Siemens SIMATIC NET PROFIBUS Owner's Manual page 271

Note
If there is a potential difference between the grounding points, an illegally high
compensating current can flow through the shield grounded at both ends. To
rectify the problem, do not, under any circumstances, open the shield of the LAN
cable.
Handling the Shield
Note the following points when handling the shield:
S Secure the braided shield with metal cable clamps.
S The clamps must make good and large-area contact with the shield (see Figure
C.3 ).
S Contact SIMATIC NET PROFIBUS cables only using the braided copper shield
and not the aluminum foil shield. The foil shield is connected to a plastic foil to
increase tearing strength and is therefore non-conductive.
S Contact the shield with the shielding bar directly at the point at which the cable
enters the cabinet.
Figure C-3
PROFIBUS Networks SIMATIC NET
6GK1970-5CA20-0AA1 Release 2 05/2000
S Install an additional bonding conductor parallel to the LAN cable
that takes over the shield current (for notes on equipotential
bonding refer to Section C.4.4)
S Use fiber-optic cable instead of electrical cable (safest solution).
Securing Shielded Cables with Cable Clamps and Ties (schematic
representation).
Installing LAN Cables
C-9