LPI Faraday Cage Lightning Protection System

The Faraday Cage lightning protection system is the traditional, conventional approach. A metal conductor mesh is installed across the roof, connected to several down conductors and bonded into the main earthing system. When lightning strikes the building, the current is intercepted by the air terminal or the conductor mesh, then spread across several down paths before being discharged safely into the ground.

What the Faraday Cage system does well

• A widely used conventional lightning protection system
• Works on clear physical principles
• Covers the roof area with a conductor network
• Spreads the current across several down conductors
• Suits large buildings and flat roofs
• The mesh layout can be adapted to the shape of the building
• Conductor runs and joints are easy to inspect
• Can use natural metal structures where they meet the requirements
• Needs no electronic circuitry to intercept the strike
• Designed to IEC 62305, BS EN 62305, AS 1768 or the standard the project specifies
• Individual components can be replaced or maintained separately

LPI (Lightning Protection International) groups this equipment under Conventional Systems, covering conductors, connectors, fixings, finial bases and lightning protection accessories.

How it works

The Faraday Cage system provides a continuous path for lightning current from the highest point of the building down to the earthing system, in this sequence:

1. The air terminal or conductor mesh on the roof receives the lightning current
2. The current flows through the roof conductors to the down conductors
3. Several down conductors share the current and reduce the current density in any one path
4. Bonding reduces the potential difference between metal parts
5. The current is delivered into the earth electrode or earthing network
6. The lightning energy disperses into the mass of the ground
7. Surge protective devices limit the overvoltage entering the electrical and communication systems

Methods for defining the protected area

The positions of air terminals and roof conductors can be worked out using the methods the standard sets out:

1. Mesh method

Conductors are installed as a mesh on the roof, creating a network that receives strikes across the building's area. It suits flat roofs and buildings with a large roof area. The mesh size must be set by the lightning protection level, not by the size of the building alone.

2. Rolling sphere method

An imaginary sphere is rolled across the building's surfaces. Any point the sphere touches is treated as a possible strike point, so an air terminal or conductor has to be installed there. The radius of the sphere depends on the lightning protection level chosen.

3. Protective angle method

The protected zone is defined by the angle beneath the air terminal. It suits structures or projections that are not complex in shape. The protective angle varies with the height of the terminal and the protection level.

A real design may combine more than one method, especially for buildings that are complex in shape or that carry a lot of equipment on the roof.

Lightning protection levels

IEC 62305 divides lightning protection systems into four levels.

The appropriate level has to come from a risk assessment. It should not be chosen from the building type alone.

Main components

1. Air terminal

Installed at high points, corners, roof edges and anywhere likely to receive a strike, such as:

• The top of the building
• The roof deck
• Roof edges and corners
• Rooftop plant rooms
• Antennas
• Water tanks
• Signage and metal structures
• Communication equipment
• Air conditioning and other rooftop equipment

Air terminals must be installed with bases and fixings suited to the environment and the roof material.

2. Roof conductor network

The metal conductors run as a mesh across the roof, receiving the current and carrying it to the down conductors. Common materials are:

• Bare copper
• Tinned copper
• Aluminium
• Other materials meeting the requirements of the standard

The choice of material has to take electrochemical corrosion into account, particularly where dissimilar metals meet.

3. Finial base

The base that fixes the air terminal to the roof surface or structure. Several types are available to suit concrete roofs, metal sheet roofs, walls or specific structures.

4. Conductor fixing

Fixes the conductor to the roof and walls, keeping the spacing and the run stable. It should be designed to take mechanical load, thermal expansion and the outdoor environment. LPI supplies conventional system fixings both for structural fixing and for installation that reduces roof penetrations, depending on the product and the mounting surface.

5. Conductor connector

Joins conductors in straight runs, at crossings, at branches and where they meet an air terminal or down conductor. Every joint must be mechanically strong and carry lightning current continuously.

6. Down conductor

Carries the current from the roof down to the earthing system. Several down conductors should be installed around the building so that the lightning current has parallel paths and spreads well. Key installation points are:

• Choose a short, direct route
• Avoid sharp bends
• Avoid loops or reversed runs
• Spread the conductors around the building perimeter
• Fix the conductors securely
• Keep the calculated separation distance from electrical systems and other metalwork
• Protect against damage where people can reach them

The number and spacing of down conductors must be set by the LPS level, the shape of the building and the standard in use.

7. Test joint

A test point between the down conductor and the earthing system, which can be separated to measure earth resistance and check conductor continuity. It should be installed where it can be reached for maintenance but is protected against impact, tampering and corrosion.

