lightning protection down conductors

[Zambo]

I have a 220m structure that requires a lightning protection system.

I am used to seeing 75mm2 or 120mm2 copper tape, but I want to find a way to justify the design.

ANSI/NFPA 780 gives me a minimum of 58mm2

IEC 61024-1 I think, but I'm not sure has a minimum requirement of 50mm2.

Can anyone guide me towards a comparison table of the various codes? Apart from reading directly from tables in the codes is there any calculation method for te sizing of ten copper tape?

 

[VEBill]

There's also the issue of keeping the length-to-width ratio of the tape below some value. But I recall that this might be 'recommended practice' (as opposed to Code).

 

[MiketheEngineer]

I know nothing about this - but I would use something like 6'' (150mm) diameter cable if it was my building....

Of course you do realize that lightning protection tends to draw lighting to itself. Some people won't use it!!

 

[Zambo]

VE1BLL, length to width ration doesn't sound right, do you mean width to thickness?

MiketheEngineer, unfortunately not using a lightning protection system is not an option here.

 

[7anoter4]

If VE1BLL is referring to weight/length ratio then that is right. For type II the minimum weight per m indicated is 558 g/m.It corresponds
to 62.7 sqr.mm copper. We use 95 sqr.mm stranded [round] copper for chimney lightning protection down conductor-mechanically
strong enough- and withstand corrosion longer.

 

[Zambo]

Zanoter4,

thank you for tbe response.

Are you talking about tall industrial chimneys for petrochem plants? which code do you follow to get 95mm2? Is the stranded copper PVc insulated? At the top of the chimney is it coated to protect it from the fumes (lead coated?)

How do you spece the horizontal "loop connectors"

I assume as you are using g/m and mm2 you will not be using ANSI/NFPA 780. Do you use IEC 61024-1?

 

[7anoter4]

It is for power stations. Chimney height above 700 ft.[200 m].
NFPA 780, UL 1449 and IEC 62305-3.[what ever will be more conservative]
The minimum from NFPA 780 is then 62.5.According to IEC 60228 the close to this is 70 sqr.mm
We use 95 sqr.mm the same conductor we use for grounding purpose.
For grounding purpose : IEEE Std 665-1995 and IEEE-80/2000.

In IEEE-80/2000 ch.11.3.3 Additional conductor sizing factors (b) it is indicated:
"The possible exposure to a corrosive environment should be carefully examined. Even when the correct
conductor size and the selected joining (connecting) method have satisfied all the IEEE Std 837-1989 test
requirements, it may be prudent to choose a larger conductor size to compensate for some gradual reduction
in the conductor cross-section during the design life of the installation where the soil environment tends to
promote corrosion."
In IEEE Std 665-1995 5.6.7.1 Chimney and stacks it is noted:
"All exposed copper or bronze equipment on the upper 7.5 m (25 ft.) of the chimney should have a minimum
thickness of 1.6 mm (1/16 in) lead coating to resist corrosion by sulphurous products. Such parts include
conductors, connectors, splices, fittings, and attachments."

 

[VEBill]

Length-to-width ratio is related to the inductance and thus the voltage rise. This consideration may be why wide Cu tapes are often used (?) for grounding conductors. It almost certainly doesn't matter for lightning protection of structures, but the ratio could be a consideration for the grounding conductors used at the building entrance where the protection devices would be installed. I know that some trustworthy lightning protection vendors do sell very wide Cu straps.

I wouldn't worry about it for basic structural lightning protection.