Re: BIL
1.4 x 40 microseconds describe the surge voltage wave pulse shape used to test the transformer or other insulation system. The voltage surge must reach its peak in 1.4 microseconds and decay to 50% of its peak in 40 microseconds. Other test waves are 1.2 x 50 and 250 x 500. These are DC voltage surges, like a hypot, only a lot faster.
A 95 kV BIL transformer winding will be designed to withstand a surge voltage that hits 95 kV in 1.2 microsecond and drops to 42.5 kV in 50 microseconds. It imitates a lightning surge.
There are several other tests that use different wave shapes to test for other characteristics.
Wave shape has an impact on transformer winding, arrestor and bushing design.
Think of comparing a 12-foot tide change on a beach with a 12-foot tsunami. The tsunami wave has a lot more destructive power when it hits the seawall even though high tide reaches the same water level. It is the same with a voltage surge hitting the transformer winding impedance. A fast rise time wave, like lightning, can cause more damage than a gentle voltage wave.