Under the right conditions of over-voltage and line impedance, it may be possible to cause the MOV to burst into flames, the root cause of many fires and the main reason for NFPA s concern resulting in UL1449 in 1986 and subsequent revisions in 1998 and 2009 . Properly designed TVSS devices must not fail catastrophically, resulting in the opening of a thermal fuse or something equivalent that only disconnects MOV devices.
12.
The behavior of such a system is independent of the operating frequency, but is dependent upon details of the power conductor and its environment . " A nearby conductor other than the line itself may provide a termination point and thereby reduce energy coupled into the TM wave " . ( This has relevance to Tesla's 1891-1893 table-top demonstrations . ) As for any transmission line, at very high frequencies, the increased losses of the metal conductor, despite the advantage obtained using the surface wave mode, are increased, however because conductor losses are inversely proportional to the square of line impedance, this mode can achieve much lower losses, no more than a few percent of a 50 ohm coaxial line having the same center conductor.
13.
In electrical systems involving transmission lines ( such as radio and fiber optics ) where the length of the line is long compared to the wavelength of the signal ( the signal changes rapidly compared to the time it takes to travel from source to load ) the impedances at each end of the line must be matched to the transmission line's characteristic impedance ( Z _ c ) to prevent reflections of the signal at the ends of the line . ( When the length of the line is short compared to the wavelength, impedance mismatch is the basis of transmission-line impedance transformers; see previous section . ) In radio-frequency ( RF ) systems, a common value for source and load impedances is 50 antenna ( 37 ohms with an ideal ground plane; it can be matched to 50 ohms by using a modified ground plane or a coaxial matching section, i . e ., part or all the feeder of higher impedance ).
