Lightning amplitudes

For instance, when lightning of, say, a nominal discharge current of 10 kA strikes a 400 kV (r.m.s.) overhead line, having a surge impedance of 350 Q, then two parallel waves will be produced each of amplitude 10 x 350/2 or 1750 kV which may be more than the impulse withstand level of the system and cause a flashover between the conductors and the ground, besides damaging the line insulators and the terminal equipment (Table 13.2). It is therefore imperative that the system is protected against such eventualities. 

When such an arrester is installed on the system, it takes care of switching, lightning or steep to very steep rising surges, irrespective of their amplitudes. 

Although there is no interference from other NQR signals, other types of signals may interfere with NQR explosives detection. For unshielded detector coils, the primary culprit is RF interference (RFI). Most NQR frequencies of interest for explosives detection fall in the frequency range of 0.5-5 MHz. The biggest problem in this range is the amplitude modulation (AM) radio band, from 0.5 to 1.5 MHz. Besides radio transmissions, other sources of RFI can include nearby electrical equipment, power transmission lines and lightning strikes. 

Amplitude Modulation (AM). With AM radio, the amphtude (height) of the transmitted signal is made proportional to the sound amplitude captured (transduced) by the microphone. The transmitted frequency remains constant. AM transmission is degraded by static and interference because sources of electromagnetic transmission such as lightning and automobile ignitions, which are at the same frequency, add their amplitudes to that of the transmitted signal. AM radio stations in the United States and Canada are limited to 50 kilowatts (kW). Early twentieth century, U.S. stations had powers up to 500 kW, some of which could be heard worldwide. 

Measured results of lightning current amplitudes. (From IEEE Guide for Improving the Lightning Performance of Transmission Lines. 1997 Ametani, A. et al. 2002. lEE Japan WG Report. Technical Report No. 872 CRIPEI WG. 2003. Guide to transmission line protection against lightning. Report T72 Ametani, A. and T. Kawamura. 2005. IEEE Trans. Power Deliv. 20 (2) 867-875 Anderson, R. B. and A. J. Eriksson. 1980. Electro 69 65.)... 

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