Lightning protection system

Revision as of 11:14, 27 April 2017

Introduction

A lightning protection system (LPS) is intended to protect a structure or building from damage caused by being struck by lightning. The voltage currents of a lightning strike can often exceed 100 million Volt Amps.

The main purpose of an LPS is to the ensure safety to a building and its occupants by providing a low resistance path to ground for the lightning to follow and the energy to be dispersed. This is important as high voltage currents from a lightning strike will always take the path of least resistance to ground.

An LPS does not attract lightning, and cannot dissipate lightning in any way, but do provide fire and structural damage protection by preventing lightning from passing through building materials.

Buildings most at risk are those at high altitudes, on hilltops or hillsides, in isolated positions and tall towers and chimneys.

Hazards of a lightning strike

In the absence of an LPS, a lightning strike may use any conductor as a path to reach ground which could include the phone, cable or electronic lines, other utilities such as water or gas pipes, or the structure itself if it is a steel frame.

Any grounded object that provides a path to earth will emit upwards ‘positive streamers’ or fingers of electrical charge. These create a channel of plasma air for the huge downward currents of a lightning strike.

Some of the main hazards presented by a lightning strike to a building include:

• Fire: Caused by lightning igniting flammable materials or overheating electrical wiring.
• Side flashes: When lightning jumps through the air to reach a better-grounded conductor.
• Damage to building: Building components can be damaged by explosive shock waves, glass shattered, concrete and plaster fragmented, and so on.
• Damage to building contents: Any electrical appliance plugged into a circuit may be badly damaged.

If there is no low resistance path provided by an LPS, the strike will divide and follow every possible conductive path to ground. This can mean passing through otherwise insulating materials at very large heat levels. This can be dangerous for porous materials such as masonry if the air inside it expands rapidly; similarly with materials containing moisture from humidity or rain which flashes to steam. Other materials can reach their plastic limit and melt or ignite.

Types of LPS

The main types of LPS that can be used are as follows:

Rods or ‘air terminals’

A lightning rod is a tall metallic tip, or pointed needle, placed at the top of a building. One or more conductors, often copper strips, are used to earth the rod. Rods are designed to act as the ‘terminal’ for a lightning discharge.

Conductor cables

This involves numerous the placing of numerous heavy cables all around the building in a symmetrical arrangement. This is sometimes referred to as a ‘Faraday cage’. These cables are run along the tops and around the edges of roofs, and down one or more corners of the building to the ground rod(s) which carry the current to the ground.

This type of LPS is often used for buildings which are highly exposed and/or house sensitive installations such as computer rooms.

Ground rods

These are long and thick rods which are buried deep into the earth around a protected structure. They are normally made of copper or aluminium and are designed to emit positive streamers rather than the structure they are protecting.

The conductor cables and ground rods are the most important components of an LPS, whereas the lightning rods may not play a large role in the system’s functionality.

Designing an LPS

Since lightning can be unpredictable, an LPS should take this into consideration during the design stage. The design should ensure that even if lightning strikes the structure first, the large voltage currents will be drawn into the LPS before serious damage can be done.

An LPS can be designed so as to utilise parts of the building that can safely carry large current loads, and draw energy away from the parts of the building that are not able to.

An LPS should be designed and installed to try and prevent side flashes between objects from occurring. By maintaining the electrical continuity of objects to a bonding conductor, any differences in electric potential can be zeroed, allowing any voltage changes to occur simultaneously.

Failure to design proper grounding will render an LPS ineffective as safe dispersal of the energy from the strike will not be possible. Additional earthing from that provided by a utility supplier is often required.

Find out more

Related articles on Designing Buildings Wiki

• Building services.
• Concept services design.
• Environmental health.
• Fire.
• Fire safety design.
• Health and safety plan.
• M&E.
• Phase change materials.
• Risk in building design and construction.