LIGHTNING PROTECTION SYSTEMS

Each year the United Kingdom, Ireland and the surrounding seas typically experience 200,000 to 300,000 lightning counts. A ‘thunderstorm day’ may experience as many as 10,000 but on exceptional days more than 50,000 can occur, as happened on 28 June 2012 when there were 64,000 strikes. Potentially dangerous cloud-to-ground strikes make up only one-quarter of all lightning generated by thunderstorms. Most other lightning happens wholly within the cloud and is visible only as a brightening of the cloud (often called ‘sheet lightning’).

Thunderstorms come into existence when warm air masses containing sufficient moisture are transported to great altitudes. This transport can occur in a number of ways such as when the ground is heated up locally by intense insolation which causes the layers of air near the ground heat up and rise. Or if there is an invasion of a cold air front which causes cooler air to be pushed below the warm air, forcing it to rise. Orographic thunderstorms are caused when warm air near the ground is lifted up as it crosses rising ground like a mountain or large hill side. Additional physical effects further increase the vertical upsurge of the air masses. This forms up draught channels with vertical speeds of up to 100 km/h, which create towering cumulonimbus clouds with typical heights of 5 to 12 km and diameters of 5 to 10 km.

Electrostatic charge separation processes, e.g. friction and sputtering, are responsible for charging water droplets and particles of ice in the cloud. Positively charged particles accumulate in the upper part and negatively charged particles in the lower part of the thundercloud. In addition, there is again a small positive charge centre at the bottom of the cloud. This originates from the corona discharge which emanates from sharp-pointed objects underneath the thundercloud (e.g. trees and buildings) and is transported upwards by the wind. If the space charge densities, which happen to be present in a thundercloud, produce local field strengths of several 100 kV/m, leader discharges are formed which initiate a lightning discharge. If a current is formed at a single point on a homogeneously conducting surface, a potential gradient area arises. This effect also occurs when lightning strikes homogeneous ground, if living beings (persons or animals) are inside this potential gradient area, step voltage is formed which can cause electric shock. The higher the conductivity of the ground, the flatter is the potential gradient area. The risk of dangerous step voltages is thus also reduced. If lightning strikes a building which is already equipped with a lightning protection system, the lightning current flowing via the earth-termination system of the building causes a voltage drop across the earth resistance of the earth-termination system of the building. As long as all exposed conductive parts in the building are raised to the same high potential, persons inside the building are not in danger

The function of a lightning protection system is to protect structures from fire or mechanical destruction and persons in the buildings from injury or even death.

A lightning protection system consists of an external and an internal lightning protection system.

The function of the external lightning protection system is:

• To intercept direct lightning strikes via an air-termination system
• To safely conduct the lightning current to the ground via a down-conductor system
• To distribute the lightning current in the ground via an earth-termination system

The function of the internal lightning protection system is:

• To prevent dangerous sparking inside the structure. This is achieved by establishing equipotential bonding or maintaining a separation distance between the components of the lightning protection system and other electrically conductive elements inside the structure.

Lightning equipotential bonding reduces the potential differences caused by lightning currents. This is achieved by connecting all isolated conductive parts of the installation directly by means of conductors or surge protective devices (SPDs)

The four classes of LPS I, II, III and IV are determined using a set of construction rules including dimensioning requirements which are based on the relevant lightning protection level.

Each set comprises class-dependent (e.g. radius of the rolling sphere, mesh size) and class-independent (e.g. cross-sections, materials) requirements. To ensure permanent availability of complex information technology systems even in case of a direct lightning strike, additional measures, which supplement the lightning protection measures, are required to protect electronic systems against surges.