Achieving EMC Compliance

Compliance with the 1996 EMC directive for electronic products has been a costly business. Companies supplying electronic products have had to rethink designs to find the simplest and most effective way their existing products can be re-designed or upgraded. One good example of this comes from Rittal Limited, supplier of enclosures and accessories for the electrical and electronic industries.

Ripac Subracks from Rittal have been designed from the outset to provide EMC protection to IEEE and IEC standards. In order to achieve compliance, Rittal designed the new system from the printed circuit card, outwards, rather than redesigning or up-grading their older "Varioset" subrack product. The obvious EMC components in the design are the front and rear flanges, front (or rear) panels, top and bottom covers and a range of gaskets. Less visible, but just as important, is a new design range of horizontal rails into or onto which some of the gaskets fit.

Between adjacent front and rear panels and between those panels and the sides or top and bottom of the subrack exist narrow, slit-shaped gaps or slots. These act as antennae at a wide range of frequencies, especially if there is not a good conductive join between the two adjacent surfaces. To overcome this, the right hand side of each vertical component has a stainless steel spring gasket snapped into place to makes contact with the "contact strip" on the left hand side of the adjacent component. This gasket spring has been designed to provide the EMC shield without the risk of snagging adjoining boards.

Because of the "U" shape of the front panels (to accommodate the gasket and provide the "contact strip"), they are significantly more rigid than non-EMC flat panels when perforated by connector holes. This has the advantage that after a prolonged period of use, the panels retain their flatness and thus are aligned in the subrack. Flat front panels often bend with repeated use, especially where a high pin count connector is fitted to the rear of the board and extractor handles are fitted to the panel to ease insertion and extraction.

The top and bottom covers, which fit into channels in the horizontal rails, can be either ventilated or solid and may include 1/2U or lU edge fold. The edge folds allow for configurations such as a 7U card frame for 6U cards with a 1/2U above and below the card area to improve ventilation.

Early designs of cover were fixed to the side panels with mounting blocks, which minimise the slot length and ensure good EMC contact. Newer covers are designed to "snap-in" between the side plates of the subrack and have "buttons" which provide an interference fit into the existing hole matrix at 10mm pitch along the top and bottom edges of the side plates. Gaskets are fitted to ensure good contact between the covers and the horizontal rails at the front and rear.

The horizontal rail profile has been developed to allow gaskets to be either fixed to them directly, such as the one to ensure front panels are in proper contact, or to accommodate other components with gaskets attached, such as covers. The gasket between the front panels and the horizontal rails is necessary in two circumstances. Firstly, large panels, frequently with hinged opening, have few fastening points. A relatively small number of fastening points allows a longer slot between the known contact points and thus can provide a greater leakage path. The springing of the gasket reduces the path length to about 10mm, such that the majority of frequencies are blocked. Secondly, the increased use of insertion and extraction lever handles without the optional retention screws on narrow (single or double slot) front panels, removes the guarantee of electrical contact between the panel and the horizontal rail. Again it is the 10mm pitch of protruding spring section which provides the necessary contact point to contain EMI / EMC.

The EMC cage can be the total subrack envelope or limited to the space surrounding the PCB's. In this case a suitable ground plane should be incorporated in the backplane to complete the cage. Rittal have included a contact system in the Ripac range to ensure the ends of the backplane make good electrical contact with the side plates of the subrack.
 
Rittal's Ripac subracks can provide EMC protection of over 40dB at a frequency of 1GHz when properly equipped as above.
Tel: 01709 704000

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