re well matched to this frequency and couples h.f. interference straight through from one 'design' requirement. side to the other via the filter's local ground path. Common input- 3. Any components on the mains wiring side of equipment are output wiring does the same thing through stray capacitance. The exposed to an extra layer of safety regulatory requirements. cures are obvious: always mounting the filter SO that its ground node Filter manufacturers are able to amortise the cost of designing is directly coupled to the lowest inductance ground of the equip- and certifying their products to the plethora of national stand- ment, preferably the chassis, and keeping the input and output leads ards over a large number of units, thus relieving the equipment separate, preferably screened from each other. The best solution is manufacturer to some extent of this particular burden. to position the filter SO that it straddles the equipment shielding. Locating a filter directly at the mains inlet lends itself well to the provision of the whole input circuitry - connector, filter, 23.14.2 Mains filters fuse, on/off switch - as one block which the manufacturer can 'fit and forget'. RFI filters for mains supply inputs have developed as a separate 5. Many equipment designers are at a loss when it comes to r.f. species and are available in many physical and electrical forms from filter design, and prefer a bought in solution. several specialist manufacturers. A typical 'block' filter for Euro- pean mains supplies with average insertion loss might cost below On the other hand, mains filters can be designed in with the rest £5. The reasons for this separate development are: of the circuit, and this becomes a cost effective approach for high volume products and is almost always necessary if the optimum 1. Mandatory RFI emission standards have concentrated on inter- filter performance is essential. A typical mains filter circuit (Figure ference conducted out of equipment via the mains, and conse- 23.29) includes components to block