Phase Spacing, Barrier Geometry and Dielectric Clearances in MV Primary Design

There is no universal phase-spacing table that proves an MV switchgear design. Required dielectric performance comes from system insulation coordination and rated withstand voltages; the physical geometry must then withstand power-frequency and impulse stress in its actual enclosure, material, pressure, altitude, contamination and tolerance state.

This article gives a rigorous primary-design workflow for air, gas, solid-interface and barrier-assisted insulation in IEC 62271 assemblies. It separates clearance from creepage, explains field enhancement and triple points, and shows how to validate simulations and design variants without misusing outdoor-insulator guidance.

Executive conclusions

Select rated withstand voltages from the insulation-coordination study and the relevant product standard before choosing physical spacing.
IEC 60071-1 coordinates phase-earth, phase-phase and longitudinal insulation; it does not prescribe human safety clearances.
Clearance is the shortest distance through the insulating medium; creepage is the shortest distance along a solid insulating surface. A barrier changes both field/path geometry but does not automatically add its thickness to air clearance.
Power-frequency and lightning-impulse stresses distribute differently; the worst location/polarity can differ.
Sharp conductor edges, bolt ends, slots, screen terminations, floating metal and conductor-insulator-earth triple points can dominate over nominal phase-center spacing.
Air dielectric strength changes with atmospheric conditions and altitude. IEC 62271-1 normal service conditions and IEC 60060-1:2025 correction procedures must be applied appropriately.
Pollution/condensation can convert a surface into the governing path. IEC TS 60815:2025 concerns outdoor insulators and is not a plug-in indoor-switchgear creepage table.
Barriers can improve performance but can also concentrate field at their edges, trap moisture/dust, block cooling and interfere with pressure relief/IAC.
Numerical field analysis is a design tool, not proof. Validate material properties, mesh and boundary conditions against representative dielectric/PD tests.
Check tolerance stacks and every operating position: service/test/disconnected, shutters, cable terminations, earthing switch, removable links and short-circuit deflection.
Changes in gas, barrier material/shape, conductor edge, insulator, CT/VT or enclosure require IEC TR 62271-307:2024 applicability review.

1. Standards map

Reference	Purpose	Important boundary
IEC 60071-1:2019	Select standard rated withstand voltages for phase-earth, phase-phase and longitudinal insulation above 1 kV	Insulation coordination only; human safety not covered
IEC 60071-2:2023	Application guidance, overvoltages, protective devices and detailed simulation	Product committees specify equipment tests/values
IEC 62271-1:2017+AMD1:2021	Common HV switchgear service/rating/design/test requirements	Relevant product standard can add/modify
IEC 62271-200:2021+AMD1:2024	MV metal-enclosed assembly requirements/tests above 1 kV through 52 kV	Classification/test applies to represented assembly
IEC 60060-1:2025	HV AC/DC/impulse test terminology and requirements	Test-technique standard, not product spacing table
IEC TR 62271-307:2024	Guidance for extending validity of type tests after changes	Requires documented technical comparison
IEC TS 60815 series:2025	Outdoor HV insulator selection/dimensioning in pollution	Not directly a generic indoor MV switchgear rule

2. Start with insulation coordination

define highest voltage for equipment Um, system frequency and earthing;
identify temporary, slow-front, fast-front/lightning and very-fast-front stresses as applicable;
model incoming overhead line/cable, transformer, capacitor, reactor and switching phenomena;
select/location-rate surge arresters and protective level/margin;
select standard rated withstand voltages under IEC 60071 and product standard;
distinguish phase-earth, phase-phase and longitudinal/open-gap insulation;
specify normal/special service conditions including altitude/pollution/condensation;
translate ratings into the equipment dielectric test schedule;
design geometry/materials to pass with manufacturing/ageing margin.

Choosing 24 kV switchgear because the network is 20 kV is not the complete coordination. Confirm the required power-frequency and lightning-impulse withstand set, arrester coordination and open-device longitudinal duty.

3. Four insulation paths

Phase to earth: conductor to enclosure, partition, support hardware or earthed screen.
Phase to phase: adjacent phase conductors, terminals or accessories.
Longitudinal: across an open switching/isolating gap between energized systems.
Surface/interface: along solid insulation or across solid-gas/solid-air junctions.

