Editor’s note: The following article is based on a white paper written by Dion Neri and Bruce Glushakow that is currently in review as an IEEE transactions paper.

For decades, American transient voltage surge suppressor (TVSS) products have been tested using the 8/20microsec waveform according to ANSI/IEEE C62.41 guidelines. The products designed and manufactured under these guidelines have performed well in the field.

Until 1995, the 8/20microsec waveform was also accepted in virtually all other countries, including those that use the International Electrical Code (IEC). At that time, however, the IEC 61643 family introduced a “Class 1” test for surge protectors installed at service entry locations. In order to meet the impulse current (I_imp) requirements of IEC 61643-1, testing organizations now had to use the 10/350microsec test waveform. The change was based upon the assumption that the 10/350microsec waveform better simulated the parameters of a direct lightning strike, hence making it more relevant in the determination of the effectiveness of surge protective devices (SPDs).

Based upon its proven track record, the IEEE saw no reason to change its SPD testing standards. However, the introduction of the IEC 61643-1 Class 1 test opened a window of opportunity and European manufacturers engage upon a campaign to legitimize the use of this long waveform. Their message was simple: “If direct lightning strikes could only be simulated by a 10/350microsec waveform, then the SPDs produced according to ANSI/IEEE guidelines were an inadequate defense against lightning.”

According to the new testing requirements, the 10/350microsec waveform is to be used for protection against direct lightning currents while the 8/20microsec waveform is useful only for protection against indirect lightning effects.

One manufacturer produced a diagram (at right) to show that a 100kA 10/350microsec impulse delivers 125 times as much discharge in amps as a 20kA 8/20microsec pulse. And use of an MOV protector for a 100kA 10/350microsec pulse would require about 2,500kA 8/20microsec MOV capability.

In reality, there’s no ANSI/IEEE-tested, low-voltage SPD on the market rated anywhere near 2,500kA. It’s understood that a TVSS rated at 400kA 8/20microsec is more than sufficient for even the most extreme lightning environments.