify fault-current range, X/R, remanence, required operate time and CT saturation—not only accuracy at rated current.
For VTs, specify voltage factor/duration, burden/class, winding connection, residual/open-delta function, fusing and ferroresonance control.
LPITs remove conventional secondary-energy hazards but introduce interface, scale factor, power, timing, bandwidth and data-quality dependencies.
Never open a conventional CT secondary while primary current flows; never short a VT secondary.
Ratio/polarity secondary injection does not prove primary insulation, installed phase identity or the complete digital time chain. Allocate type, routine, FAT and SAT evidence correctly.

1. IEC 61869 architecture and edition transition

Part	Principal subject
IEC 61869-1:2023	General requirements for HV instrument transformers with analogue/digital signals; incorporates former LPIT general content
IEC 61869-2:2012	Additional requirements for inductive current transformers
IEC 61869-3:2011	Additional requirements for inductive voltage transformers
IEC 61869-4:2013	Combined current and inductive voltage transformers
IEC 61869-9:2016	Digital interface for instrument transformers
IEC 61869-10:2017	Low-power passive current transformers
IEC 61869-11:2017	Low-power passive voltage transformers
IEC 61869-13:2021	Stand-alone merging units with standardized analogue inputs

IEC 61869-1:2023 replaced IEC 61869-1:2007 and IEC 61869-6:2016. Several specific parts still contain normative wording tied to the older general edition. The contract and manufacturer should document the edition pairing and any transition assessment; do not mix clauses casually. The 2023 edition covers analogue and digital secondary signals, bandwidth/anti-aliasing concepts, revised test classification and commissioning tests.

2. Start with function, not transformer type

Function	Instrument-transformer priority	Failure to avoid
Revenue/precision metering	Accuracy over declared load/current range; limiting factor protects instruments	High error at low load or excessive fault output
Overcurrent/earth-fault protection	Accuracy and non-saturation through required fault/time	Delayed/missed trip from saturation
Differential/REF	Matched transient response, polarity and external-fault stability	Spurious differential current
Distance/directional	Phase/sequence magnitude and angle during transients	Reach/direction error
Power quality	Bandwidth, harmonic phase/amplitude accuracy, anti-aliasing	Misleading harmonic results
Digital process bus	Sensor + MU + time/network chain performance	Correct samples with wrong scale, phase, time or quality

3. CT primary/secondary ratio selection

maximum continuous load and foreseeable overload;
minimum load at which metering accuracy matters;
maximum and minimum fault current for protection sensitivity;
1 A versus 5 A secondary and lead burden;
multi-ratio taps and which tap is wired/marked;
future network capacity and CT replacement constraints;
relay input range and thermal withstand;
residual connection and zero-sequence sensitivity;
physical window/bushing arrangement and primary turns.

An oversized CT can reduce protection/metering resolution at normal load; an undersized CT can overload thermally or saturate during faults. Select from the complete application envelope.

4. Actual CT burden

For a conventional current-output CT, secondary burden includes the relay/meter plus the round-trip leads and series devices:

VAactual = Is² × (Rleads + Rterminals/test switch + Rdevices)

use conductor resistance at credible operating temperature;
include both outgoing and return conductors unless circuit topology proves otherwise;
include test switches, shorting links, terminals and transducers;
use actual secondary current (1 A versus 5 A greatly changes lead VA);
check every core/tap independently;
verify burden remains within the accuracy-class conditions;
do not assume lower burden always improves every transient/core behaviour—apply the product rules and manufacturer data.

5. Metering CT specification

rated ratio and multi-ratio taps;
metering accuracy class at specified burden/range;
rated output/burden and power factor conditions;
instrument security factor or applicable limiting characteristic;
extended current rating where required;
revenue-metering authority requirements and sealing;
core allocation isolated from protection/transient demand;
calibration/type/routine certificates traceable to exact serial.

