The Need for Lightning Protection

 

Lightning consitutes a serious risk for people, structures and equipment.

The lightning stroke follows its kilometer-long channel coming from the sky. Any lightning protection system is the final part of this channel. Such a part however is in the neighbourhood of building structures and electronic and equipment.

The immediate effect of primary overvoltages caused by the passage of lightning current can result in damage to the structure itself and to its occupants and contents, including failure of equipment especially of electrical and electronic systems.

The damages and failures may extend to the surroundings of the structure and may even involve the local environment. The scale of this extention depends on the characteristics of the structure and those of the lightning flash.

Figure 1 shows the statistics of transients due mainly to lightning strokes in different environments in Switzerland [1].

 

The point of strike on the lightning protection system may be raised to a high potential which is hazardous to touch (touch potential risk). To reduce such hazard to a tolerable level, insulation of the exposed down conductor shall be provided giving a minimum 100kV, 1.2/50ms impulse withstand in accordance with the recommendation of lightning protection standard BS EN 62305-3 Clause 8.1.

Additionally, there is a risk of side flash from the protection system to any other metal on or in the structure if large potential difference exists between them.

When side flash occurs, part of the lightning current may be discharged via a path of air, wood, or concrete, and through internal installations such as pipes and wiring, causing potential threats to the occupants and the fabric of the structure before it terminates at the ground.

If the path is through steel encased in concrete, moisture in this path can turn into steam, which can crack or damage the structure. The resulting explosion can start a fire.

Equipotential bonding can remove side flash and the possibility of fire risk, but it is not effective in reducing the touch voltages. On the contrary, the degree of touch potential risk increases since both the lightning protection system and all metal within the low voltage equipotential zone become hazardous to touch after equipotential bonding when lightning current flows.