3.3 EMI/EMC Management

Electromagnetic behaviorCompatibility inside(EMC) anis enclosurea mandatory safety and functional requirement ensuring a system operates correctly in its intended electromagnetic environment without causing or suffering unacceptable degradation. Electromagnetic Interference (EMI) management in Box Build is primarily a mechanical function: establishing a perfect Faraday cage and enforcing routing protocols that maintain signal integrity. Failure to control segregation and bonding results in crosstalk, data loss, and failure of regulatory testing.

3.3.1 Grounding Strategy: Chassis vs. Circuit Reference

EMC performance is dictated less by circuit intent than by the physicalgrounding layoutstrategy. The chassis serves as the protective shield and safety earth, while the 0 V rail is the circuit reference. These must be bonded deliberately.

A) Bonding Strategy Mandates

The choice of metalsbonding andtopology cables.depends Currents follow geometry, not schematics, andon the wrong return path can turn a neat harness into an antenna. Effective EMI/EMC control comes from deliberate grounding schemes, robust shield terminations, and disciplined segregation of noisy and sensitive zones. When clamps, gaskets, and routing are treated as functional components rather than afterthoughts, assemblies leave the line already stable, reducing the need for late fixes or costly compliance surprises.

3.3.1 The goal (in one line)

Keep noise in the noisy places and outfrequency of the quiet ones—by giving currents short, obvious return paths and making cables poor antennas.noise:

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• 3.3.2 Know your players: sources vs victims

Noisy stuffSingle-Point (emitters)LF Analog): switching PSUs, motor/solenoid drivers, DC–DCs, relays, PWM lines, long unshielded harnesses, poor shield bonds.

Sensitive stuff (susceptors): radios/GNSS, analog sensors, high-impedance nodes, USB/Ethernet PHYs, high-speed clocks, long signal runs.

First rule: shorten loop area (route out and back together) and give noisy currents a nearby chassis to hug.

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3.3.3 Grounding strategy (decide once, build to it)

• Chassis vs 0V: treat chassis/earth asTie the EMC0 sink; treat V0V ascircuit reference to the circuit reference. Bond them deliberately, where designed.
• Single-point (low-freq analog): tie 0V↔chassischassis/earth at one slot only (star). This is used to avoid low-frequency hum loops.and DC ground loops (mains ripple).
• Multi-pointPoint (RF/fastFast edges)Edges): useUse short, wide bonds at several points to lowerachieve HFa impedance.low-impedance path for high-frequency (RF) noise.
• Equipotential bonding:Bonding: allAll metalwork (lids, frames, trays) must read < 0.1 Ω to the chassis at their bond pads (logverified at first article).

B) Earth Joint Integrity

Earth joints must maintain the low-ohm bond permanence.

• Earth joints:Mandate: Use serrated/star washerwashers on bare metal bond pads (23.1/23.2),paint torquemust be scraped or masked, Section 5.6).
• Verification: The joint must be torqued per the map, and the bond resistance must measure < 0.1 Ω. to the chassis safety earth.

3.3.2 Segregation and Routing Protocol

RuleRouting ofcontrols thumb:how ifsignals interact with noise sources and how much energy the problemharness iscan radiate.

A) Zoning and Separation

Internal components must be segregated into logical zones: low-frequency ground loop (mains ripple), think single-point. If it’s RF spray, think multi-point/short bonds.

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3.3.4 Cable shields & terminations (where the magic happens)

• 360° termination at entry: clamp braid/foil to chassis as the cable enters the box (backshell, gland, or clamp). Avoid pigtails; if unavoidable, keep ≤ 10 mm.
• Drain wires: bond alongside the 360° clamp, not to a distant PCB pad.
• Which end(s) to bond?
• • HF noise / radiated control: both ends (360°) to kill common-mode.
  • Low-freq instrumentation (e.g., 4–20 mA): single-end at the quiet end to avoid DC loops—but still add a high-frequency path (capacitor/RC or 360° with DC-isolation hardware) if the spec allows.
• Coax: treat the shield as signal return; bond shells properly (U.FL click, SMA torque).
• EMI gaskets: ensure clean bond lands; target 20–30% compression; verify seam < 0.1 Ω (23.4/23.5).

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3.3.5 Segregation & routing (turn geometry into a filter)

• Zoning inside the box:
• 1. Noisy zoneZone (PSUs, motors,motor relays).drivers),
  2. Digital zoneZone (processors,processors), high-speed).and
  3. Quiet/analog/Analog/RF zoneZone (sensors, radios, GNSS)radios).
     
     • Mandate: Keep short paths to their exits; do not meander.
• Separation: keep noisy power/switching harnesses ≥ 100 mm away from low-level signals;signals. whenWhen crossing,crossing nets is mandatory, do itso at a 90°90˚ angle.
• Pairing: runMinimize the radiating area by running the supply withwire and its return (twistedground) iftogether possible)(paired or twisted) to shrink the loop area.area.

B) Shielding and Return Paths

• Chassis-hugging:Hugging: clampClamp long harness runs along metal rails (24.1)chassis-hugging) to reduce the potential radiation and pickup.pickup areas. Clamps should be spaced at ≤ 200 – 300 mm intervals.
• Shield Termination (360°): The cable shield (braid/foil) must be terminated 360˚ (circumferentially) to the chassis wall or backshell immediately upon entry. Prohibited: Using long pigtails (> 10 mm) to connect the shield to a distant ground point, as these compromise the shield effectiveness.
• Drain Wires: If a drain wire is present, it must be bonded alongside the 360˚ clamp, not routed to a distant PCB pad.

3.3.3 Interface and Verification Mandates

Noise must be controlled at the point of entry/exit to the enclosure (the interface panel).

