All About Lightning

Lightning Primer

Most lightning that reaches the ground (75% to 90%) is negatively charged. It begins to intercept the ground by lowering a stepped leader - a precursor to the actual lightning discharge.

This leader progresses in steps toward the ground and is comprised of electric charge. It completes this process in a length of time measured in tens of milliseconds.

This leader progresses in steps toward the ground and is comprised of electric charge. It completes this process in a length of time measured in tens of milliseconds. Below the leader is a region of very high electric field.

As the leader approaches the earth, the high electric field induces objects on the ground to emit leaders of opposite polarity charge. Since opposite charges attract, the path of the downward leader is influenced by an upward leader of opposite polarity. Upon connection, the actual current discharge associated with lightning begins as shown in figure below

Types of Lightning Strikes

Direct Strike

A direct lightning attachment to an unprotected structure usually causes fire and electrical damage. Occasionally, explosive damage will occur if lightning attaches to a chimney or other porous structural component. Typically, an attachment will be to a roof or protrusion, and arcing within the structure will cause ignition of structural materials.

Since lightning prefers the path of least impedance once it attaches to a structure, it will often flow to the electrical system and thence to earth, causing severe damage to the wiring.

Indirect Electrical Effects

Several damaging effects can occur from lightning other than the effects from the direct strike. Flashover occurs when lightning attaches to something that has a relatively high impedance path to ground. A good example of such an event is lightning striking a tree.

The tree presents a path to ground, but the path has a high impedance compared to that of a properly installed, well-grounded lightning protection system.