Each path has different geometry and overvoltage distribution. The shortest geometric distance may not be the highest electric-field path, particularly near electrodes and dielectric interfaces.

4. Clearance versus creepage

Quantity	Path	Driven by	Typical misconception
Clearance	Shortest path through air/gas/liquid	Electric field, medium density, waveform, geometry and atmospheric/service conditions	Center-to-center phase spacing equals clearance
Creepage	Shortest path along a solid insulating surface	Surface material, pollution, moisture, profile, orientation and field	More ribs always improve performance
Solid thickness	Through dielectric material	Dielectric strength, void/PD, ageing, thermal/mechanical stress	Datasheet kV/mm × thickness proves the assembly
Combined path	Air/gas + surface/solid interfaces	Permittivity, triple points, screens and field grading	Distances can simply be added arithmetically

5. Why nominal phase spacing misleads

bar thickness/width and orientation change nearest surfaces;
bends, tee plates and risers can project closer than straight bus;
bolt heads, washers, threads and split pins become electrodes;
CT/VT terminals, cable lugs and separable connectors alter geometry;
enclosure ribs, weld studs and door hardware reduce phase-earth distance;
insulator inserts/screens create high-field interfaces;
bar sag, thermal bow and short-circuit displacement reduce clearance;
manufacturing and installation tolerances accumulate;
bar edge radius changes peak field at the same spacing.

Create a three-dimensional minimum-distance/field map for every compartment and operating state. A single plan-view dimension is not a dielectric design.

6. Field enhancement at electrodes

sharp bar corners and poorly controlled edge radii;
cut/blanked burrs and plating spikes;
exposed bolt threads and abrupt step in joint plates;
small-radius cable-lug palm/connector screen ends;
floating metal labels, clips, springs or unused inserts;
shield/semicon termination not at designed location;
thin barrier edges near energized conductor;
air voids in cast insulation and adhesive interfaces.

Control by smooth electrode contours, adequate radius, screened/graded interfaces, consistent hardware orientation, removal of burrs and elimination/earthing of floating metal. Never solve a field problem solely by increasing nominal phase-center distance while leaving the local electrode unchanged.

7. Triple points and partial discharge

A triple point occurs where conductor/electrode, solid insulation and gas/air meet. Permittivity differences concentrate field in the lower-permittivity medium; microscopic gaps or contamination can initiate partial discharge (PD) below bulk flashover voltage.

recess or screen the metal edge as the insulation system requires;
avoid air gaps beneath bonded insulation or around inserts;
control casting/curing, voids, moisture and surface finish;
use stress grading for cable/separable-connector interfaces;
prevent conductive dust at high-field edges;
define PD test/acceptance where applicable to the component/assembly/specification;
investigate PD phase pattern/location rather than accepting one aggregate number;
include operating voltage, temperature, pressure and sensor bandwidth in interpretation.

8. Barrier functions

intercept a direct air flashover path;
increase/reshape creepage or combined path;
redistribute electric field through permittivity/geometry;
separate pollution/condensation zones;
provide partitioning/contact protection and direct arc products;
support conductors or screens where designed.

A barrier is an electrical component, not simply plastic sheet. Specify material, thickness, CTI/tracking/erosion behavior as relevant, flammability/fire products, thermal rating, moisture absorption, mechanical strength, ageing, edge radius, mounting, tolerances and cleaning compatibility.

9. Barrier failure modes

field concentration at edge or fixing hole;
surface tracking from dust/condensation;
void/delamination/poor adhesive bond;
warping that reduces clearance;
cracking from short-circuit pressure or thermal ageing;
blocked airflow causing conductor/insulation overheating;
trapped moisture/inaccessible contamination;
unintended floating fastener or screen;
obstruction of IAC pressure-relief path;
incorrect field replacement material/thickness/orientation.

10. Air-insulated design

use rated withstand/test evidence, not a generic spacing table;
apply altitude/atmospheric correction to external air insulation as required;
control edge radius, hardware and conductor surface;
account for enclosure walls/partitions and door deformation;
consider dust, condensation, insects and conductive fibers;
provide heaters/ventilation/anti-condensation controls without creating hot spots;
maintain access for inspection/cleaning while preserving LSC/IAC;
check clearances at maximum bar short-circuit deflection.

