Lightning protection system

INTRODUCTION

LIGHTNING PROTECTION SYSTEM

-A lightning protection system is designed to protect a structure from damage by intercepting such strikes and safely passing their extremely high voltage currents to "ground."

-This system includes a network of air terminals, bonding conductors, and ground electrodes designed to provide a low impedance path to ground against potential strikes.

-Each year thousands of properties are damaged or destroyed by lightning.

-The lightning protection system (LPS) used in Malaysia and around the world is basically divided into two types:

1) Conventional or standard LPS i.e. that which comply with the technical standard.

2) Unconventional or non-standard i.e. those that do not comply with the standard.

Components on Lightning Protection System

The difference on Conventional & Unconventional LPS System

Conventional

-The conventional lightning protection is combination of the franklin rods and faraday cage method.

-As Benjamin franklin discovered the lightning should have a specific point to strike before safely flowed the electrical charge into earth.

-The air terminals associated with the unconventional LPS are the so-called ‘active’ air terminals.

-E.g. the early streamer emission (ESE) and the charge transfer system(CTS).

Unconventional

 

-In the 1970s, two types of unconventional air terminals had been commercially reinvented and introduced in the world market. They are the lightning prevention air terminals and the lightning air terminal.

-Only one unconventional air terminal is usually installed centrally on the roof of a building such as a bungalow or a high-rise apartment block.

-However, for buildings with a larger roof area, two or more unconventional air terminals may be installed and they are normally spaced at some distance apart from one another.

-In reality, the inventors of these unconventional air terminals have never been able to provide any scientific basis for their invention.

-In addition to this, these inventors have never been able to provide any independently validated proof that their inventions work.

Damages by lightning

 Lightning strikes should be taken seriously. A lightning strike is a discharge of atmospheric electricity which is generated by an accumulation of differing charges within a cloud. It is one of the most common causes of weather-related deaths in the U.S.  Each year lightning strikes cause billions of dollars in property damage and complete devastation. Lightning can start a direct fire because the core temperature of a lightning strike is approximately 50,000 degrees Fahrenheit. The harm to structures, electronics, building systems and vehicles can be extensive, causing loss of product and major downtime.

Major hazards caused by lightning

• Fire damage: The biggest threat lightning poses to a structure is fire. Wood and other flammable construction materials can easily explode when exposed to the high temperature of a lightning strike. Lightning current travelling through wires and pipelines instantly burns them up causing complete damage to property.
• Power surge damage: If lightning chooses electrical wiring as its primary or secondary path, the explosive surge can damage all the appliances it is connected to.
• Shock wave damage: Lightning produces shock waves that can be destructive. These shock waves can fracture concrete and brick and stone chimneys severely.

Basic Principle of Lightning Rod

A lightning conductor is a device invented by Benjamin Franklin to protect buildings from lightning strikes. It provides a low-resistance path for the lightning current to flow to the ground, preventing any damage that would have resulted if the lightning current had passed through the building to the ground. Let us now consider how a lightning conductor acts during a lightning strike.

Lightning rods were originally developed by Benjamin Franklin. A lightning rod is very simple -- it's a pointed metal rod attached to the roof of a building. The rod might be an inch (2 cm) in diameter. It connects to a huge piece of copper or aluminum wire that's also an inch or so in diameter. The wire is connected to a conductive grid buried in the ground nearby.

The purpose of lightning rods is often misunderstood. Many people believe that lightning rods "attract" lightning. It is better stated to say that lightning rods provide a low-resistance path to ground that can be used to conduct the enormous electrical currents when lightning strikes occur. If lightning strikes, the system attempts to carry the harmful electrical current away from the structure and safely to ground. The system has the ability to handle the enormous electrical current associated with the strike. If the strike contacts a material that is not a good conductor, the material will suffer massive heat damage. The lightning-rod system is an excellent conductor and thus allows the current to flow to ground without causing any heat damage.

Lightning can "jump around" when it strikes. This "jumping" is associated with the electrical potential of the strike target with respect to the earth's potential. The lightning can strike and then "seek" a path of least resistance by jumping around to nearby objects that provide a better path to ground. If the strike occurs near the lightning-rod system, the system will have a very low-resistance path and can then receive a "jump," diverting the strike current to ground before it can do any more damage.

As you can see, the purpose of the lightning rod is not to attract lightning -- it merely provides a safe option for the lightning strike to choose. This may sound a little picky, but it's not if you consider that the lightning rods only become relevant when a strike occurs or immediately after a strike occurs. Regardless of whether or not a lightning-rod system is present, the strike will still occur.

If the structure that you are attempting to protect is out in an open, flat area, you often create a lightning protection system that uses a very tall lightning rod. This rod should be taller than the structure. If the area finds itself in a strong electric field, the tall rod can begin sending up positive streamers in an attempt to dissipate the electric field. While it is not a given that the rod will always conduct the lightning discharged in the immediate area, it does have a better possibility than the structure. Again, the goal is to provide a low-resistance path to ground in an area that has the possibility to receive a strike. This possibility arises from the strength of the electric field generated by the storm clouds.

EXTERNAL LIGHTNING PROTECTION SYSTEM

Functions of an External Lightning Protection System

• Interception of direct lightning strikes via an air-termination system.
• Safe discharge of lightning current to earth via a down-conductor system.
• Distribution of the lightning current in the ground via an earth-termination system.

