CT, VT and Surge-Arrester Mounting in MV Primary Compartments

CTs, VTs and surge arresters are not accessories that can be “placed wherever space remains.” Their mounting determines measurement/protection performance, insulation coordination, fault-current forces, thermal behaviour, secondary-circuit safety, maintainability and internal-arc consequences. The primary compartment must be designed around their exact electrical and mechanical interfaces.

This guide covers inductive and low-power sensors, fixed or withdrawable VTs, metal-oxide arresters, support structures, conductor routing, earthing, fusing, test access and assembly-level verification.

Executive rules

Freeze exact device type, ratio/rating, insulation, terminals, mass, orientation and allowable loads before compartment layout.
Apply IEC 61869-1:2023 together with the relevant specific instrument-transformer part; a general-part certificate alone is incomplete.
Never open-circuit an energised conventional CT secondary; provide shorting/test facilities and safe terminal architecture.
Protect VT primary/secondary circuits and prevent backfeed; coordinate fuses, disconnecting means, earthing and ferroresonance risk.
Select arresters from system temporary-overvoltage and energy/charge duty, not Um alone.
Keep arrester line and earth connections short, direct and low inductance; connection lead voltage can consume protective margin.
Transfer short-circuit/seismic mass and conductor reactions into verified structural members, not thin partitions.
Maintain assembly dielectric clearances through device tolerance, conductor movement and removable/test positions.
Preserve IEC 62271-200 LSC, partition, accessibility and IAC pressure paths.
Design replacement/testing routes and isolation before approving the layout.

1. Standards map

Reference	Relevant scope
IEC 62271-1:2017+AMD1:2021	Common switchgear ratings, service conditions and tests
IEC 62271-200:2021+AMD1:2024	MV metal-enclosed assembly, compartments, accessibility, dielectric/thermal/short-circuit/IAC performance
IEC 61869-1:2023	General requirements for new instrument transformers above 1 kV AC/1.5 kV DC with analogue/digital signals
IEC 61869-2:2012 + ISH1:2022	Additional requirements for inductive current transformers
IEC 61869-3:2011	Additional requirements for inductive voltage transformers
IEC 61869-10/-11	Low-power passive current/voltage transformers where applicable
IEC 60099-4:2014	Gapless metal-oxide AC surge arrester product requirements
IEC 60099-5:2018	Arrester selection and application recommendations above 1 kV
IEC 60071-2:2023	Insulation-coordination application guidance

Use the project’s frozen editions and actual documents. IEC 61869-1:2023 replaced the 2007 general edition and merged the former IEC 61869-6:2016 general LPIT requirements; specific product parts remain necessary. Device conformity does not by itself prove the modified switchgear assembly.

2. Build the device/interface schedule

system Um, frequency, earthing and insulation levels;
device technology, make/type, dimensions, mass and centre of gravity;
rated primary/secondary values, ratio/taps, classes, burden and frequency/bandwidth;
short-time/peak current or short-circuit withstand, thermal limits and pressure-relief behaviour;
terminal orientation/material/hardware and maximum axial/lateral/bending loads;
mounting plane, fasteners, tightening, allowed orientation and seismic qualification;
clearance/creepage and earth-screen requirements;
secondary connector/terminal, cable/shield/earth arrangement and test links;
withdrawal/replacement envelope, lifting points and special tools;
connected protection/metering function and polarity/phasing convention.

Maintain one source-controlled schedule linking electrical schematics, mechanical layout, relay calculation, test plan and nameplate. A ratio change can alter CT core/package size, secondary terminals and thermal performance; it is not only a drawing-text edit.

3. Current-transformer placement and primary path

place CTs so the protection zone boundary matches the single-line and breaker/cable/bus arrangement;
orient P1/P2 (or equivalent) consistently with protection polarity and physical power direction convention;
route primary conductor concentrically/as qualified through window CTs;
avoid bus offsets that impose side load on resin bodies or terminals;
support conductor short-circuit reaction independently or through qualified CT support points;
maintain phase spacing and clearance at terminals, shields and secondary boxes;
provide access to nameplate, secondary terminals and test points without unsafe primary access;
consider replacement of the largest permitted CT without dismantling unrelated live-adjacent barriers.

The CT’s rated short-time thermal current and dynamic current must suit the system duty and time. The assembly support structure must also withstand the conductor/CT forces. Do not infer bracket capacity from the CT nameplate.

