EMC-Compliant Secondary Wiring: Shielding, Segregation and Earth Connections

EMC compliance of an IED type test does not guarantee a reliable installed protection scheme. Cable loops, shield pigtails, mixed trays, weak bonding, coil transients and unintended multiple signal earths can couple disturbances into perfectly compliant relays. Installation EMC must control source, coupling path and victim while preserving electrical safety and protection accuracy.

This guide covers MV switchgear secondary wiring: zoning, segregation, routing, shielding, equipotential bonding, CT/VT earthing, DC coil suppression, analogue/binary/network circuits, panel entry and FAT/SAT troubleshooting.

Executive rules

Classify disturbance source, coupling mechanism, victim and functional consequence before choosing a shield or filter.
Minimise loop area and shared impedance; route outgoing and return conductors together.
Separate high-energy/noisy power and coil circuits from CT/VT, low-level analogue and communications by engineered distance/barrier/route.
Specify shield termination by signal bandwidth, common-mode/earth-potential conditions and equipment instructions—there is no universal “one end only” rule.
Bond panels/doors/gland plates with short, wide, low-inductance connections suitable for the disturbance frequency.
Keep protective earth, functional/equipotential bond, cable shield and CT/VT secondary earth conceptually distinct.
Use fibre where galvanic isolation and high-frequency immunity materially reduce risk, while protecting transceiver power and routing.
Verify installation under representative switching, radio, ESD and failure conditions; continuity alone is not EMC proof.

1. Standards hierarchy

Reference	Application
IEC 60255-26:2023	EMC emission/immunity requirements and tests for protection schemes including control, monitoring, communication and process interfaces
IEC 61000-6-5:2015	Generic immunity for equipment used in power-station/substation environments
IEC 61000-4-18:2019	Damped oscillatory wave immunity phenomena particularly relevant to HV/MV substation cables
IEC 61000-5-6:2024	Installation/mitigation guidelines for external electromagnetic influences
IEC 61850-3:2013	General/environmental requirements for utility communication/automation IEDs aligned with protection-equipment EMC/safety
IEC 62271-200:2021+AMD1:2024	MV metal-enclosed assembly and earthing/compartment context
Manufacturer/project EMC study	Actual port class, cable/shield/bonding/filter limitations and installation environment

Use the product-family standard when applicable; generic standards help define the environment. Type tests demonstrate equipment performance at defined ports/setups, not every field cable topology.

2. Coupling mechanisms

Mechanism	Driver	Primary mitigation
Capacitive/electric-field	High dv/dt and mutual capacitance	Distance, electrostatic screen, lower impedance, short route
Inductive/magnetic-field	High di/dt and loop mutual inductance	Small loop/twisted pair, distance, route orientation, magnetic containment
Common-impedance	Shared return/earth impedance	Separate returns, equipotential bonding and controlled earthing
Conducted differential/common mode	Surge/EFT/ripple/earth-potential difference	Protection/filter/isolation and correct bonding
Radiated RF	Transceiver/arc/switching fields and antenna-like wiring	Enclosure/shield termination, filtering, fibre and distance
Electrostatic discharge	Personnel/tools/insulators	Bonding, enclosure, ESD practices and protected ports

At low frequency a bond may look like a short circuit; at high frequency its inductance can dominate. Long thin “earth” wires and shield pigtails can therefore be ineffective even though they pass a DC continuity test.

3. EMC zoning and boundaries

Primary bus/cable and breaker/interrupter compartment: highest switching field/transient exposure.
Mechanism/coil/motor/heater circuits: high di/dt, inductive and conducted noise.
LV control compartment: IEDs and conditioned auxiliary supplies.
CT/VT analogue zone: safety-critical measurement circuits.
Low-level sensor/transducer zone.
Station/process network copper/fibre zone.
External cable-entry boundary: surge, earth-potential and shield/armour transition.

Define what crosses each boundary and the required barrier, filter, surge protection, isolation, bonding or shield termination. Keep noisy components near their source and interfaces near the boundary to avoid carrying disturbance through the clean zone.

