A Guide to BS EN 62305:2006 Protection Against Lightning Part 6 pps

If the metallic and electrical services enter the

structure at different locations and thus several

bonding bars are required, these bonding bars should

be connected directly to the earth termination system,

which preferably should be a ring (Type B) earth

electrode arrangement.

If a Type A earth electrode arrangement is used then

the bonding bars should be connected to an individual

earth electrode (rod) and additionally interconnected

by an internal ring conductor.

If the services enter the structure above ground level,

the bonding bars should be connected to a horizontal

ring conductor either inside or outside the outer wall

and in turn be bonded to the external down

conductors and reinforcing bars of the structure.

Where structures are typically computer centres or

communication buildings where a low induced

electromagnetic field is essential, then the ring

conductors should be bonded to the reinforcing bars

approximately every 5 metres.

Protection measures for roof mounted equipment

containing electrical equipment

This is an issue that has already caused some debate.

Applying the guidance from BS 6651 the

designer/installer would bond the metallic, roof

mounted casing into the mesh air termination system

and accept that if the metallic casing suffered a direct

lightning strike, then the casing, if not sufficiently

thick, could be punctured.

What it did not address to any great degree was the

solution to the possibility of partial lightning currents

or induced overvoltages entering into the structure,

via any metallic services that were connected to the

roof mounted equipment.

BS EN 62305-3 significantly elaborates this topic.

Our interpretation of the lightning protection

requirements can be summarised by the flow chart

shown in Figure 4.41.

There are several scenarios that can occur:

a) If the roof mounted equipment is not protected

by the air termination system but can withstand a

direct lightning strike without being punctured,

then the casing of the equipment should be

bonded directly to the LPS. If the equipment has

metallic services entering the structure (gas, water

etc) that can be bonded directly, then these should

be bonded to the nearest equipotential bonding

bar. If the service cannot be bonded directly

(power, telecom, cables) then the ‘live’ cores

should be bonded to the nearest equipotential

bonding bar, via suitable Type I lightning current

SPDs.

BS EN 62305-3 | Lightning equipotential bonding

64

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b) If the roof mounted equipment cannot withstand

a direct lightning strike then a separation

(ie isolation) distance needs to be calculated

(explained in more detail, later in this section). If

this separation distance can be achieved, (ie there

is sufficient space on the roof) then an air rod or

suspended conductor should be installed (see

Figure 4.19). This should offer sufficient protection

via the protective angle or rolling sphere method

and is so spaced from the equipment, such that it

complies with the separation distance. This air

rod/suspended conductor should form part of the

air termination system. If the equipment has

metallic services entering the structure (gas, water

etc) that can be bonded directly, then these should

be bonded to the nearest equipotential bonding

bar. If the other electrical services do not have an

effective outer core screen, then consideration

should be given to bonding to the nearest

equipotential bonding bar, via Type II overvoltage

SPDs.

If the electrical services are effectively screened

but are supplying electronic equipment, then

again due consideration should be given to

bonding, via Type II overvoltage SPDs.

If the electrical services are effectively screened

but are not supplying electronic equipment, then

no additional measures are required.

c) If the roof mounted equipment cannot withstand

a direct lightning strike, then again a separation

distance needs to be calculated. If this separation

distance cannot practically be achieved, (ie there is

insufficient space on the roof) then an air rod or

suspended conductor should be installed. This still

needs to meet the protective angle or rolling

sphere criteria but this time, there should be a

direct bond to the casing of the equipment.

Again, the air rod/suspended conductor should

be connected into the air termination system.

If the equipment has metallic services entering

the structure (gas, water etc) that can be bonded

directly, then these should be bonded to the

nearest equipotential bonding bar. If the service

cannot be bonded directly, (power, telecom,

cables) then the ‘live’ cores should be bonded to

the nearest equipotential bonding bar, via suitable

Type I lightning current SPDs.

The above explanation/scenarios are somewhat

generic in nature and clearly the ultimate protection

measures will be biased to each individual case.

We believe the general principle of offering air

termination protection, wherever and whenever

practical, alongside effective equipotential bonding

and the correct choice of SPDs where applicable, are

the important aspects to be considered when deciding

on the appropriate lightning protection measures.

BS EN 62305-3 Physical damage to

structures and life hazard