Q283 : Standards for Safety with Transients

Question
Are there any standards regarding safe values of touch and step voltages for fast transient phenomena?

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Answer
There are no agreed standards for the allowable safe values for touch and step voltages during a fast transient such as a lightning strike. It is therefore very difficult to evaluate whether a given installation is safe from this point of view. Roughly speaking, the fast transient induces two main effects, which tend to cancel somewhat. The impedance of the grid is replaced by its surge impedance, which is normally considerably larger. This yields larger GPR, therefore larger touch and step voltages. This effect can be completely accounted for by proper modelling with HIFREQ and FFTSES. The second effect is that the human body appears to tolerate larger peak (and rms) currents for short duration pulses. Therefore, the safe touch and step voltages during a lightning stroke are usually larger than for a 50 Hz fault.

Note also that while you can get spectacularly large earth potentials and touch voltages at high frequencies, these higher frequencies carry usually a very small fraction of the total energy of the lightning signal and may not contribute a lot to the time domain potentials. The quantity of interest in the frequency-domain is the modulated spectrum, i.e. the product of the response of the grid (and tower) with the spectrum of the transient. This quantity will be plotted at the same time as you generate the time-domain results in FFTSES.

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