3.1 back flashover (lightning): A flashover of insulation resulting from a lightning stroke to part of a
network or electric installation that is normally at ground potential.
3.2 basic impulse insulation level (BIL) (rated impulse withstand voltage) (surge arresters): The crest
value of a standard lightning impulse for which the insulation exhibits a 90% probability of withstand (or a
10% probability of failure) under specified conditions.
3.3 cloud-to-ground lightning flash: A lightning discharge to ground consisting of a first return stroke
that may be followed by subsequent strokes and other impulsive or continuing currents.
3.4 critical impulse flashover voltage (CFO) (insulators): The crest value of the impulse wave that,
under specified conditions, causes flashover through the surrounding medium on 50% of the applications.
3.5 direct flash: A lightning flash with one or more return strokes terminating directly to any part of a
network or electric installation.
3.6 direct stroke: A lightning stroke directly to any part of a network or electric installation.
3.7 distribution line: Electric power lines which distribute power from a main source substation to
consumers, usually at a voltage of 34.5 kV or less, but possibly at a phase-to-phase voltage of up to 69 kV.
3.8 flashover (general): A disruptive discharge through air around or over the surface of solid or liquid
insulation, between parts of different potential or polarity, produced by the application of voltage wherein
the breakdown path becomes sufficiently ionized to maintain an electrical arc.
3.9 ground electrode: A conductor or group of conductors in intimate contact with the ground for the
purpose of providing a connection with the ground.