Earthing measures are generally imperative in order to fulfil the statutory requirements of the EMC and Low Voltage directives. They are a prerequisite for the effective use of other measures, such as shielding and filters. Other measures are of no benefit without good earthing. For this reason, the earthing arrangements must be checked and verified for proper EMC implementation before retrofitting shielding or filters and as the first step in troubleshooting.
Electrically conductive materials
Operators must ensure that metallic surfaces are earthed with low-impedance connections. In terms of EMC, the decisive factor is not the cross-section of the conductor but instead its surface area, since high-frequency currents flow on the surface due to the skin effect. The portion with the smallest conductor surface area is what limits the ability to drain leakage currents. Earthed surfaces have a shielding effect and reduce the amplitude of ambient electromagnetic fields.
Star-configured earthing system
All earthed points and components must be connected to the central earthing point as directly as possible, such as by means of a potential equalisation rail. This results in an earthing system in which all connection points are connected radially to the earthing point. This earthing point must be defined unambiguously.
After paint and corrosion have been removed, connections must be made to contact points using a large surface area. Serrated washers are better for this purpose than plain washers. Tin-plated, zinc-plated or cadmium-plated components should be used in preference to painted components. Multiple contacts for the shield connection must be provided in connectors.
Conductor surface area
A large conductor surface area for draining high-frequency currents can be obtained by using fine stranded wire, such as high-flexibility instrument wire, or by using special earthing straps or cables. Braided earthing straps are often used nowadays in practice; they replace the rigid conductors used in the past. These straps have a significantly higher surface area with the same cross-section.
Note: System earthing has a substantial effect on smooth, trouble-free facility operation. Ground loops must be avoided. Good potential equalisation is essential. Generate a suitable earthing plan as early as the design and projecting engineering stage.
Article by Gregers Geilager