, SO that surfaces do not mate perfectly, or because of painting, anodising or corrosion, SO that an insulating 5. Another way of improving shielding contact between two sur- layer is present on one or both metal surfaces. Consequently, the faces that frequently mate and un-mate is to use beryllium shielding effectiveness is reduced by seams just as much as it is by copper 'finger' stock either as a continuous strip or spaced at apertures. The reduction of shielding effectiveness depends on the intervals subject to the limitations on spacing given above for longest dimension of an aperture applies equally to a non-conduc- fasteners. tive length of seam. The problem is especially serious for hinged front panels, doors and removable hatches that form part of a screened enclosure. The penalty of poor contact is mitigated to some extent if the conductive 23.14 Filtering sheets overlap, since this forms a capacitor which provides a partial current path at high frequencies. An enclosure made of several ill fitting painted panels with large holes and few fastenings will give The purpose of filtering for EMC is almost invariably to attenuate little or no shielding at typical frequencies. high frequency components while passing low frequency ones. It is There are a number of design options which will improve the also, almost invariably, to block interfering signals which are shielding effectiveness at the seams: coupled onto cables which enter or leave the equipment enclosure. There is little point in applying good shielding and circuit design 1. Ensuring that supposedly conductive surfaces remain conduc- practice to guard against radiated coupling if interference is then tive. They must not be painted or anodised; alochrome is a allowed to be coupled in or out via the external connections. suitable conductive finish for aluminium. Interference can be induced directly onto the cable or can be 2. The area of overlap of two joined sheets is maximised. This can coupled through the external connection, and a proper filter can