Should there be isolation betweeen base of Air terminal and the metal roof on which it is installed?

[Roshan07]

In one project, we are installing Lightning protection System on a tall building, which comprises of both Concrete Building sections and metal roofed sections.
There are Air Terminals installed on supports fixed to handrails. There are Air terminals installed on metallic columns and the metal roof.

Our client has commented that we should provide a non conducting insulation between the base of the air terminals and the metal roof/column in such areas.

We have followed NFPA 780 for the design of the System.
In NFPA 780 ,4.9.3 Substitution of Main Conductor - it is indicated that in certain conditions the metal members can be substituted for the main conductor.

I do agree to the effect of bimetallic reactions of dissimilar metals of the air terminal and the metal roof. this can be solved. But,

My question is - if NFPA 780 itself states that metal members such as metal roofs can be function as Substitutes for main conductor, isn't the client's requirement provide isolation between the base of air terminal & the metal roof silly?

 

[MikiBg]

Depends on is your metal roof capable/allowed to carry lightning current. Here in Europe, IEC provides conditions for metal structures to be used as part of air termination system. If your roof has adequate material and thickness, and it isn't within any explosive zone, then it can be used as "natural" air termination, and should be connected to separate air termination conductor, if any. Otherwise, separate air termination conductor shall be installed on required safety distance and isolated from the roof.
Above applies for lightning current carrying capacity. Specially, if there is electrolytic incompatibility between roof material and air termination conductor material, isolation should be considered.