Two tests using short rise times are used in a similar
way to the destructive hipot test. A simulated lightning
impulse is applied across the main insulation to determine
its ability to withstand a fast, high-energy wave. A shorter
rise time applied to a coil with its surface ungrounded
establishes a nonequilibrium voltage condition among the
turns (Figure 2). Under ac, turn-to-turn insulation does
not come into play, but at a rise time of lower than about
0.5 μs, it appears as an insulator between two areas of differing
high potential. This test simulates a very rapid electrical
switching surge on the motor winding, such as that
from a breaker closure [13], [14].