Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <A123X-at-aol-dot-com> One of my NST's failed with a protection filter. Another I never used in TC use. The one that lasted longest used a resonant cap and no filter. The other is still living. It would end up being cheaper just to get a potential transformer and ballast it with the NST's. Mark