The protection of buildings and structures against direct lightning strikes is well understood. All the recognised world standards on lightning protection present techniques for the protection of buildings and structures through the use of air terminals, down conductors and earthing systems. All these techniques are largely unchanged from the days of Benjamin Franklin and still the most common air terminal is the Franklin Rod. It is only now that other non-conventional systems are gaining some acceptance although much controversy still surrounds their effectiveness.

A lightning strike is an electrical discharge resulting from the build up of electrical charge in a thundercloud. Discharge may occur from cloud to cloud, or from cloud to ground. The current which flows in a cloud to ground strike ranges from about 2,000A to 200,000A with a log normal distribution.

The current in most ground flashes is from a negatively charged cell, although positive discharges also occur. The current flow is unidirectional with a rise time of less than 10 microseconds and a decay time of 100 microseconds or less. Multiple flashes along the same ionised path are likely. These may be spaced by around 50 to 100 milliseconds.

Conventional lightning protection systems work well at protecting buildings and structures from the direct effects of a lightning strike and adequately protect the occupants of these structures.

The indirect effects of these events must also be considered. It is not only lightning that can create damage but a host of other events caused by power line disturbances and load switching transients. These are caused by inductive load switching, capacitor banks, thyristor power supplies and other electrical switching operations.

Electrical: As a current discharge occurs, voltage differentials raise the potential of the structure and the surrounding ground leading to dangerous ‘step and touch’ potentials.

Side Flashing: The point of strike on the structure will be raised to a high potential. Side-flashing to adjacent metal in or on the structure or to service pipes and electrical services will occur.

Thermal: The instantaneous temperature of material within the discharge path will rise, leading to possible fire.

Mechanical: A peak power of about 100MW/m can be attained in a lightning discharge. The shock wave close to this strike can readily dislodge tiles from roofs of unprotected structures.

Protection Against Direct Effects

A properly designed and installed lightning protection and earthing system will overcome the effects of a direct lightning strike. The likely attachment points of lightning to a structure can be determined by a technique known as the ‘Rolling Sphere’ analysis.

Air Terminals are the collectors of the lightning discharge. These may be vertical rods, known as finials, horizontal strap, or the material itself of an all metallic structure. The standards state that an all metallic structure, such as a communications tower, is self protecting and requires no additional protection. The same applies to an all metal building, tanks and pipework.

Downconductors carry the lightning discharge to ground. These may be purpose designed conductors, the frame of an all metal building, reinforcing steel, or the legs of a communications tower.

Earthing System dissipates the energy into the ground. This may consist of an array of buried horizontal strap and vertical rods, the foundation steel of a building as well as bonded connections to metallic service pipes and electrical grounds.

Despite claims to the contrary, none of the recognised world lightning standards accept the concept that any construction or device can prevent a lightning strike.

Earth Potential Rise: A direct strike to a protected structure will flow to ground via that structure’s earthing system. Due to the high current amplitude and the fast rate of rise of current, both conductor resistance and inductance play a part in raising the local earth potential. This potential rise causes equipment damage by voltage breakdown and transfer at both ends of the cable.

Magnetic Field Coupling: The magnetic field associated with current flow in a direct or distant lightning discharge, will induce currents into exposed conductors causing equipment damage at both ends of the conductors. Conductors running parallel to lightning down conductors are to be avoided.

Electric Field Coupling: The electric field collapse associated with direct or distant lightning strikes is capacitively coupled into exposed conductors causing equipment damage at both ends of the conductors in a similar fashion to magnetic field coupling.

Protection Against Indirect Effects

Protection against the indirect effects of a lightning strike require a systematic approach to the installation of protection devices.

Define Protection Boundary. Define a protection boundary, which can be the perimeter of a building, or a section of that building such as a computer room. In the case of a multi storey building, each floor can be defined as a boundary. Any service, such as power, signal or data lines crossing the boundary must be fitted with suitable protection devices.

Protect the Structure. To minimise the indirect effects of a lightning strike, it is essential to provide adequate structural protection with air terminals and down conductors in accordance with appropriate standards.

Install a Bonded Earthing System. This is critical to minimise differences in earth potentials within the protected area.

