Ground Types
| Ground Type | Purpose | Why Kept Separate |
|---|---|---|
| Earth / ground rod (safety) | Stabilizes system voltage to earth; dissipates lightning/surge energy; gives fault current a path to trip overcurrent devices | Carries high-energy transients — must not share a conductor with sensitive signal returns |
| Chassis / equipment ground | Bonds exposed metal enclosures/frames for personnel shock protection | Can carry fault current and motor/relay switching noise |
| Signal / reference ground | Low-noise 0V reference for a circuit's signals | Any shared current on this path shows up directly as signal noise (a "ground loop") |
Bonding — NEC Article 250
NFPA 70 (NEC) Article 250 governs grounding and bonding of electrical installations. Pick your device or service conductor to read the required conductor size straight from Table 250.122 (equipment grounding conductor) or Table 250.66 (grounding electrode conductor).
Equipment Grounding Conductor (EGC) — Table 250.122
Grounding Electrode Conductor (GEC) — Table 250.66
Cable Shield Types
| Shield Type | Coverage | Best Frequency Range | Notes |
|---|---|---|---|
| Foil (Al/Mylar + drain wire) | ~100% | High frequency | Higher DC resistance; excellent HF coverage, weak alone at low frequency |
| Braid | ~70–95% | Low frequency (<~1 MHz) | Low DC resistance, good flex life; small weave gaps leak HF |
| Foil + braid combo | Near-complete, broadband | Low through high frequency | Combines braid's LF strength with foil's HF coverage |
| Spiral/serve (wrapped wire) | ~95% | Audio/low frequency (<100 kHz) | Very flexible, low cost; inductive, HF performance is poor |
EMI/EMC Standards
| Standard | Scope | Region/Body | Status |
|---|---|---|---|
| FCC Part 15 Subpart B | Unintentional radiators (digital devices). Class A = commercial/industrial (10m, looser limits); Class B = residential (3m, ~10dB stricter) | US — FCC (47 CFR 15) | Current |
| CISPR 11 / EN 55011 | Industrial, scientific & medical (ISM) equipment RF disturbance limits | International (IEC/CISPR) | Current |
| CISPR 22 / EN 55022 | Information technology equipment — radio disturbance limits | International/European | Withdrawn — superseded by CISPR 32/EN 55032 (2017) |
| CISPR 32 / EN 55032 | Multimedia equipment emissions — replaces CISPR 22 & CISPR 13 | International/European | Current |
| IEC 61000-4-2 | Electrostatic discharge (ESD) immunity — contact and air discharge | International (IEC) | Current |
| IEC 61000-4-4 | Electrical fast transient/burst (EFT) immunity | International (IEC) | Current |
| IEC 61000-4-5 | Surge immunity (1.2/50µs + 8/20µs waveform) | International (IEC) | Current |
| MIL-STD-461 | US military EMI/EMC — covers CE/RE (emissions) and CS/RS (susceptibility) test methods | US DoD | Rev H current, supersedes Rev G |
Common EMI Mitigation Techniques
| Technique | Addresses | When to Use |
|---|---|---|
| Ferrite bead / common-mode choke | Broadband HF (bead); common-mode specifically (CM choke) | Cables/power leads carrying HF noise (>~1 MHz); CM choke where differential signal integrity matters |
| Twisted pair | Differential-mode radiated emission/pickup (cancels loop area) | Signal/power pairs; combine with a shield for common-mode/capacitive coupling |
| Shielding (cable/enclosure) | Radiated emissions & immunity | High-frequency radiated coupling — pick shield type per the table above |
| Filtering / decoupling capacitors | Conducted noise (differential-mode at IC pins; common-mode at cable/connector entry) | IC power pins (bypass); power/signal entry points |
| Physical separation / spacing | Radiated E/H-field near-field coupling (inductive/capacitive crosstalk) | Route noisy (switching, motor, clock) traces/cables away from sensitive analog/signal lines |