Grounding & bonding

Protective earthing, signal reference and EMC bonding

Protective earthing provides a fault-current path so protective devices can operate and limit the risk from exposed conductive parts.

Signal reference defines the voltage against which a circuit measures its signals. Differences between circuit references can drive unwanted currents.

EMC bonding connects metalwork to provide controlled RF current paths through screens, enclosures and gaskets. Its performance depends on impedance across the relevant frequency range, including the effects of conductor geometry. See the bond strap impedance calculator.

Meeting protective earthing requirements does not, by itself, establish EMC performance. Both requirements must be addressed in the installation design.

The problem: common-impedance coupling

When two circuits share a return conductor, current from one develops a voltage across the shared impedance. That voltage can couple into the other circuit:

A switching return current of 10 A through a shared impedance of 0.1 Ω develops 1 V of interference, which can overwhelm a millivolt-level sensor signal.

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.

Star and mesh arrangements

A star arrangement gives each unit a dedicated conductor to a common reference point, reducing shared-path coupling at low frequencies. At RF, the inductance of long conductors limits its effectiveness. Typical applications include low-frequency audio and sensitive instrumentation.

A mesh arrangement bonds equipment to a common conductive structure, such as a ground plane, floor grid, cabinet frame or cable tray network. Short connections and parallel paths reduce effective impedance at RF.

Hybrid installations combine a meshed structural bonding network with controlled reference arrangements for sensitive circuits. The complete topology should be defined and documented.

Cables: the parallel earthing conductor idea

A bonded cable tray, duct or parallel earthing conductor provides an additional low-impedance path between connected areas. This can reduce the current carried by a cable screen:

Bonding continuity at cable tray joints supports this current path. Cable screen termination should be selected for the installation and frequency range; leaving one end unconnected is not a universal solution.

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.