Magnetic-Field and Eddy-Current Effects in MV Switchgear Enclosures

In MV switchgear, a steel enclosure can become a short-circuited turn, a lossy magnetic core or an electrodynamic force path. Poor conductor/plate geometry creates local hot spots, current derating, vibration and unintended circulating currents even when the copper busbar is correctly sized by DC resistance.

This article explains magnetic-field, skin/proximity and eddy-current mechanisms around main buses, branches, single-core cables, gland plates, partitions, bolts and frames, then gives an analysis–test workflow for controlling them.

Executive rules

Analyse the complete AC current loop and nearby conductive structure; DC bus resistance cannot predict AC loss.
Keep phase and return conductors geometrically compact/balanced where dielectric and short-circuit requirements permit.
Never pass individual high-current single-core AC conductors through independent closed ferromagnetic apertures without analysis.
Use a proven non-magnetic plate or a common opening/slot arrangement when required to break magnetic loops.
Check bolts, frames and partitions as possible loop-closing members—not only the obvious plate.
Include skin/proximity loss in busbars, joint hardware, flexible links and parallel paths.
Thermal hot spots often occur at aperture edges, seams and fasteners rather than on the conductor.
Short-circuit peak fields impose transient forces in both conductors and induced-current structures.
Validate 3D electromagnetic/thermal models with temperature, current or magnetic-field measurements under representative current and phase balance.
Control material grade, permeability, conductivity, coating, seams and assembly details in production.

1. Standards and evidence boundary

Reference	Relevant role
IEC 62271-1:2017+AMD1:2021	Continuous-current temperature limits, service conditions and common tests
IEC 62271-200:2021+AMD1:2024	Assembly temperature-rise, short-time/peak withstand and enclosure design evidence
IEC TR 60943:1998+AMD1:2008	Guidance on permissible temperature/temperature rise for electrical parts/connections
IEC 60909-0:2026	Short-circuit current and peak-current basis
IEC 60865-1:2011	Mechanical/thermal effects of short-circuit currents
IEC 60287-1-1:2023	Cable current-rating/loss equations; useful cable-system context, not enclosure proof
IEC TR 62271-307:2024	Assessment after influential assembly changes

Product standards require assembly performance; they do not give a universal safe distance from a busbar to steel or a universal plate alloy. Geometry, material and current determine the result. Cable-loss formulae cannot be substituted for switchgear enclosure temperature-rise evidence.

2. The physics in practical terms

Skin effect: alternating current concentrates toward conductor surfaces, raising effective resistance.
Proximity effect: nearby phase/return fields redistribute current, often crowding it on particular faces/edges.
Eddy current: changing flux induces closed currents in plates, frames, bolts or screens; I²R becomes heat.
Hysteresis: cyclic magnetisation of ferromagnetic material adds loss.
Circulating current: parallel bonded conductive paths form loops driven by induced voltage.
Electromagnetic force: current/field interaction produces vibration, attraction/repulsion and fault-force reaction.

At power frequency, increasing frequency, current, permeability, conductivity, loop area and flux linkage generally increases concern. Thin material is not automatically safe: current density can concentrate along narrow edges or fastener paths.

3. Map current loops and flux linkage

Draw phase and return current paths for normal, unbalanced, earth-fault and test conditions.
Identify every nearby conductive loop: gland plate, frame, partition, door, cable screen, earth bus and structural steel.
Mark where seams, bolts, welds or bonds close/open each loop.
Estimate field concentration at bends, phase separation, apertures and end regions.
Assign material conductivity, magnetic permeability and temperature dependence.
Calculate AC loss/induced current and couple it to the thermal model.
For short circuit, calculate peak current/force and verify structural reaction.

Model the actual return path. Three balanced phases close together give much lower external field than widely separated conductors or a return conductor routed elsewhere. Residual/zero-sequence current produces different enclosure flux from balanced positive-sequence current.

4. Main-bus phase geometry

flat, vertical, triangular/trefoil or enclosed phase arrangements have different fields/loss/current sharing;
phase spacing balances dielectric clearance against force and external field;
multiple bars per phase need controlled spacing/transposition/current distribution;
steel roof, rear sheet or partition proximity can heat asymmetrically;
end panels and bus bends interrupt symmetry and may govern hot spots;
bus section joints/tees/flexible links change local current density;
ventilation alters temperature but does not remove electromagnetic loss.

