Copper Earth Tape Down Conductors & Lightning Protection To BSEN 62305

By Chris Dodds on 12th November, 2014

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Copper Earth Tape Down Conductors & Lightning Protection To BSEN 62305

Down Conductors

General considerations for the copper down conductors (copper earth tapes) are as follows unless the Lightning Protection System (LPS) is isolated:

  • There should be at least two down conductors around the building
  • The down conductors should be as equally spaced as possible
  • Wherever possible the down conductors  should be installed at each exposed corner of the building
  • The down conductors  can be fixed to any wall or surface which is non-combustible, if the surface is combustible refer to BS EN 62305 for guidance
  • Down conductors must not be sited in gutters or down pipes
  • The down conductor ideally will follow the shortest and most direct path to earth
  • The down conductor should as far as possible be straight and vertical
  • Wherever possible avoid any re-entrant loops where this is not possible the separation distance shown in figure 3 below is required as a minimum. If this cannot be achieved another design for the down conductor system should be considered
  • A test joint should be fitted on each down conductor to enable a disconnect  from the earth network and provide access for earth resistance measurement and maintenance

Copper Down Conductors

No test joint is used where natural copper down conductors are combined with foundation earthing.

The bottom 3 metres of the copper down conductor should be protected within a metal guard or PVC covering at least 3 mm thick to deter vandalism and theft, see figure 2. The test clamp should be fixed above the capping wherever possible, see figure 2.

Pic: Figure 2 Test Clamp Fixed Above The Capping

If the air termination is a metal rod on a non conductive mast, at least one down conductor is needed for each mast. No additional down conductors are required for masts made of metal or interconnected reinforcing steel.

If the air termination consists of one or more catenary wires at least one down conductor is needed at each supporting structure.

Pic: Figure 3 Test Joint Fitted To Provide Access For Earth Resistance Measurement & Maintenance

Copper Tape

 

 

Positioning The Down Conductors

There should be multiple copper down conductors following the shortest possible path to earth. Typical values of the distance between copper down conductors, subject to architectural and practical constraints are given in the table below.

Table - Typical Copper Down Conductor Spacing’s & Distance Between Ring Conductors

Class of LPS Typical Ring Distances (M)
I 10
II 10
III 15
IV 20

 

It may not be practically possible to space the copper down conductors exactly as required so the spacing’s can be adjusted by ±20% but the average spacing of all down conductors must conform to the typical distances for the class of LPS.

If it is not possible to place copper down conductors at a side or part side of the building the down conductors that should be on that side should be placed as additional down conductors  compensating for the other sides. The distance between the installed down conductors  should not be less than one-third of the required down conductor distances dependent upon the class of LPS.

Equipotential bonding to conducting parts of the structure should be performed according to BS EN 62305-3, 6.2.

The distance between the down conductor and the internal services must satisfy the distance requirements covered in the table above.

If the separation distance required to avoid dangerous sparking between the down conductor and the internal services cannot be satisfied, the number of down conductors should be increased until the required separation distance is met.

Re-Entrant Loops On A Cantilevered Structure

There is a risk that when the down conductor goes into a cantilever that a strike could flash over to the person standing underneath as shown in figure 3. To reduce this risk the separation distance, h in metres, should satisfy the following conditions.Copper Down Conductor Going Into A Cantilever

Pic: Figure 3 Down Conductor Going Into A Cantilever

H > 2.5 + S 

S - Is the separation distance in metres calculated.

2.5 – Represents  the height of a typical person with their hand in the air.

 

Using Natural Components As Down Conductors

The natural components can be used as copper down conductors provided the components comply with the requirements of BS EN 62305.

The natural components can be used provided there is electrical continuity, where the joints are tightly bolted they can be considered as electrically continuous figure 4.

Electrical Continuity Where The Joints Are Tightly BoltedPic: Figure 4 Electrical Continuity Where The Joints Are Tightly Bolted

The facade elements, profile rails and metallic sub-constructions of facades can be used provided their dimensions conform to the requirements for down conductors and metal sheets or metal pipes, not less than 0.5 mm thick.

If the metal façade of the building is to be used as the down conductor then BS 62305 offers specific guidance.

Each overlapping vertical joint at each down-conductor position should be bridged by flexible metal strapping. See figure 5.

Connections between the sheet metal panels should have a minimum contact surface area of 50sqmm and be capable of  withstanding the mechanical forces of a lightning discharge.

If access to the rear of the façade is not possible and the only type of fixing available for connections between facade sheets and the air termination or down-conductor tapes is pop rivets then these should be at least four 5mm diameter rivets and used on a length minimum of 20mm long. 

Pic: Figure 5 Overlapping Vertical Joint At Down Conductor Bridged With Flexible Metal Strapping

Overlapping Vertical Joint At Down Conductor Bridged With Flexible Metal Strapping

Down Conductors Not Using The Natural Components Of The Building

Where copper down conductors  are installed on the building not using the natural components then consideration needs to be given to the separation distance between the internal columns and internal partition walls with conductive parts.

If these conductive columns and partitions do not satisfy the separation distance conditions they must be connected to the air termination system at roof level and to the earthing network at ground level.




Category:  Earthing & Lightning Protection

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