Earthing Bus and Fault-Return Path Design for MV Switchgear Assemblies

The switchgear earthing bus is a deliberately engineered fault-current circuit, not a convenient copper strip for attaching green-and-yellow wires. Every credible fault must have a continuous, thermally and mechanically adequate, low-impedance path from the faulted conductive part to the station earthing system for the full protection clearing and backup duration.

This guide develops the complete path through enclosures, equipment frames, cable screens/armour, earthing switches, removable parts, shipping splits and external grid connections. It distinguishes protective bonding, functional/reference earths and high-current fault paths so that a continuity buzzer is never mistaken for a fault-duty proof.

Executive design rules

Draw each fault-return loop from the fault location through every joint to the grid; do not size only the main bar.
Use a network short-circuit study to determine current, current division, peak and duration. The three-phase bus fault is not automatically the governing earth-path case.
Verify both thermal I²t duty and peak electrodynamic forces, including joints and branches.
Do not credit painted sheet joints, hinges, door braids, rollers or incidental bolted contact unless specifically designed and verified.
Rate earthing-switch paths for their declared making/short-time duty and coordinate them with the assembly.
Make shipping-split earth joints accessible, keyed/identified, corrosion-controlled and independently inspected before energisation.
Keep protective-earth continuity independent of optional control wiring and removable covers where access could expose hazardous voltage.
Coordinate cable screen/armour bonding with cable ampacity, sheath-voltage, protection/CT and ground-fault studies.
Provide at least the specified robust connections from the lineup earth bus to the station grid; analyse actual current sharing.
After a major earth fault, inspect and test the complete affected path before return to service.

1. Standards and design boundary

Reference	Relevant role
IEC 62271-1:2017+AMD1:2021	Common switchgear specifications, earthing provisions, short-time/peak withstand framework
IEC 62271-200:2021+AMD1:2024	Earthing circuits, metal-enclosed assembly construction, compartments and type/routine verification
IEC 62271-102:2018+AMD1:2022	Earthing-switch ratings, making/operating duties and tests
IEC 60909-0:2026	Short-circuit current calculation for three-phase AC systems
IEC 60865-1:2011	Thermal and mechanical effects of short-circuit currents
IEC 61936-1:2021	Power-installation earthing and safety coordination above 1 kV AC
IEEE 837-2024	Permanent substation grounding connections when the IEEE scope/regime applies

IEC 62271 conformity of the assembly and the station earthing design are complementary. The switchgear supplier must declare and prove the internal earthing circuit and connection interface; the installation designer must prove connection to the site grid, touch/step safety and system fault-current distribution. IEEE 837 evidence for a permanent grounding connector does not automatically prove an internal switchgear joint or the whole return path.

2. Separate the functions before combining conductors

Function	Examples	Primary design concern
Protective bonding	Enclosure, doors/covers requiring bonding, breaker frame, CT/VT frames	Touch voltage and automatic fault clearance
Main fault-current path	Main earth bus, branch bonds, earth-switch path, grid leads	Peak force, I²t, joints and impedance
Cable metallic-screen/armour path	Screen wires, sheath, armour, gland bonds	Fault duty, induced/circulating current, sheath voltage
Functional/reference earth	Relay reference, VT secondary, EMC/shield drain	Noise, single/multiple-point architecture, dangerous transferred potential
Temporary operational earthing	Portable-earth studs or integral earthing switch	Making duty, sequence, access and interlocking

They may connect to a common earthing system at defined points, but their conductors, terminals and routing are not interchangeable. A small instrument-screen wire must not become a primary fault path; a high-current earth connection must not be routed through a relay reference terminal.

3. Map credible fault zones

main-bus or branch conductor to enclosure/partition;
cable termination or surge arrester to earth;
breaker pole/stab to moving truck frame;
VT, CT, fuse or auxiliary transformer primary fault;
earthing switch closing onto an energised circuit;
cable conductor-to-screen/armour fault;
control/auxiliary supply fault to a panel or door;
external cable/trench fault returning through the switchgear earth bus;
internal arc attaching to different metal parts as it moves.

For each zone draw a single-line or impedance network showing source, fault, parallel metallic routes and grid return. Identify what protection clears it and the maximum backup time if the primary relay, trip coil or breaker fails.

