Enclosure openings Ideal shielding can be achieved by placing the electronic com-ponents in a metal enclosure welded all round. This is only veryrarely possible, however, in practical applications. Openings are required for operation and indications, for cable entries, for ven-tilation and for the mounting and removal of components.The individual parts of the enclosure must be electrically con-ductive with good continuity across the joints in order to preventa reduction of the shielding effectiveness. Without conduc- tive gasket or contacts with closely situated contact points (e.g. screws) gaps are produced, which produce antenna effectsowing to their length. Gaps lengths less than Cable entries Careful consideration should be paid to the cable entries in the enclosure. Errors in cable runs can result in the required shielding effectiveness being almost entirely lost. The followingpossibilities are available for correct execution of cable entries and exits.
Decouplingcapacitors 1 /
20 of the wavelength only have a negli-gible effect on enclosure shielding effectiveness. In units subjectto strong radiation, the gap length should be kept shorter than 1
1 /
50 of the wavelength.
st possibility Decoupling capacitors. Decouple all cable wires with a capacitor of low value (10nF) and very short connections to the enclosure. Emc gaskets The primary function of an EMC gasket is the reduction of theinterface resistance across a joint of the enclosure. The lower the interface resistance, the better the industrial seal. The interface resistance depends on the surface resistance ofthe enclosure parts, the conformability of the seal, the contactpressure and the conductivity of the seal.Elma industrial enclosures are provided with copper-beryllium contact strips and a plastic foam seal, sheated with a conductive metal.These seals ensure low interface resistance.The effectiveness of the EMC gasket under extreme environ-mental conditions, e.g. humid heat, is extremely important.
Shield contact all roundShielded cableMetallicconnector body Filter 2
nd possibility: Shielded cables. The shield contacts the enclosure all round viaa cable fitting or a metallic connector body. 3
rd possibility: Filter. The filter must be screwed to a good conducting surfaceof the enclosure.
201