11. Gas-insulated and sealed-pressure design

dielectric strength depends on gas composition, density/pressure and temperature;
design for minimum functional/lockout density, not nominal fill alone;
verify mixture condensation/ratio and low-temperature behavior;
control particles, protrusions and cleanliness during sealed manufacture;
grade field at spacers, inserts and interfaces;
monitor pressure/density and define alarm/lockout behavior;
assess leakage, moisture and service gas handling;
do not transfer SF6 geometry to dry air/alternative gas without validated analysis/test;
separate atmospheric-altitude effects on external interfaces from sealed internal insulation.

12. Solid-insulated interfaces

bulk dielectric strength is not the same as PD-free service strength;
voids, delamination, cracks and conductive contamination are critical;
thermal expansion mismatch drives interface stress;
casting inserts/screens must be positioned and earthed as designed;
surface flashover at exposed ends can govern;
ageing from heat, PD, moisture and mechanical cycles must be considered;
repairability and diagnostic access can be limited;
process control and production dielectric/PD tests are essential where specified.

13. Altitude and atmospheric conditions

IEC 62271-1 normal service conditions include an altitude limit; above the applicable normal value, external air insulation may require correction or a higher withstand design. IEC 60060-1:2025 provides atmospheric-correction principles for HV tests. Engineering must distinguish:

air clearance outside sealed gas/solid insulation;
internal sealed-gas density maintained by enclosure;
external bushings/cable boxes and open disconnector gaps;
reduced air cooling at altitude, which raises temperature and can affect dielectric ageing;
test-site versus service-site pressure/temperature/humidity;
manufacturer altitude rating and any derating/modified test voltage.

Do not multiply every internal dielectric distance by one altitude factor. Apply the product/insulation-system method to the affected air paths and verify temperature-rise consequences separately.

14. Pollution and condensation

conductive dust, salt, cement, carbon/metal fibers and industrial films;
high humidity, temperature cycling and dew-point crossing;
water paths from roof, cable trench, ventilation or cleaning;
heater failure and cold-soak energization;
surface orientation that retains contamination;
creepage profile too tight to clean/inspect;
hydrophobicity/material ageing and tracking/erosion;
vent/filter strategy and room pressure/housekeeping.

IEC TS 60815-1/-2/-3:2025 provide methods for outdoor insulators in polluted conditions. Their scope must be respected; use product tests, service experience and a project environmental assessment for indoor enclosed switchgear rather than importing outdoor unified-specific-creepage values without justification.

15. Moving and temporary geometries

breaker/service/test/disconnected positions;
shutter opening/closing and incomplete travel;
disconnector/earthing-switch open gaps and blades;
racking misalignment and worn guide rails;
portable test plugs/ground-and-test devices;
cable test with surge arrester/VT disconnected;
temporary barriers and tools;
busbar thermal expansion and fault deflection;
transport/installation before final bracing.

Check longitudinal/open-contact insulation and position indication/interlocks together. A nominal disconnected position is not a dielectric calculation unless contact gaps, shutters and tolerances are verified.

16. Tolerance stack

Source	Potential dielectric effect
Bar width/thickness/bend angle	Nearest surface/edge shifts
Support/insert/hole position	Phase and phase-earth spacing changes
Enclosure/partition flatness	Earthed surface approaches conductor
Joint plate/bolt orientation	Local electrode radius/distance changes
Insulator shrinkage/warp	Creepage/field/triple-point geometry changes
Breaker/bushing alignment	Disconnect contact and shutter clearance changes
Cable lug/connector assembly	Screen termination and phase spacing shift
Thermal/fault movement	Dynamic minimum clearance

Use worst-case or statistically justified stack consistent with manufacturing control. Do not combine all nominal dimensions in the favorable direction.

17. Electric-field simulation workflow

select power-frequency/impulse/longitudinal boundary conditions from the coordination study;
build actual 2D/3D electrodes, enclosure, barriers, insulators, screens and terminals;
assign frequency/temperature-dependent permittivity/conductivity where material;
define conductor/earth/floating boundaries correctly;
refine mesh at edges/triple points without interpreting mathematical singularities literally;
check equipotential/field vectors and energy/equilibrium sanity;
run polarity, phase combination, position and tolerance extremes;
include pressure/density/atmospheric case through validated breakdown criterion;
compare with known analytical geometry and tested reference design;
correlate model hotspots with dielectric/PD test observations;
document software/version, mesh, assumptions, uncertainty and acceptance basis.