8. Earth electrode

The earthing system discharges lightning current into the ground. It may consist of:

• Earth rods
• A ring conductor around the building
• Earth plates
• An underground grid
• Building foundations designed as a foundation earth
• Soil resistance improvement compounds

The design has to take soil resistivity, the installation area, corrosion and impulse current distribution into account. It should not be judged on a single earth resistance figure.

9. Earth pit

An inspection pit for checking and maintaining the earth connection. It protects the joint from damage and keeps the test point accessible.

10. Equipotential bonding

Bonds metal parts and the earthing system to a similar potential, reducing the risk of side flash. Items that may need bonding include:

• Steel structure
• Water pipes and metal pipework
• Cable trays
• Air conditioning
• Roof framing
• The electrical earthing system
• Communication systems
• Fire protection systems
• Metalwork entering the building from outside

The lightning protection earth and the electrical system earth should be bonded in line with the standard, to reduce dangerous potential differences. LPI states that earthing and bonding exist to create a low-impedance reference point and to reduce the potential difference between conductive parts.

11. Surge protection device

The Faraday Cage handles a direct strike outside the building, but it cannot fully protect against surge voltage entering power, telephone or data lines. Surge protective devices should be installed as well, at points such as:

• Main distribution board
• Sub-distribution boards
• Critical equipment circuits
• Telephone lines
• Network and data cabling
• CCTV systems
• Fire alarm systems
• Nurse call systems
• Building control systems
• Signal cabling from outside

A UPS is not a direct substitute for an SPD, because an ordinary UPS may not handle the fast-fronted surge current of a lightning event.

The LPI four-step protection concept

LPI recommends that a complete lightning protection system cover four parts:

1. Direct strike protection – intercepting the strike itself
2. Surge protection – protecting against surge voltage and current
3. Earthing and bonding – discharging the current and reducing potential differences
4. Personal protection – reducing the risk to people

Installing air terminals on the roof without proper down conductors, earthing, bonding and SPDs therefore does not count as a complete protection system.

Points to consider

• Uses conductors and equipment at many points on the roof
• May affect the appearance of the building
• Positions have to be coordinated with the roofing and architectural work
• Roof penetrations and water ingress need care
• Corrosion has to be controlled where dissimilar metals are used
• Complex building shapes need a detailed protected-zone analysis
• New rooftop equipment may mean the protection design has to be reviewed
• Conductor continuity and joint condition need periodic inspection
• The external system has to work together with the SPDs and bonding inside the building
• The separation distance must be maintained, or bonding carried out as designed, to prevent flashover

Buildings this system suits

• Hospitals and healthcare facilities
• Office buildings
• Industrial factories
• Warehouses and distribution centres
• Data centres
• Communication buildings
• Schools and universities
• Shopping centres
• Hotels
• Airports and transport terminals
• Buildings with flat roofs or large roof areas
• Buildings with a lot of critical equipment on the roof deck

Design and installation steps

1. Survey the building and the site conditions
2. Assess the lightning risk
3. Set the standard and lightning protection level
4. Check the structure and roof materials
5. Define the protected zone using mesh, rolling sphere or protective angle
6. Position the air terminals and roof conductors
7. Set the number and positions of the down conductors
8. Calculate the separation distance
9. Design the earthing and equipotential bonding
10. Design the SPDs for the electrical and signal systems
11. Install with compatible equipment and materials
12. Test and produce a report before handover

Inspection and maintenance

The system should be inspected on schedule and after significant events, such as a severe strike, building extension work or new rooftop equipment. Check:

• The condition of the air terminals
• Conductor continuity
• Every joint and connector
• Fixings and conductor runs
• Signs of heat or arcing
• Corrosion
• Damage caused by roofing work
• Test points and earth pits
• The bonding system
• Earthing system readings
• The status of the SPDs
• That the as-built drawings match the site

Relevant standards

The design should follow the standards required by the project specification or by law, such as:

• IEC 62305 – Protection Against Lightning
• BS EN 62305
• IEC 62561 – Lightning Protection System Components
• IEC 61643 – Surge Protective Devices
• AS 1768 – Lightning Protection
• NFPA 780 – Standard for the Installation of Lightning Protection Systems
• UL 96 – Lightning Protection Components

LPI products reference several sets of standards, but the certification documents for each individual part number have to be checked. Do not assume every product carries the same certification.

In short, the LPI Faraday Cage system is a conductor-network lightning protection system covering the building. It receives the lightning current at designed points, spreads it across several down conductors and discharges it into the earthing system. Installed together with equipotential bonding and surge protection, it gives more complete protection for the building, the people in it and the electrical equipment inside.