A Quarter Century or Experience in Insulation Co-Ordination: Basic Philosophy, Application, and Operating Experience on the AG&E System

1. The BILs set initially in the 1941 report of the AIEE-EEI-NEMA Committee on Insulation Co-ordination10 have proved to be sound, with adequate flexibility for economic application. The authors have worked effectively with this framework. The 60-cycle rating of the lightning arrester is the base point for determining a suitable BIL, and therefore must be selected only after full and careful consideration of all factors affecting the arrester operation in service. 2. Aimed for margins between lightning-arrester protective levels and BIL have generally been 30%, but 20% margins have been used. With the newer (1955) arresters, much larger margins have become available. 3. Systems with grounded neutrals can, in general, be protected by lightning arresters rated 80% or less than those used on isolated neutral systems. Since system rated voltages follow a pattern in which any system voltage is roughly 80% of the next higher one, the required BIL follow approximately the same pattern, assuming similar margins of protection between BIL and lightning-arrester protective levels. Because of this, one level down in BIL has been a logical and safe step where station-type arresters are used, so far as safe lightning protection is concerned. 4. With the newer arresters (1955) having some 20% lower impulse level, two steps down in insulation is in some cases possible. 5. Substantial economies in transformer first cost are possible in applying reduced insulation on transformers in insulation classes of 92 kv and over; below the 92-kv class the saving is very small or nonexistent.