18. Simulation pitfalls

using ideal sharp corners that generate nonphysical infinite field;
omitting bolts, screens, cable lugs or earthed enclosure details;
treating floating metal as earth or ignoring it;
using one material permittivity without tolerances/temperature;
applying uniform breakdown kV/mm to a nonuniform field;
ignoring surface/interface/PD mechanisms;
modelling nominal geometry only;
claiming pass because peak field is below a supplier datasheet value;
using a 2D section for a three-dimensional tee/bend/edge;
not validating against representative type tests.

19. Dielectric test plan

exact IEC 62271 edition, test duty and applicable IEC 60060-1:2025 technique;
power-frequency withstand phase-earth/phase-phase and auxiliary circuits as required;
lightning impulse polarity/sequence and longitudinal configuration as required;
representative worst panel, bus, CT/VT, breaker, cable/plug and barrier geometry;
minimum gas pressure/density and specified temperature state;
shutters, removable parts and devices in required positions;
altitude/atmospheric correction and test-site measurements;
PD test where specified/applicable, with background noise and calibration;
flashover/puncture/PD location and post-test inspection;
report photographs/drawings and offered-versus-tested applicability.

20. Production controls

bus/barrier/insulator material and drawing revision;
edge radius, burr and surface/plating inspection;
phase/earth distance gauges at defined critical points;
support, insert and screen position;
clean-room/particle/moisture controls for sealed insulation;
barrier orientation, fastener and torque;
gas identity, purity, moisture, ratio, fill density and leak test;
routine power-frequency/PD tests per product/specification;
controlled repair/cleaning/rework and full relevant retest;
serial traceability to dielectric routine record.

21. Installation and lifecycle controls

room/environment within specified pollution/humidity/temperature/altitude;
shipping braces/foreign materials removed without damaging barriers;
field bus splices/lugs oriented to retain clearances;
cable screen/semicon/termination length per manufacturer;
heaters, ventilation, filters and dew-control alarms operational;
enclosure/duct seals preserve IP/IAC without trapping moisture;
visual/PD/thermographic condition program appropriate to design;
cleaning materials/method compatible with insulation;
investigate tracking, corona marks, deposits, cracks or abnormal PD;
review dielectric evidence after retrofit, fault or water ingress.

22. Design-change triggers

rated voltage/withstand set or arrester coordination;
phase order, bar width/orientation, bend/tee/riser;
edge radius, bolt, joint plate or hardware finish;
barrier material/thickness/profile/fixing;
insulator material/profile/insert/screen;
gas composition/pressure/temperature range;
CT/VT/sensor/breaker/cable termination;
enclosure/partition/vent/pressure-relief geometry;
altitude/pollution/condensation/service condition;
support spacing or short-circuit deflection;
manufacturing process/factory or supplier.

Use IEC TR 62271-307:2024 for a structured extension assessment and retest when the new geometry/medium is not demonstrably bounded.

23. Common mistakes

using one universal phase-spacing table;
confusing center distance with minimum clearance;
adding barrier thickness arithmetically to air distance;
treating creepage and clearance as interchangeable;
using material bulk kV/mm as assembly proof;
ignoring bolt edges, triple points and floating metal;
applying outdoor IEC TS 60815 values directly to indoor switchgear;
applying one altitude multiplier to every insulation path;
checking nominal service position only;
ignoring tolerance, thermal bow and short-circuit movement;
accepting an unvalidated colorful FEA field plot;
changing gas/barrier/insulator without type-test applicability review.

Minimum design dossier

insulation-coordination study and selected withstand voltages;
service altitude, environment, pollution/condensation specification;
3D clearance/creepage/tolerance map for every state;
material, gas, pressure and barrier/insulator specifications;
field/PD analysis and validation report;
short-circuit displacement/clearance calculation;
dielectric type/routine reports and complete test object identity;
offered-versus-tested/IEC TR 62271-307 assessment;
production inspection/test plan and gauges;
installation/cleaning/condition-monitoring limits and change triggers.

Primary references

Safety/engineering note: Dielectric design and HV tests require qualified engineers, controlled laboratories and approved test safety procedures. IEC insulation coordination does not define personnel approach/work clearances.