4. CT secondary safety and grounding

A conventional CT attempts to maintain secondary current. Opening an energised secondary can produce dangerous voltage, insulation stress and core magnetisation. Provide:

shorting-before-opening test terminal/block with verified sequence;
clearly identified core, ratio/tap, polarity and terminal numbers;
a defined secondary earthing point—avoid accidental multiple earths unless deliberately engineered;
adequate wire area, burden, insulation and terminal current rating;
secure links for unused cores/taps per manufacturer and scheme design;
segregation from control power and high-noise conductors;
safe access without exposing primary compartments;
warnings and commissioning procedure for injection/ratio/polarity tests.

For differential/REF schemes, physical CT location, polarity and secondary-earth topology are protection-zone parameters. For core-balance CTs, route all intended phase/neutral conductors and screen-earth returns correctly; an earth lead passing through the CT in the wrong direction can cancel or create residual current.

5. CT accuracy, saturation and thermal layout

calculate connected burden including lead resistance, terminals and relay inputs;
select measurement/protection class from required accuracy and transient/fault performance;
check remanence/transient specification where high-speed or differential protection requires it;
avoid heat sources or blocked airflow that push CT beyond declared ambient/temperature;
account for multiple cores and continuous primary current;
validate conductor proximity and stray fields for sensitive/LPIT devices;
keep secondary cable length/impedance within the qualified measurement chain.

Mechanical convenience must not increase secondary lead length until protection CT performance is lost. The protection engineer and primary designer must close the burden/saturation calculation together.

6. Voltage-transformer primary mounting

fixed, drawout or plug-in design and exact service/test/disconnected positions;
phase-earth versus phase-phase connection and neutral/earth terminal;
primary fuse type/rating, striker/indication and replacement isolation;
contact engagement, shutters, interlocks and fuse/VT truck earthing;
terminal load, conductor flexibility and short-circuit support;
ventilation and temperature rise from core/winding losses;
safe fuse/VT withdrawal envelope, weight handling and stored charge;
secondary isolation to prevent back-energising the primary during testing;
ferroresonance/damping arrangement where system/study requires it.

Primary fuses limit some faults but do not remove the need for compartment short-circuit/IAC assessment. A fuse operation can produce hot gas, debris or an unbalanced VT condition. The mounting and barriers must accommodate the qualified device/fuse behaviour.

7. VT secondary circuits and backfeed control

provide secondary protective devices/disconnects sized and selectively coordinated;
define secondary neutral earthing and avoid unplanned parallel earths;
separate metering/protection windings and burdens as specified;
identify phase sequence, residual/open-delta connections and polarity;
interlock or procedure-control test plugs so injection cannot energise the primary;
check secondary cables/terminals for thermal and short-circuit duty;
use position auxiliary contacts that accurately represent primary and secondary connection;
prove discharge and absence of voltage before access.

8. Low-power sensors and digital interfaces

LPCT/LPVT or combined sensors reduce secondary energy but add signal-chain constraints. Control sensor orientation, rated transformation ratio, analogue output/cable, shielding/earthing, connector pinning, bandwidth and calibration data. Where digital outputs/merging units are used, coordinate IEC 61869-9/IEC 61850, time synchronisation, network redundancy and dataset/configuration management. Do not apply conventional CT open-circuit assumptions or test equipment blindly to LPIT outputs; follow the exact device test method.

9. Surge-arrester electrical selection

maximum continuous operating voltage at the arrester terminals;
system neutral earthing and temporary overvoltage magnitude/duration;
lightning/switching surge environment and insulation coordination;
rated voltage, continuous operating voltage and residual/protective voltage;
nominal discharge current and charge/energy capability;
repetitive duty, cable/transformer switching and harmonics where relevant;
short-circuit/failure mode and pressure-relief/disconnector behaviour;
environment, altitude, pollution and housing/interface;
equipment withstand levels and protective margin including leads.

Use IEC 60099-5 application guidance and a system insulation-coordination study; selecting an arrester merely because its rated voltage matches switchgear Um can lead to thermal runaway under TOV or inadequate protective margin.

10. Arrester placement and lead inductance

The protected terminal experiences arrester residual voltage plus dynamic voltage across connection inductance. Keep the line lead from protected conductor to arrester and the earth lead to the main earth path short, direct, separated from sensitive wiring and free of loops. Mount the arrester near the asset/interface it protects, subject to insulation and failure-containment needs.

avoid routing the earth lead around the compartment perimeter;
use adequate conductor area and mechanically supported terminals;
do not share a long/common inductive lead that couples phases;
bond monitoring/disconnector leads without adding harmful series length;
check earth-current path and enclosure forces;
preserve phase-earth/phase-phase clearance around the housing/terminals;
locate failure vents/disconnectors away from personnel and critical insulation.

