EMC - Earthing principles and structures

This section deals with the earthing and equipotential bonding of information-technology devices and other similar devices requiring interconnections for signalling purposes.
Earthing networks are designed to fulfil a number of functions. They can be independent or operate together to provide one or more of the following:

• Safety of persons with respect to electrical hazards
• Protection of equipment with respect to electrical hazards
• A reference value for reliable, high-quality signals
• Satisfactory EMC performance

The system earthing arrangement is generally designed and installed in view of obtaining a low impedance capable of diverting fault currents and HF currents away from electronic devices and systems. There are different types of system earthing arrangements and some require that specific conditions be met. These conditions are not always met in typical installations. The recommendations presented in this section are intended for such installations.
For professional and industrial installations, a common bonding network (CBN) may be useful to ensure better EMC performance with respect to the following points:

• Digital systems and new technologies
• Compliance with the EMC requirements of EEC 89/336 (emission and immunity)
• The wide number of electrical applications
• A high level of system safety and security, as well as reliability and/or availability

For residential premises, however, where the use of electrical devices is limited, an isolated bonding network (IBN) or, even better, a mesh IBN may be a solution.
It is now recognised that independent, dedicated earth electrodes, each serving a separate earthing network, are a solution that is not acceptable in terms of EMC, but also represent a serious safety hazard. In certain countries, the national building codes forbid such systems.
Use of a separate “clean” earthing network for electronics and a “dirty” earthing network for energy is not recommended in view of obtaining correct EMC, even when a single electrode is used (see Fig. R3 and Fig. R4). In the event of a lightning strike, a fault current or HF disturbances as well as transient currents will flow in the installation. Consequently, transient voltages will be created and result in failures or damage to the installation. If installation and maintenance are carried out properly, this approach may be dependable (at power frequencies), but it is generally not suitable for EMC purposes and is not recommended for general use.

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Fig. R3: Independent earth electrodes, a solution generally not acceptable for safety and EMC reasons

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Fig. R4: Installation with a single earth electrode

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The recommended configuration for the earthing network and electrodes is two or three dimensional (see Fig. R5). This approach is advised for general use, both in terms of safety and EMC. This recommendation does not exclude other special configurations that, when correctly maintained, are also suitable.

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Fig. R5: Installation with multiple earth electrodes

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In a typical installation for a multi-level building, each level should have its own earthing network (generally a mesh) and all the networks must be both interconnected and connected to the earth electrode. At least two connections are required (built in redundancy) to ensure that, if one conductor breaks, no section of the earthing network is isolated.
Practically speaking, more than two connections are made to obtain better symmetry in current flow, thus reducing differences in voltage and the overall impedance between the various levels in the building.
The many parallel paths have different resonance frequencies. If one path has a high impedance, it is most probably shunted by another path with a different resonance frequency. On the whole, over a wide frequency spectrum (dozens of Hz and MHz), a large number of paths results in a low-impedance system (see Fig. R6).
Each room in the building should have earthing-network conductors for equipotential bonding of devices and systems, cableways, trunking systems and structures. This system can be reinforced by connecting metal pipes, gutters, supports, frames, etc. In certain special cases, such as control rooms or computers installed on false floors, ground reference plane or earthing strips in areas for electronic systems can be used to improve earthing of sensitive devices and protection interconnection cables.

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Fig. R6: Each level has a mesh and the meshes are interconnected at several points between levels. Certain ground-floor meshes are reinforced to meet the needs of certain areas