Are US concrete a better insulator?

[Gymnast]

According to both NFPA780 and IEC 62305-3 you need a minimum distance from the LPS to be exceeded if you can avoid making equipotential bonding to other metalic structures in a building.

According to IEC 62305-3 (section 6.3) you need the double distance, if concrete is used as insulating material with reference to air. This international code is normaly used in Europe and Australia.

According to NFPA780 (section 4.21.2.4) you need half the distance, if concrete is used with reference to air. So it seems like US concrete is a better insulator...

Does other US codes consider the same as NFPA780?

Can anyone explain this difference between the two codes?

 

[bphgravity]

I'm not sure you understand the intent of the section you reference.

Section 4.21.2.4 of the NFPA 780 is one method for determining if a long or large grounded metal body needs to have additional bonding when its connection to the LPS occurs at one point or at one of its extremities.

Flashover or sideflash doesn't occur as readily through dense objects such as concrete or brick as compared to the air alone. The intensity of the electric field in air is 2 times that of concrete, thus the reduction allowance provided by this section.

I am not familair enough with the IEC document, therefore cannot comment on it.

 

[Gymnast]

Section 4.21.2.4 of the NFPA 780 is one method for determining if a long or large grounded metal body needs to have additional bonding when its connection to the LPS occurs at one point or at one of its extremities.

Yes, it is these sideflashes you want to avoid with this kind of requirement in both standards.

Flashover or sideflash doesn't occur as readily through dense objects such as concrete or brick as compared to the air alone. The intensity of the electric field in air is 2 times that of concrete, thus the reduction allowance provided by this section.

I am not familair enough with the IEC document, therefore cannot comment on it.

One "specialist" (using the IEC document) I asked claimed, that concrete could not be considered as a good homogene insulator, and therefore twice the distance was needed using concrete compared to air. It is this "strange" difference I ask you about. Does other US standards (UL and others) on lightning have the same interpretation? Possibly the US representation in the IEC committee might have objected to the IEC formula.

 

[bphgravity]

I wish I could comment on the IEC requirements, but I can't.

The statement made by your specialist is in driect conflict with the comments made in the NFPA 780. See Annex B of the NFPA which gives some explanatory detail of bonding requirements for lightning protection systems.