A) Filtering and Feedthroughs

• Placement: Filters (ferrites, common-mode chokes) must be placed directly at the bulkhead entry point, not mid-span, to prevent internal re-radiation. The choke's shield/can must be tied to the chassis nearby.
• TVS/RC: Transient Voltage Suppressors (TVS) and RC networks must be located right at the connector to shunt ESD transients to the chassis; requiring the shortest possible leads.
• Ferrites: Ferrite cores are a "last-resort band-aids;aid." pickThey coresmust be correctly sized for the cable and placeplaced rightonly at the entry/exit points to choke common-mode.mode noise.

B) Quick Verification and Functional Audit

• Acceptance Cues: The functional check must include watching sensitive circuits (radios, GNSS, analog noise) while toggling noise sources (relays, power lines) to verify no data errors occur.
• AntennaBond keepouts:Resistance: keepAudit digital and power wiring away from RF modules/antennas; avoid routing behind antenna ground cuts.

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3.3.6 Interface panels & filters (stop noise at the wall)

• Feedthroughs / filtered connectors: use at the bulkhead for nasty lines (PWM, long I/O).
• Common-mode chokes on I/O just inside the entry; tie the choke’s shield/can to chassis nearby.
• RC/TVS right at the connector where ESD enters; shortest leads to chassis.

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3.3.7 Inside the enclosure (bonds that actually work)

• Paint scrape pads under every intended bond (23.1).
• Board-to-chassis bonds: short, wide straps or mounting posts near the I/O shield.
• Lid seams: conductive gasket or fingers around the perimeter, even compression (23.5).
  

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3.3.8 Quick verification & pre-compliance sanity

• Bond resistance:confirms seam/strap/chassis points read < 0.1 Ω (low-ohm meter)meter log).
• ShieldNear-Field continuity:Sniff: shellUse ↔a shell and braid ↔ chassis low ohms.
• Near-field sniff:small, handheld probe (or small loop) over hot zones—comparezones “clamp(PSUs, on/off”,high-speed “ferritelines) in/out”to deltas.
• Common-modeverify EMC control effectiveness. The current clamp:must ondrop externalsignificantly cables during function—look for drops after 360°when bonds/ferrites.
• Functionalferrites abuseare test: toggle relays/PWM while watching radios/GNSS/analog noise; verify no resets or data errors.applied.

Final

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3.3.9 Acceptance cues (fast eyes)Checklist

FeatureMandate	AcceptCriteria	RejectVerification Action
ShieldGrounding terminationsStrategy	360°Single-point clampfor atLF entry;(0V pigtailstie); ≤multi-point 10for mmHF/RF only if spec’dshields.	FoilBond braidpads twistedverified clean; all structural metal reads < 0.1 Ω to long pigtail; clamp far from entrychassis.
Segregation	BondPower/switching padsruns separated ≥ 100 mm from quiet signals.	Bare,Audited clean;cross-points < 0.1 Ωconfirmed to chassis	Paintedbe pad;at star90˚ washer on paint; high Ωangles.
Shield Termination	SegregationCable shields terminated 360˚ at the enclosure entry.	Prohibited:Power/relay harnessesPigtails awaylonger fromthan analog/RF;10 90°mm crosses	Paralleling noisy and quiet runsused for longbonding spansshields.
Routing Protocol	Pairing/returnsLong runs clamped along metal rails; return wires co-routed.	TwistedClamps orspaced tapedat pairs;≤ returns200 co-routed	Return– separated300 →mm bigintervals loopalong areathe main trunk.
Filter Placement	GasketsFerrites and chokes placed directly at the bulkhead entry/exit.	ContinuousChoke's contact,shield/can evenbonded squeeze	Gaps/crushedimmediately foam;to inconsistentthe seamschassis.
Gasket Integrity	FerritesConductive gaskets verified for even compression (e.g., 20% – 30%).	AtLow-ohm entry/exit;meter sizedaudit toacross cable	Randomthe mid-spanconductive beadsseam “justverifies because”< 0.1 Ω.

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3.3.10 Common traps → smallest reliable fix

Trap	Symptom	First move
Shield bonded to PCB 0V only	Radiates on cable	Move to 360° chassis bond at entry; add HF decoupling if single-end needed
Long drain pigtails	Fails radiated emissions	Replace with 360° clamp; shorten to ≤ 10 mm if constrained
Ground loop fear everywhere	Hum fixed, RF worse	Use single-point for LF, multi-point for HF; add DC-isolated HF bond
Paint under earth lugs	High leakage, ESD upsets	Scrape pad; serrated washer; verify < 0.1 Ω
Big harness loops	Analog noise, resets	Pair with return; route along chassis; add clamp spacing ≤ 200–300 mm
Filters far from wall	Internal re-radiation	Move chokes/feedthroughs to the bulkhead
Ferrite scattergun	Costs rise, little effect	Measure current; place few, correct cores at entries

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3.3.11 Pocket checklists

Before build

• Ground strategy chosen (single- vs multi-point) and drawn
• Bond pads exposed; Ω meter at station; star washers in kit
• Shield hardware (360° clamps/backshells) present; ferrites (if any) sized

During routing

• Noisy runs segregated; crosses at 90°; power paired with return
• Shields bonded at entry; pigtails ≤ 10 mm (only if spec’d)
• Gaskets seated; seam compression even; lid bonds in place

Quick verify

• Chassis bonds < 0.1 Ω logged (tray, lid, straps)
• Sniff/common-mode check shows improvement vs open case
• Radios/GNSS/analog behave while PWM/relays chatter

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By locking down ground bonds, shield entries, and cable routing as part of the build flow, assemblies gain inherent immunity against interference. This approach shortens debug cycles, improves regulatory margins, and ensures that both noisy and sensitive electronics coexist without conflict.