When should a BIL rating
be specified?
Devices built to a given BIL (basic insulation level or basic impulse level) rating are designed and tested to withstand high impulse levels. To meet a given BIL rating a device must be able to withstand a simulated lightning wave that has a wave shape of 1.2 by 50 µsec and in both positive and negative polarities. The wave reaches its peak in 1.2 µsec and decays to ½ of the peak value in 50 µsec. BIL ratings are expressed in
kilovolts (KV).
   
Power control and/or power management subsystems in medium to high voltage applications at the PCB level are being increasingly exposed to surges of varying time and voltage levels. The results can be “false triggers”, repetitive on – off cycling, reduced MTBF and catastrophic failure. BIL ratings, typically, had previously been limited to gas and liquid filled transformers usually located outdoors and most often with BIL requirements exceeding 100 KV. This has now changed as internal as well as external grid saturation coupled with lightning strikes, inductive loading, load variances, and on grid switching create more frequent grid perturbations in the form of voltage surges. The menu bar on the left links to data sheets describing the nine basic design platforms for BIL rated devices now available along with applicable KV ratings.

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