11. Mechanical support and conductor loads

device dead mass and centre-of-gravity cantilever;
primary conductor thermal movement and assembly tolerance;
peak short-circuit electromagnetic reaction;
breaker/mechanism shock transmitted through frame;
seismic acceleration and modal amplification;
transport shock/vibration and temporary braces;
installation/maintenance tool and lifting loads.

Request allowable terminal and mounting loads from the device manufacturer. Use flexible links where needed, but validate their current, thermal, short-circuit and dielectric behaviour. Transfer brackets into main frame members; assess thin-sheet buckling, fastener pull-through and welds.

12. Dielectric geometry and barriers

rated power-frequency/impulse withstand and assembly insulation level;
phase-phase, phase-earth and longitudinal clearances at all positions;
device terminal field enhancement, shields and sharp hardware;
creepage, condensation, dust/pollution and material tracking;
altitude correction and atmospheric condition;
manufacturing/installation tolerance and fault/seismic movement;
barrier material, tracking/thermal/arc performance and fastening;
test leads/adapters and temporary configurations.

Do not count a grounded screen or resin surface as an arbitrary clearance shortcut. The complete device–conductor–barrier geometry must be covered by design and dielectric evidence.

13. Thermal and magnetic interaction

include CT/VT core and winding losses plus arrester leakage/temporary energy duty;
avoid trapped hot zones and blocked ventilation;
check neighbouring bus/conductor radiation and convection;
assess ferromagnetic brackets/plates around single-phase currents for eddy heating;
control CT stray-field influence and phase proximity;
maintain temperature within device accuracy/insulation and assembly limits;
validate substituted device mass/loss/size against temperature-rise evidence.

14. Accessibility, LSC and internal arc

define whether replacement requires bus/cable shutdown and how adjacent compartments remain safe;
provide isolation, proving-dead, discharge and earthing points;
ensure VT withdrawal/fuse access is interlocked and position-indicated;
keep CT terminal access on the safe side of partitions where feasible;
do not obstruct pressure-relief flaps/ducts or reduce compartment volume without assessment;
secure heavy devices/debris against internal-arc projectile risk;
restore covers, gaskets, partitions and shutters after maintenance.

Adding an arrester or changing from fixed to drawout VT can alter arc initiation location, pressure volume and gas flow. Evaluate IEC 62271-200 type-test applicability rather than assuming a smaller device is benign.

15. FAT and commissioning

Device	Key checks
CT	ID, ratio/taps, polarity, winding resistance/excitation where specified, secondary earth, shorting links and primary orientation
VT	Ratio/vector/polarity, winding/insulation tests, fuses, secondary protection, earthing, withdrawal/interlocks and backfeed control
LPIT	Sensor/IED pairing, scaling, polarity, cable/shield, calibration/configuration and time/network where digital
Arrester	Nameplate/rating, position, line/earth lead length, connection/earth continuity, monitor/disconnector and housing condition
Assembly	Fasteners, terminal loads, clearances, barriers, thermal/IAC features, schematics and protection functional tests

Apply only tests permitted by the manufacturer; inappropriate megger, withstand or injection voltage can damage LPIT electronics, arresters or connected relays. Isolate sensitive devices and discharge stored energy according to the approved plan.

16. Common design failures

Failure	Correction
CT fits window, so accepted	Verify zone/polarity, class/burden, fault duty, support and secondary safety
VT secondary isolated, primary assumed dead	Prevent backfeed and prove primary isolation/discharge
Arrester selected by Um only	Perform TOV, energy/charge and insulation-coordination study
Long coiled arrester earth lead	Use short direct low-inductance route
Heavy VT hung from terminal	Provide verified mounting/support and flexible primary link if needed
CT test terminals inside unsafe compartment	Provide segregated shorting-before-opening test access
Device substitution by dimensions	Reassess electrical, thermal, mechanical, dielectric and IAC influence

17. Required deliverables

device/interface and protection-function schedule;
instrument-transformer calculations: ratio, class, burden, saturation/transient and earthing;
VT protection, isolation/backfeed and ferroresonance assessment;
arrester selection/TOV/energy and protective-margin study;
dimensioned mounting, conductor, clearance and removal drawings;
terminal-load, bracket, short-circuit/seismic verification;
secondary wiring, test/shorting, shielding and grounding diagrams;
assembly type-test/extension-of-validity assessment;
FAT/SAT test plan, records and settings/configuration baseline;
safe maintenance/replacement method and approved spares.

References

Safety note: CT secondary opening, VT backfeed and arrester/primary access can cause lethal voltage. Qualified personnel must isolate, prove dead, discharge and earth the primary, control CT shorting and follow exact device/switchgear procedures before testing or maintenance.