4. Cable segregation and routing

Group circuits by EMC class: power/motor/heater, trip/close coil, AC/DC control, CT/VT, low-level analogue, high-speed communication/fibre.
Use separate ducts/trays or grounded barriers for incompatible classes.
Do not quote a universal separation distance; derive from voltage/current/dv/dt/di/dt, parallel length, shield/barrier and project/vendor rules.
Where routes cross, minimise shared length; near-right-angle crossing can reduce inductive/capacitive coupling.
Route forward and return together; use twisted pair for balanced circuits and minimise untwist at termination.
Avoid large loops through separated positive/negative or phase/neutral routes.
Keep secondary wiring away from primary conductors, breaker arcs, spring motors, contactors, heater thermostats and VFD outputs.
At unavoidable parallel routes, use increased separation, metal containment/barrier and suitable cable construction.

5. Panel bonding and equipotential reference

Bond enclosure sections, gland plates, mounting plates, doors and cable trays to the switchgear earth/equipotential system.
Use short, wide straps or direct metal contact for high-frequency bonds; control paint/coating and corrosion.
Door hinges alone are not a dependable HF bond unless demonstrated.
Bond across removable sections/transport splits as specified and verify after site assembly.
Keep protective-earth conductors sized/installed for fault and touch-voltage safety; EMC bonding is additional where needed.
Avoid daisy-chain high-frequency bonds that force interference current through electronics mounting paths.
Document the functional reference/0 V relationship to PE; do not create accidental multiple bonds through communications, test equipment or power supplies.

6. Shield termination philosophy

Application condition	Engineering tendency—not universal rule
Low-frequency, high-impedance analogue with small earth-potential difference	Single-point shield connection may avoid low-frequency loop current
High-frequency/RF environment	Low-inductance 360° termination, often at both boundaries, improves screen performance
Long inter-building/substation route	Earth-potential rise and lightning common-mode require explicit bonding/isolation/surge study
Ethernet copper	Use specified screened/unscreened cable/connector and switch/IED bonding; avoid improvised pigtails
Armoured cable	Armour bonding follows safety/fault/EMC study; armour is not automatically the signal screen

Identify overall screen, individual pair screen, drain and armour separately.
Terminate at a defined bar/gland/connector close to cable entry; minimise exposed unscreened length.
Do not connect shields to random terminal PE points through long wires.
Insulate the unconnected shield end safely; prevent accidental contact.
Show both ends and every marshalling boundary on drawings.
Verify that surge protectors/filters have a short low-impedance route to the reference earth.

7. CT and VT circuits

Maintain one intentional secondary earth per galvanically connected CT/VT circuit unless a specifically engineered alternative exists.
Do not confuse that earth with cable-screen bonding at one or both ends.
Route each phase and return together; avoid large star-point/neutral loops.
Use adequate conductor/twisting and segregate from coil/motor/AC heater wiring.
For differential/REF, keep polarity/branch routes and earth arrangements controlled; induced/common-mode effects can create spill current.
VT secondary selection must avoid double earth/backfeed through alternate panels.
Use CT shorting and VT isolation test hardware without long exposed injection leads.

8. Binary inputs and long interlock circuits

Select wetting voltage/current and IED pickup/dropout to reject induced/leakage voltage.
Route positive and return together; avoid using distant shared returns across panels.
Check cable capacitance can delay dropout of high-impedance inputs.
Use interposing isolation/bleed/filter only with calculated thresholds and timing.
Debounce can reject contact chatter but not a persistent induced voltage; keep SOE requirements.
Supervise critical interlocks; loss/invalid proof must not look safe.
Test with adjacent noisy circuits operating and field radio exposure as approved.

9. Trip/close coils, contactors and suppression

Inductive loads produce voltage/current transients when switched. Place the approved suppression near the source/load as specified, with short connections. But suppression is a functional component:

A simple flyback diode can greatly slow DC current decay and relay/contactor release.
Diode polarity failure can short the supply; voltage rating and reverse transients matter.
TVS, diode-zener, RC or varistor choices change clamp voltage, energy and release time.
Check breaker trip/close mechanism timing, anti-pumping and output-contact duty after suppression.
Keep coil circuits physically separate from sensitive inputs and avoid common returns.
Verify suppression is not duplicated inside the coil/output module.
Document part, polarity, location, energy/lifetime and test method.

