Variation in Lightning Simulations to Assess Grounding Safety of Corrugated Stainless Steel Tubing (CSST)

Abstract

Codes and standards for lightning often rely on simulations due to the difficult nature of lightning testing, as in the case of corrugated stainless steel tubing (CSST). A small set of simulations by CSST manufacturers were previously used to justify the suggestion that grounding CSST would make it safe from perforation in the presence of lightning. Such a small set of simulations does not account for the uncertainty of lightning and the situations where it may interact with CSST. We account for these uncertainties in this work by performing thousands of simulations that use different combinations of simulation parameters. For example, for one scenario we run 2560 simulations with a variety of different waveforms and different impedance values. The waveforms follow IEC 62305 with rise times ranging from 0.25 $\upmu$s to 10 $\upmu$s and fall times ranging from 100 $\upmu$s to 1000 $\upmu$s and the impedance values were varied by ± $25\%$. Our results show that there are cases where grounding may prevent perforation, cases where grounding may reduce the damage but not prevent perforation and cases where grounding increases the chances of perforation. Our results further show that for lightning strikes with peak current greater than the median, there was never a case where grounding could have prevented perforation. Our methods provide a way to perform more comprehensive simulations to replicate what may happen in nature and better inform decisions made about codes and standards. In particular we show grounding of CSST will not prevent fires when assaulted by lightning with any reasonable degree of certainty.