The amplitude of the current of the lightning stroke that, upon terminating on the phase conductor, would raise the voltage of the conductor to a level at which flashover is likely.

The downward leader of a subsequent stroke of a multiple stroke lightning flash.

The average number of lightning strokes per unit area per unit time at a particular location.

The average annual number of thunderstorm days or hours for a given locality.

1) A daily keraunic level is called a thunderstorm-day and is the average number of days per year in which thunder is heard during a 24 hr period.

2) An hourly keraunic level is called a thunderstorm-hour and is the average number of hours per year that thunder is heard during a 60 min period.

The complete lightning discharge, most often composed of leaders from a cloud followed by one or more return strokes.

A column or narrow base structure containing a vertical conductor from its tip to earth, or that is itself a suitable conductor to earth. Its purpose is to intercept lightning strokes so that they do not terminate on objects located within its zone of protection.

A wire suspended above the phase conductors positioned with the intention of having lightning strike it instead of the phase conductor(s).

Static discharge that propagates from a cloud into the air. Current magnitudes that are associated with stepped leaders are small (on the order of 100 A) in comparison with the final stroke current. The stepped leaders progress in a random direction in discrete steps from 10 to 80 m in length. Their most frequent velocity of propagation is about 0.05% of the speed of light, or approximately 500 000 ft/s (150 000 m/s). It is not until the stepped leader is within striking distance of the point to be struck that the stepped leader is positively directed toward this point.

The length of the final jump of the stepped leader as its potential exceeds the breakdown resistance of this last gap.

The ratio between voltage and current of a wave that travels on a conductor.

A day on which thunder can be heard, and hence lightning occurs.

An hour during which thunder can be heard, and hence when lightning occurs.

That which permits lightning strokes no greater than those of critical amplitude (less design margin) to reach phase conductors.

A geometrical representation of a facility, that, together with suitable analytical expressions correlating its dimensions to the current of the lightning stroke, is capable of predicting if a lightning stroke will terminate on the shielding system, the earth, or the element of the facility being protected.

The theory describing the electrogeometric model together with the related quantitative analyses including the correlation between the striking distances to the different elements of the model and the amplitude of the first return stroke.

Lines on a map connecting points having the same keraunic level.

The shielding angle formed when the shield wire is located beyond the area occupied by the outermost conductors.

The shielding angle formed when the shield wire is located above and inside of the area occupied by the outermost conductors.

A simplified technique for applying the electrogeometric theory to the shielding of substations. The technique involves rolling an imaginary sphere of prescribed radius over the surface of a substation. The sphere rolls up and over (and is supported by) lightning masts, shield wires, fences, and other grounded metal

objects intended for lightning shielding. A piece of equipment is protected from a direct stroke if it remains below the curved surface of the sphere by virtue of the sphere being elevated by shield wires or other devices. Equipment that touches the sphere or penetrates its surface is not protected.

Shielding Angle

(of shield wires with respect to conductors)

The angle formed by the intersection of a vertical line drawn through a shield wire and a line drawn from the shield wire to a protected conductor. The angle is chosen to provide a zone of protection for the conductor so that most lightning strokes will terminate on the shield wire rather than on the conductor.

Shielding Angle

(of a lightning mast)

The angle formed by the intersection of a vertical line drawn through the tip of the mast and another line drawn through the tip to earth at some selected angle with the vertical. Rotation of this angle around the

structure forms a cone-shaped zone of protection for objects located within the cone. The angle is chosen so that lightning strokes will terminate on the mast rather than on an object contained within the protective zone so formed.

The return stroke is the extremely bright streamer that propagates upward from the earth to the cloud following the same path as the main channel of the downward stepped leader. This return stroke is the actual flow of stroke current that has a median value of about 24 000

A and is actually the flow of charge from earth to cloud to neutralize the charge center [B70]. The velocity of the return stroke propagation is about 10% the speed of light, or approximately 100 · 10^6 ft/s (30 · 10m/s).