10. Auxiliary power and functional 0 V

Use dedicated/segregated feeds and local filtering/surge protection according to IED port requirements.
Route DC positive/negative or AC line/neutral together.
Define floating, earthed or monitored DC system and avoid unintended 0 V–PE connections.
Separate noisy motor/heater/contactors from IED supplies or provide coordinated filtering.
Check charger ripple, dips, interruption, common-mode and earth faults.
Bond filter/surge components to the correct low-impedance reference; long leads can defeat performance.
Protect devices selectively without sharing one noisy branch with redundant protection channels.

11. Communications and fibre

Fibre removes galvanic conducted coupling along the link and is preferred across high earth-potential/noise boundaries where architecture permits.
Fibre transceivers, IEDs and switches still need clean power, bonding and ESD control.
Separate fibre mechanically from sharp edges/high heat and respect bend radius/connector cleanliness.
For copper Ethernet/serial, use the specified balanced cable, connector, screen/bond and maximum length; preserve pair twist.
Avoid mixing copper communications in ducts with coil/motor/power wiring.
Use surge/isolation devices only if compatible with bandwidth/protocol and port EMC.
Monitor link errors, CRC, packet loss and redundancy counters as early EMC indicators.

12. Cable entry, glands and marshalling

Segregate noisy and sensitive cable-entry zones/gland plates.
Terminate armour/screen at entry with the specified circumferential/terminal method.
Keep unshielded internal tails short and route them immediately into their EMC class duct.
Bond metal gland plates and maintain continuity across removable transport sections.
Install surge protective devices/filters at the boundary, not deep inside after a long exposed wire.
Maintain enclosure IP/internal-arc/fire requirements while adding EMC penetrations.
Control spare cable/shield/core termination so it cannot become an antenna or unintended earth.

13. Design verification and calculations

EMC zone/source-victim matrix and circuit-class route review.
Maximum loop area and parallel exposure check for sensitive circuits.
Earth-potential rise/common-mode and isolation/surge rating assessment for external links.
Input induced/leakage voltage versus guaranteed pickup/dropout margin.
Shield/armour current and bonding/conductor thermal/fault capability where relevant.
Filter/suppression clamp energy, repetitive duty and functional timing.
Port/immunity level compatibility with installed environment and cable length.
Reliability/common-mode effect of added filters, protectors and shared earth points.

14. FAT/SAT and troubleshooting

Inspect route segregation, twists/loops, shield/armour ends, bonds and filters against drawings.
Measure protective-earth/bond continuity using the approved method; inspect high-frequency geometry.
Verify CT/VT secondary earth and absence of unintended second earths.
Operate breaker coils, spring motors, contactors, heaters and adjacent power equipment while monitoring I/O and networks.
Apply approved ESD/radio/switching susceptibility checks without exceeding equipment/site safety limits.
Monitor relay event flags, analogue noise, binary chatter, Ethernet errors and time offset.
Test DC/AC supply dips, earth faults and surge-protection alarms where specified.
For a symptom, correlate waveform/event to source operation; change one coupling path at a time.
Re-test functional timing after adding suppression/filter/bleed/isolation.
Record as-built bond/shield/filter configuration and baseline measurements.

15. Frequent mistakes

Mistake	Why it fails	Correction
Relay passed EMC test, installation assumed safe	Field coupling differs from test setup	System zoning/routing/bonding verification
All shields earthed one end	HF screening/common-mode may be poor	Application-specific termination study
Long PE/shield pigtail	High inductive impedance	Short wide/360° bond
Positive/return separate ducts	Large magnetic loop	Route together/twist
Flyback diode added to trip coil	Release/timing changed	Approved suppression with timing test
Shield and CT earth confused	Multiple secondary earth/circulating current	Separate documented functions
Continuity-only FAT	Dynamic interference not exercised	Representative operation/failure monitoring

16. Design-release checklist

EMC environment/zones and source-victim paths identified?
Circuit classes and route/barrier/separation rules approved?
All forward/return loops minimised?
Panel/door/gland/tray high-frequency bonding defined?
PE, functional bond, signal earth and shields distinguished?
Shield/armour/drain terminations shown at every boundary?
CT/VT one-point secondary earth preserved?
Coil suppression/filter effect on timing/duty verified?
Binary input leakage/induction margins demonstrated?
External links/common-mode/surge/earth-potential covered?
Product EMC port class matches installation?
Dynamic FAT/SAT and baseline monitoring specified?

References and further reading

Engineering note: Separation distances and shield bonds are application-specific. Use the actual equipment ports, earthing study, cable routes and project EMC plan.