Shieldinhg effectiveness

[emiliano.menghi@xxxxxxxxx]

Hi. I use femm to calculate shielding effectiveness of various 
material (such as alluminium, copper, iron, mumetal etc,) at 50 Hz, 
but i have some problem.
My configuration is simple:
A alluminium wire with 1000 A 50 Hz (r=7.1 mm)
A dielectric around the wire (external radius r=16.5 mm)
A tubolar shield around (external radius r= 19.5 mm)

I use A=0 as boundary condition

If i don't simulate the shield all is OK, but when i use for example 
an alluminum shield, i have these problems:

1- Shielding effectiveness depends on the position of the boundary 
condition
2- I compared femm results with real measures: in the measure setup 
the wire and the shield lenghts were 10 mt. The wire was grounded 
(for femm is grounded at infinity, i think)
Results ( distance from wire: 20 cm)

without shield
B femm= 323 microT B measured=321

with alluminium shield 
B femm= 101 microT B measured= 275 microT
I in the shield femm= 200A measured=35 A
with shield floating B measured= 303 microT
Why?
Is The problem, perhaps, the lenght of the wire (and the shield) 
infinite for femm and finite (10 mt) for the measure?
Can i simulate with femm terminations of shield (floating or 
grounded)?
Thanks and exuse me for my poor english

Menghi Emiliano