OpenSpecEngineering

— Grounding, Bonding & EMI/EMC

Ground Types

Single-Point vs. Multi-Point Grounding

Single-point (star) grounding — every circuit's return wired individually to one common point — is preferred at low frequency (roughly below 1 MHz) since it guarantees only one path exists, eliminating loops between ground points at different potentials; typical in audio and low-frequency instrumentation. Multi-point / ground-plane grounding — each circuit grounds locally to the nearest point on a continuous low-impedance plane — is preferred for high-speed digital systems, where star-topology wires become significant inductors/antennas at high frequency and a plane keeps every connection short with the lowest-inductance return path directly under the signal trace.

Bonding — NEC Article 250

NFPA 70 (NEC) Article 250 governs grounding/bonding of electrical installations. Table 250.66 sizes the grounding electrode conductor (GEC) from the largest service conductor; Table 250.122 sizes the equipment grounding conductor (EGC) from the upstream overcurrent device rating.

Per 250.66(A)/(B): where the GEC connects only to a ground rod/pipe/plate electrode it's capped at 6 AWG Cu regardless of the table value; for a concrete-encased (Ufer) electrode it's capped at 4 AWG Cu.

If ungrounded conductors are up-sized for voltage drop, the EGC must be increased proportionately per 250.122(B).

Shielding Techniques

Ground the shield at one end only for low-frequency signals (roughly <100 kHz–1 MHz, e.g. audio/instrumentation) — grounding both ends creates a loop between two chassis points at different potentials, driving a ground-loop current onto the signal conductor. Ground both ends for high-frequency signals (above ~1 MHz, or cable length > ~1/20 wavelength) — the shield must be a low-impedance path to ground at both terminations, since an open end behaves like an antenna stub and shielding effectiveness collapses at high frequency. A small pigtail capacitor (~1–10 nF) at the otherwise-floating end is a common hybrid technique, passing HF noise to ground while blocking the LF loop current.

EMI/EMC Standards

Common EMI Mitigation Techniques