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Surge current withstand ratings, also called surge capacity ratings, are divided into two categories. Those that simulate accidental contact with AC power lines and those that simulate the pulse caused on cables by nearby lightning strikes. First, the lightning simulation surges will be looked at. The waveforms used to simulate lightning are called double exponential waveforms. One classic waveform used in telephone circuits is a ten by one thousand waveform. That means the wave will take ten microseconds to achieve its peak value and will take one thousand microseconds to decay to half of the peak value. Specifying three things, the peak value, the rise time and decay time, fully define the waveform. Most of the time the waveform is defined for the short-circuit current. Sometimes the open circuit voltage is also defined. The voltage and current waveforms may have different rise times and decay times. The most common current waveforms are ten by one thousand, eight by twenty and two by ten. Others are ten by seven hundred, ten by five sixty, and five by three twenty.
