Driving High Surge Currents into Long Cables: More Begets Less

Significance: Part 2 Development of Standards – Reality checks Part 4 Coupling and propagation of surges In the propagation of a surge current injected at the service entrance of a building, two significant factors can prevent the propagation of a postulated " large " surge current to the end of the branch circuits of the facility. 1. The combination of the inherent inductance of the wiring and the high rate of current change for such a current to begin flowing into the branch circuit results in a high voltage at the driving end (V = L x di / dt). 2. In the absence of a surge=protective device at the service entrance, the withstand voltage of the wiring devices at the driving end – the service entrance – is very likely to be exceeded by the voltage that this rising current will develop along the branch circuit. The resulting flashover will abort further propagation of the surge current toward the far end, thus establishing a limit to what is physically possible. If there is a surge-protective device at the service entrance, the scenario becomes a matter of cascade coordination. Provides quantitative information on this limitation, as a function of wiring length and current rate of rise. Abslract-Reality checks can and should be applied to proposals for characterizing the surge environment and application of surge-protective devices (SPDs) to end-user, low-voltage power systems. One such check is the fact that driving a large current with steep front toward an SPD installed at the far end of a branch circuit cable could require such a high voltage that the connections at the near end of the cable will flashover, limiting the stress applied to the far-end SPD. Tests and numerical modeling were performed to support this thesis. The results of real-world measurements and modeling, presented in the paper, are in good agreement and validate each other. From that point on, the model allows parametric variations of cable length and surge current amplitude and waveform, of which several examples are presented.