he amount of interference that can be test results. These are: fed onto the mains via the power input leads. A great deal of work has been done to characterise the impedance of the mains supply, The apparatus continues to operate as intended with no degra- and perhaps surprisingly measurements in quite different environ- dation below a performance level specified by the manufac- ments show close agreement. This has allowed the development of turer. the CISPR16 artificial mains network (LISN) referred to in the last 2. The apparatus continues to operate as intended after the test, but section. The impedance can be simulated by 50S in parallel with during the test some degradation of performance is allowed. 50H with respect to earth (Figure 23.21) and Figure 23.22 shows 3. Temporary loss of function is allowed, provided that it is self its impedance-frequency plot. Power cables tend to act as low loss recoverable or operator recoverable. transmission lines up to 10MHz SO that interference can propagate quite readily around the power distribution network, mainly attenu- ated by the random connection of other loads rather than by the 23.11 Interference coupling mechanisms cable itself. Interference can be coupled into or out of equipment over a number 23.11.2 Radiated of routes (Figure 23.20). At low frequencies the predominant modes of coupling are directly along the circuit wires or by induction, but When source and victim are near one another (in the 'near field', d at high frequencies each conductor, including the equipment hous- being given by inequality 23.2), radiated coupling is predominantly ing if it is metallic, acts as an aerial in its own right and will due either to magnetic or electric induction. contribute to coupling. (23.2) 23.11.1 Conducted A very frequent cause of conducted interference coupling is the existence of a common impedance path between the interfering and victim circuits. This path is usually though not invariably in the ground return. typical case of common impedance coupling might Equ