Grounding & Bonding
Grounding & bonding, explained properly
"Ground" is the most overloaded word in electrical engineering. It's a safety path, a signal reference, and an EMC tool — and confusing those three jobs causes more real-world interference problems than almost anything else. This page untangles them, with the diagrams I draw on whiteboards.
Three jobs, one word
Safety earthing exists so that a fault current has a low-resistance path back to the source, tripping the protection before an exposed metal case becomes dangerous. It cares about ohms at 50 Hz and nothing else.
Signal reference is the "zero volts" that circuits measure everything against. Two circuits that disagree about where zero is will exchange unwanted currents to settle the argument.
EMC bonding ties metalwork together so RF currents can flow where you want them (screens, enclosures, gaskets) rather than where you don't (cables, apertures, your measurement). It cares about impedance across the whole spectrum — which, as the bond strap tool shows, is mostly about geometry, not resistance.
A system can be perfectly safe and an EMC disaster at the same time. The safety earth is non-negotiable; the question is how everything else is arranged around it.
The problem: common-impedance coupling
Here is where most "grounding problems" actually come from. When two circuits share a stretch of return conductor, the current from one develops a voltage across that shared impedance — and the other circuit sees that voltage added directly to its signal:
Feel the sizes involved. A motor drive dumping 10 A of switching return current through a shared conductor with just 0.1 Ω of impedance at the frequency of interest puts a full 1 volt of noise in series with whatever else uses that conductor — enough to swamp a millivolt-level sensor completely. Try it:
Remember that the "impedance" here rises with frequency because of inductance — so a return path that's harmless at 50 Hz can be a strong coupling mechanism for a switching converter's harmonics at hundreds of kilohertz.
Two answers: star and mesh
The star (single-point) approach eliminates sharing by giving every unit its own dedicated conductor back to one reference point. No shared path, no common-impedance coupling — at low frequency. Its weakness is that those long dedicated conductors have inductance, so at RF each one becomes a high impedance and the "single point" stops meaning anything. Stars suit low-frequency, high-sensitivity systems: audio, instrumentation, some analogue plant.
The mesh (multipoint) approach goes the other way: bond everything to a shared metal structure — a ground plane, floor grid, cabinet frame, cable tray network — at every opportunity. Individual paths are short, so their inductance is low; and the many parallel paths divide the current, so the effective impedance stays low right up through the RF range. This is how modern electronics, PCBs, vehicles, aircraft and EMC test facilities are built.
Real installations usually end up hybrid: a meshed bonding network for structure, cabinets, trays and screens, with particular sensitive circuits treated as controlled islands within it. The dangerous option isn't star or mesh — it's the accidental topology nobody designed.
Cables: the parallel earthing conductor idea
One concept from installation practice deserves its own picture. When a cable runs between two areas, any RF or fault current flowing between those areas will use the cable's screen — unless you give it a better path. Running the cable on a bonded metal tray, duct or a dedicated heavy conductor alongside it (a parallel earthing conductor) gives structural currents a low-impedance route of their own, and dramatically reduces what flows on the screen:
This is the everyday reason cable trays get bonded at every joint, and why "just float the screen at one end" is not the universal fix it's sometimes claimed to be — at RF, a floated screen end simply re-radiates.
Where to read more
The reference document for earthing and bonding of installations — including bonding networks, earthing of cable screens and equipment, and the reasoning behind meshed structures — is IEC 61000-5-2 (Electromagnetic compatibility, Part 5-2: Installation and mitigation guidelines — Earthing and cabling). For defence platforms, DEF STAN 59-411 Part 4 covers bonding classes and installation practice. The diagrams on this page are my own; for formal design work, get the actual standards.