Do not extrapolate a temperature-rise test from a symmetric straight bus to a narrow variant with a bend close to steel. Use validated computation or additional evidence and assess extension under IEC TR 62271-307.

5. Busbar AC resistance and current sharing

For the same area, one thick bar can have different AC loss from several thinner spaced laminations. Adjacent phase fields and parallel-path geometry can make equal DC resistances carry unequal AC current. Analyse:

skin depth relative to bar thickness;
proximity between same-phase bars and other phases;
joint/link current distribution and contact geometry;
edge/corner crowding at bends and bolt holes;
ferromagnetic hardware near high-field zones;
harmonics, which increase skin/proximity/eddy loss;
temperature feedback as conductor resistance rises.

6. Single-core cable entry through metal

A single AC conductor through a closed ferromagnetic opening links substantial flux around that opening, inducing circulating current and hysteresis loss. Practical solutions include:

all three phase conductors through one common opening;
non-magnetic conductive gland plate of proven alloy/thickness;
radial slot from individual apertures to a plate edge or between holes to interrupt the loop;
non-metallic qualified entry system where mechanical/IP/fire/IAC needs allow;
balanced phase grouping with verified spacing and cleats.

The slot must remain electrically open. A steel cover, reinforcing angle, cable gland, braid, bolt row or field-added seal can bridge it and recreate the loop. Non-magnetic stainless grades can still conduct eddy currents; “non-magnetic” is not “lossless.” Check plate heating and structural/pressure/cable-cleat duties.

7. Gland plates, cable screens and armour

coordinate solid/single-point/cross bonding and screen circulating-current study;
avoid accidental parallel armour/gland/enclosure loops;
keep screen-earth conductors short and correctly routed through core-balance CTs;
rate glands/plates for cable short-circuit reaction and IAC/IP boundary;
check induced heating in metallic cleats, brackets and trench steel;
maintain corrosion compatibility at aluminium/stainless/steel/copper interfaces;
measure representative plate/gland temperatures at rated current.

8. Ferromagnetic fasteners and structural members

Bolts, washers, clamps and frame members may be small but sit in intense local fields. Heating can degrade coating/gaskets, loosen joints or damage insulation. Controls include relocating the loop, using a qualified non-magnetic material set, breaking the conductive path or reducing flux linkage. Do not replace structural fasteners merely for magnetics without rechecking strength, galling, corrosion, fire and preload.

9. Partition and enclosure seams

continuous welds can create strong circulating-current loops;
bolted seams have distributed nonlinear contact and uncertain current paths;
insulated breaks may reduce induced current but affect protective earth/IAC strength;
doors/hinges/braids can form secondary loops;
vent openings and folds concentrate current at narrow ligaments;
pressure-duct or bus-duct steel may couple to bus fields;
shipping-split changes can alter loop continuity.

Protective earthing must remain reliable. Never interrupt a required fault-return path to solve eddy heating; redesign geometry or provide a deliberate low-impedance protective bond that does not recreate the harmful flux-linked loop.

10. Short-circuit transient effects

IEC 60909 peak current drives the highest electrodynamic event. Induced currents in screens/enclosures interact with conductor fields and may modify force distribution. Check:

busbar/support forces and enclosure reaction;
plate/slot edge force and fastener load;
cable cleat/gland plate reaction;
temporary magnetic attraction of ferromagnetic parts;
minimum dielectric clearance during dynamic displacement;
thermal I²t in unintended parallel paths;
post-test/post-fault residual distortion and joint condition.

11. EMC and secondary-circuit implications

Power-frequency magnetic fields and high di/dt transients can induce voltage in secondary loops. Keep CT/VT, trip and communication routing away from high-field bus apertures; minimise loop area, use paired/twisted conductors and apply the project shield/earth scheme. Do not solve a primary eddy-current issue by adding uncontrolled shields that create a new secondary-current path. LPIT/merging-unit placement requires device EMC evidence and assembly routing discipline.

