look this image:
http://etyekfilm.hu/badnormals.jpg
and the wire:
http://etyekfilm.hu/badnormals_wire.jpg
as you see this is lowpoly model with tangent normalmap calculated from fine hipoly model
but as we see, the we can see the lowpoly faces… but why?
with organic models I have no problems with tangent normal, and most of the non-organic (see my Boom project)
The problem is with the geometry of the low-poly model, which has dense poles and tris in so many places that it will be hard to make those surfaces appear completely smooth.
TS normal mapping does not completely replace all normal information in the model, it adds to it, so the condition and appearance of the base model is still very, very important. Render the low-poly with no normal mapping and use that as a guide to clean up the base geometry. Once it’s looking all smooth and nice you can re-apply the TS normal map to add back in the finer details
Thank you for the fast answer.
So you say the model is very low poly and/or it has bad topology?
Im sure you thought of it but if you didnt use normal mapping creating the maps from high res and put on low res mdls
The number of polys is alright I suppose, I didn’t count them (j/k), but the topo does looks rather messy – the many poles created with triangles are bound to cause some rendering issues, because afaik the renderer is best suited for quads. Not sure how you might consolidate the tris into quads, but one thing you might try first is to use an Edge Split modifier on the edges where the greebles meet the main body of the low-rez doohickey (dontcha just love tech talk 
).
i have these problem with triangles and setting as smooth so if you want it smooth use quads 
I will test the normal calculation with XSI…