Protect the Power Lines. Install power surge diverters or power surge filters wherever power lines cross a defined boundary.

Protect Signal Lines. Install signal and data line protection wherever these lines cross a defined boundary.

Regardless of the type of coupling, the method of protection is the same.

Lightning strikes are an unpredictable natural phenomenon.  However the way equipment can be protected from lightning strikes is predictable. The ‘Novaris Systematic Approach’ is a step-by-step solution to lightning and surge protection that can be applied to any application, via the following steps:

Define Protection Bounderies

A boundary may cover an entire building, in the case of a small telecommunications installation or remote telemetry site, it may cover a single floor of a multistorey building, a single room or even a single piece of equipment. Generally the boundary will divide areas of different potential.

In the case of a multistorey building, a lightning strike to the top of that building will create a potential gradient from the top to the bottom of the building. Metallic services running from top to bottom of the building will also be subjected to this potential gradient. It is for this reason that we choose to establish boundaries at each floor.

All services crossing the boundary require protection whether they be power, data or RF signals. It may also be necessary to provide further staged protection for some services within the boundary. For example surge diverters on incoming power may be augmented with power surge filters downstream to protect selected equipment. A fax or modem is a good example where a combined power surge filter and telephone line protector would be installed directly at the fax or modem itself.

Protect the Structure

The protection of buildings and structures against direct lightning strike is covered extensively in many of the world’s lightning protection standards. Novaris recommends the Modified Rolling Sphere Analysis as providing the best method of assessing lightning strike points.

Install a bonded earthing system

For effective lightning protection it is not necessarily the absolute value of earth resistance that is important, rather the way the earthing system is installed and how it is connected to the structure and the equipment to be protected.

It is often thought that earthing problems can simply be solved by improving or reducing earth resistance. Unfortunately this is not the case. Despite attempts to reduce the absolute earth resistance there will always be appreciable earth potential rise.

Using the Novaris Systematic Approach the creation of protection boundaries divides a structure into areas of equipotential where all equipment and protective earths are referenced to a single point within the boundary.

One earthing system is essential, this means all earths must be bonded together either directly or by the use of transient earthing clamps.

Protect the power lines

Power quality in the past has been taken for granted because it has been generally believed that the generating and power distribution authorities can provide us with quality power, suitable for all applications. Unfortunately this is not the case. Modern control technologies such as PLCs, computers and power electronics are very susceptible to minor disturbances and fluctuations of the electricity supply.

The most damaging interference is that of a direct or indirect lightning strike to the power line or a nearby strike which causes earth potentials to rise or induce interference onto power lines. The Novaris Systematic Approach to protecting a structure and its electrical services from these damaging effects includes two types of power protection products.

SURGE DIVERTERS are shunt connected devices which are installed between each phase and neutral or earth on main switch boards. They are designed to provide high energy diversion at the point of entry to a building or major power distribution point. The length of connecting leads to these devices can have a dramatic effect on increasing let-through voltage. So we advise downstream Surge Filters to protect sensitive electronic equipment.

SURGE FILTERS comprise single and three phase hard-wired filters plus plug-in units in a wide range of operating voltages and surge ratings. These are connected in series with the load and because connecting lead lengths are not an issue, provide an excellent level of surge protection with low let-through voltages. Novaris series connected Surge Filters have three stages of protection, so if one section should fail, connected sensitive electronic equipment is not exposed to unprotected power.

Protect signal and data lines

The amount of damage suffered by equipment connected to signal and data lines is rapidly increasing with the high installation density of computers, telecommunication and process control systems.

LAN networks, video, CATV and other coaxial cable applications all suffer from the fact that the coaxial cable can provide an alternative earth return path for transient overvoltage disturbances, either induced on the cable itself or on the power supply anywhere in the network.

The same applies for single or multi pair signal lines which connect process control, telemetry, PLC, telecommunications and information technology systems. They all need protection from lightning induced overvoltages on their signal lines.

Modern digital switching systems such as those found on telephone networks are highly susceptible to transient voltage disturbances and old techniques for protecting this type of circuitry are no longer valid. However, Novaris offers a complete range of protection devices to provide effective protection for each of the above mentioned applications.

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