4. Determine current, division, peak and duration

Use IEC 60909-0:2026 (or the contractually specified method) for the system short-circuit study. Required outputs include initial symmetrical current, peak current, steady/decaying contribution where relevant, protection clearing time and backup duration. For earth faults, neutral earthing impedance, transformer winding connection, zero-sequence network, cable screens and parallel earth conductors can dominate.

Do not simply assign the assembly three-phase short-time rating to every earth conductor.
Do not assume two grid leads split current 50/50; impedance and connection location control division.
Include local motor/generator contribution and transferred current where relevant.
Consider the earth switch’s rated short-circuit making current when it can close onto an energised circuit.
Use the longest credible clearing/backup time, while respecting current decay and protection logic.
State whether the rating is based on a specified current/time pair and how alternative durations are assessed.

5. Thermal sizing: conductor plus joints

Adiabatic checks often take the form S ≥ I√t/k, where S is conductor area and k depends on material, initial/final temperature and insulation/environment assumptions. Use the applicable standard/project parameters; this compact relation is not a licence to ignore heat transfer, non-uniform current, joint hot spots or repeated duty.

select copper/aluminium/steel material and actual conductivity;
declare initial temperature, permissible final temperature and duration;
consider reduced section at holes, slots, bends and flexible links;
include contact resistance and heat at bolted shipping splits;
check plated/bimetallic transitions and aluminium oxide control;
verify branch conductors to equipment, screens and grid interfaces;
evaluate multiple/repeated faults if the operating duty requires them;
ensure nearby insulation, paint, seals and control wiring survive the temperature.

A thick main copper bar is irrelevant if the branch strap, stud neck, weld, washer contact or grid lug is the bottleneck. Document the minimum net section and joint temperature evidence.

6. Peak electrodynamic force and load path

Peak asymmetrical current creates forces between parallel/loop conductors and at bends. Use IEC 60865-1 principles or validated electromagnetic/structural analysis for the actual geometry. Check the entire chain:

earth bar bending and local stress around holes;
supports/insulators, spacing and fasteners;
branch take-offs and sharp direction changes;
flexible bonds, braid end fittings and crimp/weld zones;
shipping-split links and bolts;
external cable/lug and enclosure terminal plate;
enclosure frame and foundation anchors receiving reaction.

A flexible braid relieves movement but may have less short-circuit capacity or poor terminal stress distribution. Select it from tested/verified data and control bend radius, orientation and end termination.

7. Main earth bus architecture

Run a continuous main earth bus through the lineup with clearly defined ends and shipping joints.
Place it where branch paths are short, protected and inspectable.
Provide grid terminals sized for lugs, cable bending, tool access and fault reaction.
Use multiple grid connections where required for redundancy/current distribution, but analyse them.
Avoid creating large inductive loops between outgoing and return paths where geometry can be improved.
Maintain separation/identification from neutral conductors unless a deliberate, documented connection is required.
Provide test points that do not require dismantling a current-carrying joint.
Preserve clearance from live conductors despite fault movement and manufacturing tolerance.

8. Enclosure, partitions, doors and removable parts

Metallic enclosure parts must be bonded according to their accessibility and fault exposure. Do not assume:

painted bolted seams give durable low-impedance contact;
door hinges are protective conductors;
sheet-metal screws withstand primary fault current;
breaker wheels/rails provide the sole truck earth;
a removable roof/cover remains earthed while loosened;
an internal arc will attach only near the main bar.

Use dedicated bonds/contact devices where required. For withdrawable equipment, protective-earth engagement should occur before primary connection and remain until after primary disconnection, with geometry and wear verified. Door braids must suit repeated flexing and be protected from sharp edges; doors bearing only low-voltage devices may still require bonding under the applicable construction/safety design.

9. Earthing-switch current path

Declare rated short-time withstand current/duration and rated short-circuit making current as applicable.
Trace the path from each phase blade/contact through shaft/arms or dedicated conductors to the main earth bus.
Do not credit bearings or mechanism pivots as current paths unless designed/tested for it.
Check simultaneous pole geometry, closing speed and contact bounce.
Coordinate interlocks, position indication and key logic with the operating procedure.
Verify conductor/support forces and contact condition after making-duty tests.
Provide visible indication or reliable indication in accordance with the qualified design.