12. 3D electromagnetic–thermal modelling

Input/model feature	Required control
Conductors	Actual geometry, joints/links and phase-current magnitude/angle/harmonics
Materials	Conductivity/permeability versus temperature; nonlinear steel where relevant
Enclosure	Sheets, apertures, seams, fasteners, slots and bonds
Mesh	Skin depth, edges, gaps and local hot spots; convergence study
Thermal	Loss transfer, contacts, convection/radiation, vents/fans and ambient
Boundary	Full current return and enough surrounding field domain
Outputs	AC resistance/loss, induced current, flux density, force and temperature

Check energy/current balance and sensitivity to material permeability, joint contact and mesh. A 2D cross-section can screen long uniform regions but misses bends, ends, slots, joints and asymmetric return paths.

13. Measurement and test validation

temperature-rise test at rated current, frequency, ambient and final enclosure configuration;
thermocouples/fibre sensors on predicted plate/fastener hot spots without disturbing fields;
thermal imaging with emissivity/reflection/access limitations documented;
Rogowski/current probes on accessible induced-current links where appropriate;
magnetic field probes at defined points and phase/current conditions;
phase-current balance, conductor terminal temperature and ventilation status recorded;
steady-state criterion and permissible temperatures per applicable parts/materials;
model/test correlation and conservative variant rules.

Clamp meters can miss distributed sheet currents and may be influenced by strong fields. Surface thermography can miss concealed interface heating. Use complementary methods and an uncertainty budget.

14. Production and site controls

material grade/permeability/conductivity and thickness;
slot/opening geometry and unbridged condition;
fastener material, washer stack and location;
bus/cable phase formation, spacing and support;
partition/weld/shipping-split continuity;
earth-bond location and resistance;
no field-added steel plates/clamps across designed breaks;
thermal sensor/inspection points and as-built photos;
engineering review for cable-number, current, harmonic or enclosure changes.

15. Frequent mistakes

Mistake	Correction
Copper sized by DC ampacity only	Calculate AC/proximity loss and validate temperature rise
Each single-core cable through steel hole	Use analysed common/non-magnetic/slotted entry
Slot bridged by cover/bolt/gland	Audit the complete assembled loop
“Stainless means no eddy current”	Use actual permeability and conductivity
Hot steel cured with more ventilation	Reduce flux linkage/loss source first
Remove earth bond to break loop	Preserve protective fault path; redesign geometry
Straight-bus test covers every variant	Assess bends, ends, narrow panels and material changes

16. Design-review deliverables

normal/fault/harmonic current and return-path definition;
conductive/magnetic loop map and material schedule;
AC bus/cable loss and current-sharing calculation;
3D electromagnetic/thermal model and sensitivity report;
short-circuit force/reaction assessment;
gland plate/slot/non-magnetic fastener drawings;
temperature-rise instrumentation/test/correlation report;
production inspection and site change-control checklist;
type-test extension/variant matrix;
maintenance hot-spot baseline and alarm/inspection criteria.

17. Harmonics, unbalance and abnormal operating states

Temperature-rise tests at balanced 50/60 Hz do not automatically bound converter-rich or severely unbalanced service. Higher-order currents see greater skin/proximity impedance and can disproportionately raise plate/fastener losses. Zero-sequence current produces a field/return path unlike a balanced three-phase set.

obtain phase and neutral harmonic spectra for normal and contingency operation;
check neutral/earth conductor and enclosure currents during unbalance and ground fault;
consider one/two parallel cable circuits out of service, which changes symmetry;
analyse open phase, transformer energisation and single-pole test-current configurations where credible;
state whether the certified continuous-current rating assumes sinusoidal balanced current;
set operating/monitoring limits when the actual spectrum lies outside validation.

Measure total RMS plus meaningful harmonic components and phase angles during commissioning or a representative load period. If service differs materially from the design spectrum, update the coupled model and thermal margin rather than applying an arbitrary percentage derating.

References

Safety note: Magnetic/thermal measurements near energised MV equipment require a qualified test plan, rated sensors, safe optical/isolation methods and arc-flash boundaries. Never add probes, open covers or alter bonds on energised switchgear outside approved tested arrangements.