A metering core is intentionally expected to limit secondary stress under high fault current in many applications; that behaviour conflicts with a protection core’s need to reproduce fault current. Do not share a core without a documented combined-performance assessment.

6. Protection CT accuracy and saturation

Protection performance depends on available excitation voltage and magnetic flux. The required secondary voltage rises with secondary fault current and total loop impedance:

Vrequired ≈ Is,fault × (Rct + Rsecondary-loop)

maximum symmetrical current and DC offset/X/R;
fault duration until primary/backup clearing;
remanent flux and reclosing duty;
CT secondary winding resistance at operating temperature;
lead/device burden and connection;
accuracy-limit factor/composite error for conventional protection class;
knee-point/excitation and secondary-loop parameters for PX-type application;
transient classes and time constants where required;
relay saturation detection and algorithm behaviour;
external-fault stability for differential/REF.

Protection studies should simulate or calculate worst credible saturation using manufacturer/IEC parameters. A label such as “5P20” cannot alone prove differential stability or fast high-X/R performance.

7. Differential and REF CT matching

same polarity convention and physical orientation;
correct ratios/taps and relay ratio compensation;
vector-group and zero-sequence compensation;
similar saturation/transient behaviour for external faults;
secondary resistance/lead asymmetry;
neutral CT and phase CT compatibility in REF;
high-impedance scheme knee point, stabilizing resistor and non-linear resistor requirements;
low-impedance bias/slope and maximum spill current;
single-point earthing and test-switch topology;
primary injection/polarity proof before energization.

High-impedance and low-impedance differential schemes place different demands on CTs and secondary circuits. Select the scheme first, then specify CT performance and test method.

8. CT thermal and short-circuit capability

rated continuous primary current and extended current where assigned;
rated short-time thermal current and duration;
rated dynamic current/peak withstand;
primary conductor/support integration in the panel;
temperature rise in the actual compartment;
secondary terminal short-time withstand and safe shorting provision;
future fault-level/clearing-time changes.

CT core accuracy and CT primary short-circuit withstand are separate requirements. Both must coordinate with IEC 62271-200 assembly ratings.

9. VT ratio, class and burden

primary and secondary rated voltage for phase-to-earth/phase-to-phase connection;
system earthing and maximum phase-to-earth voltage;
metering and protection accuracy classes;
burden of relays, meters, transducers and wiring;
separate windings for metering/protection/residual voltage as needed;
voltage factor and permitted duration for earth-fault conditions;
frequency and harmonic/power-quality requirement;
primary/secondary fuses or MCBs and supervision;
secondary earthing point and isolation/test facilities.

Actual VT burden is the sum of parallel device burdens and wiring losses under the applicable voltage. An under-burdened inductive VT in a lightly loaded cable network can also be more exposed to ferroresonance; selection is not a simple “lower burden is better” rule.

10. Residual/open-delta voltage

correct broken-delta winding ratio and phase sequence;
system earthing and expected residual voltage for faults;
burden/damping resistor and thermal rating;
59N/64/earth-fault function scaling;
fuse/MCB arrangement that does not create false residual voltage;
voltage-transformer supervision;
test injection for phase loss, earth fault and healthy conditions;
single-point earthing and safe isolation.

11. Ferroresonance and transient VT concerns

Inductive VTs connected to isolated or resonant-earthed systems with cable capacitance can form a nonlinear resonant circuit. Symptoms include abnormal sustained voltage, heating, noise, fuse operation and relay maloperation.

study network capacitance, VT magnetizing characteristic and switching states;
review single-phase switching/fuse-loss scenarios;
apply manufacturer-approved burden/damping on residual winding;
avoid arbitrary resistors that exceed thermal duty or impair protection sensitivity;
consider anti-ferroresonance VT design or alternative sensing;
verify voltage factor/duration and earthing method;
test supervision/alarm and operating procedures.