Earthing switches used only after proving dead still require rated capability as specified; procedural intent does not eliminate foreseeable energised-closing duty where the switch is classified for making.

10. Cable screens, sheaths and armour

Coordinate bonding with the cable system designer. Solid bonding, single-point bonding and cross-bonding have different sheath voltages, circulating currents and earth-fault paths. Within the switchgear:

rate screen wires/sheath/earth leads and their terminals for allocated fault current/time;
avoid tight coils/loops in earth leads that add inductance and force;
control armour gland/contact preparation and corrosion;
make test links accessible, labelled and safe against inadvertent open-circuit hazardous voltage;
coordinate core-balance CT placement and screen-earth return so intended residual current is measured;
separate surge-arrester earth leads and keep them short/low inductance as required;
document parallel routes through trench steel and station grid.

11. Joints, materials and corrosion

Remove paint/oxide only by the controlled process and protect the finished interface.
Use compatible copper/aluminium/plated transition systems; control galvanic exposure and moisture.
Specify bolt grade, washer arrangement, lubrication condition and preload/torque process.
Do not stack uncontrolled multiple lugs under one bolt.
Use locking that preserves preload rather than damaging the contact surface.
Mark completed joints and retain torque/tool calibration records.
Provide inspection access; hidden joints require stronger process/evidence controls.

Contact resistance, preload, creep, plating and environmental sealing determine long-term reliability. A visual copper-to-copper overlap is not a quantified joint design.

12. Verification and tests

Verification	Purpose and caution
Design review/calculation	Fault map, current division, I²t, peak force, touch/step interface and joint capacity
Short-time/peak test evidence	Proves represented assembly path; inspect all joints/supports after test
Earthing-switch making test evidence	Applicable to declared switch/design and duty
Protective-circuit continuity test	Detects missing/open bonds; does not alone prove high-current withstand
Low-resistance/millivolt-drop test	Baseline and joint consistency under defined current/temperature
Visual/torque/process records	Material, surface, fastener and shipping-joint quality
Site grid test/study	External connection and touch/step safety; separate from factory proof

Use four-wire measurements where appropriate, with probe locations, test current, temperature and acceptance method defined. A handheld buzzer confirms only that some conducting path exists. Conversely, a universal micro-ohm limit copied from another design can reject a sound long path or accept a dangerous short parallel path. Compare to design values and consistent baselines.

13. Shipping splits and site commissioning

Identify every split earth link on drawings and physically tag it.
Protect mating surfaces during transport; inspect for corrosion/damage.
Align the lineup before tightening links so they are not forced or pre-stressed.
Use specified hardware, surface preparation, compound and torque/preload.
Witness/record each joint before covers obstruct access.
Connect all specified station-grid leads and verify phase/neutral/earth segregation.
Test branch continuity and end-to-end path using defined points.
Confirm breaker-truck protective-earth timing, door bonds and earthing-switch path.
Update as-built drawings and baseline readings.

14. Post-fault and lifecycle assessment

identify the actual fault path and protection clearing record;
inspect discoloration, annealing, melting, braid damage and mechanical displacement;
check bolts/joints for preload loss and arcing;
inspect supports, welds, frames, doors and anchors for force damage;
measure continuity/resistance against baseline with the same method;
function-test earthing switch, shutters/interlocks and withdrawable-earth contacts;
assess cable-screen/armour and grid connections;
replace parts to approved criteria and repeat required dielectric/functional tests.

15. Design/procurement checklist

governing standards/editions and system earthing data;
fault-zone and protection-clearing matrix;
maximum earth current, peak, duration and division assumptions;
main bar/branch/flexible-bond/joint calculations;
earthing-switch ratings and complete current-path evidence;
enclosure/removable-part protective-bonding scheme;
cable-screen, armour, CT and surge-arrester bonding drawings;
grid terminal quantity/location/capacity and external responsibility boundary;
type/routine evidence and extension-of-validity justification;
shipping-split installation/inspection/test procedure;
baseline readings, maintenance and post-fault inspection criteria.

References

Safety note: Earthing systems can carry lethal fault or induced current and may rise to dangerous potential. Inspection, testing and temporary-earth work require qualified personnel, an approved isolation/proving-dead/earthing procedure and coordination with the station grid design.