Metal roof do not need roof conductor as lightning protection system?

[petotry]

Hi mates,

need your advice on whethre is it possible to connect the down conductor to the metal roof directly and without installing any roof conductor.

because there are people having the idea that: metal roof could be the conductor which act as if air terminal and roof conductor. my understanding is that the metal roof is not good enough to carry the high ampere of lightning strike which could be penetrated and caused phisical damage on the metal roof sheet.

kindly enllighten me on this issue. even welcome shall there be any standards or theory/calculation that could clarify my doubt.

thanks.

loke

 

[dpc]

See NFPA 780-2008. AFAIK, if air terminals are installed, you must run dedicated down conductors. Whether or not any lighting protection is even required for a metal building is another matter and may be dependent on the building function and location in the world.

The steel or aluminum roof is probably a reasonably good conductor, but you must also consider how this roof is bonded to the rest of the building and to earth. I would expect significant side flash issues. It's always good to remember that you're dealing with an arc that traveled maybe 30,000 feet through the air - so our attempts to control it often fail.

 

[7anoter4]

As dpc said you have to follow NFPA 780 Instructions. I don't have 2008 Edition but in 2004 edition you
could see:
4.6.1.2 Metal parts of a structure that are exposed to direct lightning flashes and that have a metal thickness of 4.8 mm ( in.) or greater shall require only connection to the lightning protection system.
4.6.1.3 Such connections shall provide a minimum of two paths to ground.

 

[kithiljr]

A lightning strike to rods on roof or to roof itself still may cause structural damage and/or fire. Instead of a "direct" system, an "indirect" system is suggested. Overhead grounded wires ala electric power lines or adjacent free-standing metal masts may be a better option. In either case, protection of people and of sensitive electrcial/electronic equipments will NOT be obtained. For these you need to consider Surge Protection Devices and equipotential bonding/grounding. See more at:

http://www.lightningsafety.com