Skip to main content

3.3 EMI/EMC Management

Electromagnetic Compatibility (EMC) is a 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 by the grounding strategy. 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 bonding topology depends on the frequency of the noise:

  • Single-Point (LF Analog): Tie the 0 V circuit reference to the chassis/earth at one slot only (star). This is used to avoid low-frequency hum and DC ground loops (mains ripple).
  • Multi-Point (RF/Fast Edges): Use short, wide bonds at several points to achieve a low-impedance path for high-frequency (RF) noise.
  • Equipotential Bonding: All metalwork (lids, frames, trays) must read < 0.1 Ω to the chassis at their bond pads (verified at first article).

B) Earth Joint Integrity

Earth joints must maintain the low-ohm bond permanence.

  • Mandate: Use serrated/star washers on bare metal bond pads (paint must 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

Routing controls how signals interact with noise sources and how much energy the harness can radiate.

A) Zoning and Separation

Internal components must be segregated into logical zones: Noisy Zone (PSUs, motor drivers), Digital Zone (processors), and Quiet/Analog/RF Zone (sensors, radios).

  • Mandate: Keep noisy power/switching harnesses ≥ 100 mm away from low-level signals. When crossing nets is mandatory, do so at a 90˚ angle.
  • Pairing: Minimize the radiating area by running the supply wire and its return (ground) together (paired or twisted) to shrink the loop area.

B) Shielding and Return Paths

  • Chassis-Hugging: Clamp long harness runs along metal rails (chassis-hugging) to reduce the potential radiation and 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-aid." They must be correctly sized for the cable and placed only at entry/exit points to choke common-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.
  • Bond Resistance: Audit confirms seam/strap/chassis points read < 0.1 Ω (low-ohm meter log).
  • Near-Field Sniff: Use a small, handheld probe over hot zones (PSUs, high-speed lines) to verify EMC control effectiveness. The current must drop significantly when bonds/ferrites are applied.

Final Checklist

Mandate

Criteria

Verification Action

Grounding Strategy

Single-point for LF (0V tie); multi-point for HF/RF shields.

Bond pads verified clean; all structural metal reads < 0.1 Ω to chassis.

Segregation

Power/switching runs separated ≥ 100 mm from quiet signals.

Audited cross-points confirmed to be at 90˚ angles.

Shield Termination

Cable shields terminated 360˚ at the enclosure entry.

Prohibited: Pigtails longer than 10 mm used for bonding shields.

Routing Protocol

Long runs clamped along metal rails; return wires co-routed.

Clamps spaced at ≤ 200 – 300 mm intervals along the main trunk.

Filter Placement

Ferrites and chokes placed directly at the bulkhead entry/exit.

Choke's shield/can bonded immediately to the chassis.

Gasket Integrity

Conductive gaskets verified for even compression (e.g., 20% – 30%).

Low-ohm meter audit across the conductive seam verifies < 0.1 Ω.


Back to top