12. LPIT application

Low-power instrument transformers can use passive sensors such as Rogowski-type current sensing or resistive/capacitive voltage dividers with standardized low-energy outputs. Benefits can include wide dynamic range, lower burden, compact insulation and improved safety. Engineering checks include:

applicable -10/-11 sensor standard and IEC 61869-1:2023 transition;
rated transformation ratio/scale factor and connector coding;
amplitude/phase accuracy over temperature, frequency and range;
bandwidth, transient response and anti-aliasing;
cable length/type, shielding, grounding and connector integrity;
sensor power (if active) and failure indication;
relay/MU input compatibility and calibration data;
interchangeability rules—never assume all low-power interfaces match;
commissioning injection adaptor and traceable test method.

13. Digital output and merging units

IEC 61869-9 digital interface/profile and IEC 61850 engineering;
Sampled Values stream identity, dataset/channel order and scale;
sample rate/profile, frequency and phase reference;
time synchronization, PTP profile/domain and quality;
analogue chain plus MU error and total uncertainty;
latency, jitter, lost/reordered/duplicate sample behaviour;
test/simulation and quality flag handling;
PRP/HSR/network redundancy where specified;
power supply, reboot, firmware and cybersecurity;
SAMU requirements under IEC 61869-13 for conventional analogue inputs.

A digital message can be syntactically correct while representing the wrong phase, ratio or timestamp. Test the physical primary/sensor through the subscriber protection function, not only packet decoding.

14. Earthing, wiring and safety

earth conventional secondary circuits at one deliberate point unless the approved scheme specifies otherwise;
bond screens according to the project EMC philosophy;
provide CT shorting-before-opening test blocks;
provide VT isolation and protected secondary circuits;
label core, ratio/tap, polarity, phase, destination and earth point;
segregate protection/metering circuits and prevent accidental parallel earths;
never open an energized CT secondary—dangerous voltage can develop;
never short a VT secondary—excessive current/fire risk;
treat LPIT/MU connectors and fibre as protection-critical circuits;
restore every short/link/fuse/test switch and document as-left state.

15. Type and routine evidence

exact standard parts/editions and interpretation sheets;
type designation, insulation system, dimensions and terminal arrangement;
dielectric/partial-discharge and thermal/short-circuit tests as applicable;
accuracy/error and phase displacement across specified range/burden;
protection/transient/excitation parameters;
environmental, EMC and mechanical evidence;
digital/LPIT bandwidth and interface evidence;
serial routine-test certificate for ratio, polarity, winding resistance and applicable tests;
calibration traceability and test uncertainty;
differences between tested and offered design.

16. FAT and commissioning workflow

Verify nameplate, core/winding schedule, ratios, classes and drawings.
Inspect terminals, shorting links, fuses/MCBs, earth points and labels.
Test CT continuity, ratio, polarity and excitation/secondary resistance where specified.
Test VT ratio, polarity/phase and winding connections; verify open delta.
Verify actual connected burden or calculate from as-built circuits.
Inject through test switches to each relay/meter and confirm scale/phase.
For LPIT/SV, test channel map, scale, phase, time/quality and failure response.
Primary-inject representative circuits to prove phase identity and end-to-end path where feasible.
Test CT/VT supervision and fuse/MCB alarm logic.
Remove shorts/forces/test flags, restore earth/fuses and archive results.

17. Common mistakes

specifying ratio/class without burden and application range;
using a metering core for protection without assessment;
assuming 5P/ALF alone proves differential transient stability;
ignoring lead resistance and temperature;
mixing phase/neutral CT types in REF without stability analysis;
selecting VT voltage factor without system-earthing review;
omitting ferroresonance assessment in isolated cable networks;
treating all LPIT connectors/scale factors as interchangeable;
testing SV packets but not physical scale/phase/time;
leaving multiple secondary earths or open CT links;
continuing to cite withdrawn IEC 61869-6 as current general requirements;
mixing 2023 and older editions without a controlled transition statement.

Primary references

Engineering note: Instrument-transformer parameters must be derived from the actual protection/metering study and manufacturer data. Use licensed IEC documents and a controlled edition-pairing decision.