The external lightning protection system which includes:

1. Air termination system
2. Down conductor system
3. Earth termination system

Down conductors

Down conductor positioning and distancing is often dictated by architectural constraints. There should be one down conductor for every 20m or part there of the building perimeter at roof or ground level (whichever is greater). These should be evenly spaced and distances apart of more than 20m avoided if possible.

If the building is above 20m in height or of an abnormal risk this distance should be reduced to 10m.They should be routed as directly as possible from the air termination network to the earth termination network to avoid risks of side flashing. Re-entrant loops are also to be avoided. BS 6651 recommends that the length of conductor forming the loop should not exceed eight times the width of its open side.

Earth termination networks

Each down conductor must have a separate earth termination. Moreover, provision should be made in each down conductor, for disconnection from the earth for testing purposes. This is achieved with a test clamp. With the test clamp disconnected, the resistance of each individual earth should be no more than ten times the number of down conductors in the complete system. For example, a system with 15 down conductors, the individual earth readings should be no more than 10 x 15 = 150 ohms.

Several types of earth electrode are permissible, but by far the most commonly used are deep driven earth rods. The combined earth rod length of a system should be no less than 9m whilst each individual earth rod should be no less than 1.5m in length.

There are two basic types of earth electrode arrangement:

Type ‘A’ arrangement

The type A earth arrangement uses vertical or horizontal earth electrodes. Practically, it uses both connected to each down conductor, installed outside the structure to be protected and housed in a plastic or concrete earth pit for ease of inspection. The minimum number of electrodes is 2 regardless of the perimeter of the structure.

Type ‘B’ arrangement

The type B earth termination arrangement is most suitable for structures build on rocky ground and structures housing sensitive electronics. The type B Earth electrode arrangement is recommended as either a ring conductor outside the perimeter of the structure which it’s recommended should be in contact with the soil for at least 80% of its total length. The alternative is to use a foundation earth electrode which can be in a mesh form. It is recommended that the type B earth termination network whichever method is chosen should be integrated as a meshed network buried to a minimum depth of 5 meters. The reinforced concrete floor slab can be used around the structure. If the required resistance cannot be achieved by this method the vertical or radial electrodes can be added into the network.

Air Termination System

                                                                

A conducting device mounted above the highest point of a building and it provides protection by discharging atmospheric electric charges to down conductor. Air termination networks may consist of vertical or horizontal conductors or combined of both. A single point air termination is considered to give a zone of protection defined by a cone; the apex of the cone is formed by the air termination and the base of the cone has a radius equal to the height of the air termination.

The element of air termination system can be composed;

• Rods (including free-standing masts)
• Centenary wires (suspended)
• Meshed conductors

Air termination components installed on a structure shall be located at corners, exposed points and edge in accordance with one or more of the following method.

The protection angle method

This method is a mathematical simplification of the rolling sphere method. The protection angle is measure by the radius of the rolling sphere and has varies with the height of the air termination & thus the class of LPS. The protection angle method is suitable for simple-shape buildings but it is subject to limits of air termination height.

                              

The rolling sphere method

This is a simple method to find area needs to be protected and used to determine side strikes. In this sphere is rolled over the surface of structure. The point where sphere touches is vulnerable to lightning and needs air termination. The rolling sphere method is suitable in all cases.

                                            

The mesh method

This method is for the protection of plan or flat surface. This method is used only if the meshed conductors are;

1. Positioned on the edges of the surface
2. The mesh size is in accordance with the standard table
3. No metallic structure should protrude outside the volume
4. From each point at least 2 separate path should exists to ground.

                                                     

INTERNAL LIGHTNING PROTECTION SYSTEM

Functions of an Internal Lightning Protection System is prevention of dangerous sparking in the structure by establishing equipotential bonding or keeping a separation distance between the LPS components and other electrically conducting elements

Components of the internal lightning protection system

1. Equipotential bonding system

Main Equipotential Bonding

Equipotential bonding is simply the electrical interconnection of all appropriate metallic installations/parts, such that in the event of lightning currents flowing, no metallic part is at a different voltage potential with respect to another because if the metallic parts are essentially at the same potential then the risk of sparking or flash over is nullified.

This electrical interconnection can be achieved by:

• Using natural/fortuitous bonding or
• Using direct connection by specific bonding conductors that are sized according to BS EN 62305-3,
• Using non-direct connection, where the direct connection with bonding conductors is not suitable, by using surge protection devices (SPDs) which must conform to BS EN 62305-4.

General Notes For Equipotential Bonding:

• In small buildings, bonding bar should be located close to the main distribution board (MDB) and also closely connected to the earth termination system with short length conductors.
• In large facilities, several interconnected bonding bars may be needed. Interconnection should be via a dedicated internal ring (or partial ring), or via the internal reinforcing of the concrete construction
•  An isolated LPS only requires bonding to the structure at ground level.
•  Non-isolated LPS’s require bonding to the structure at ground level, and at locations where separation distance requirements cannot be maintained.
• For structures taller than 30m the standard recommends that equipotential bonding is carried out at basement/ground level and every 20 m above that.

2. Surge Protection System

Surge Protector Device

Surge protector is an appliance or device designed to protect electrical devices from voltage spikes.A surge protector attempts to limit the voltage supplied to an electric device by either blocking or shorting to ground any unwanted voltages above a safe threshold.

The terms surge protection device (SPD) are used to describe electrical devices typically installed in power distribution panels, process control systems, communications systems, and other heavy-duty industrial systems, for the purpose of protecting against electrical surges and spikes, including those caused by lightning.