Siemens SIMATIC 500 Series Description And Operating Instructions page 28

4 Setting Up
Using bus line shields:
It is important to observe the following when shielding bus lines:
- Use only fully shielded NET PROFIBUS lines. The shields of these lines are of sufficient thickness to satisfy
the legal requirements for interference radiated and interference received.
- Always attach the shields at both ends of the bus lines. The legal requirements vis-à-vis interference radiated
and interference received for your system will only be satisfied if shields are connected at both ends
(CE symbol).
- Attach the shield for the bus line at the connector plug housing or at the cable clamps provided.
- In the case of steady-state operation, it is advisable to strip the shielded line entirely and connect it with the
shielding bus / protective conductor rail.
Note:
If differences in potential occur between the grounding points, an inadmissibly high compensating current could
flow across the shielding connected at both ends. Never eliminate this problem by removing the shielding from
the bus line!
The following solution is permissible:
Lay an additional equipotential bonding cable parallel to the bus line. This additional cable will carry the shield
current.
Shield connections
Fig. 7: Securing shielded lines using cable clamps and tube clips
(schematic diagram)
Version 2.1 10/01
Always observe the following points when installing bus
line shielding:
Secure the shield braid using metal cable clamps.
The clamps must fully enclose the shield and
make good contact (see Fig. 7).
Only connect SIMATIC NET PROFIBUS lines via
the copper braid shield, and not via the aluminum
foil shield. One side of the foil shield is attached to
a plastic film to increase its tearing strength, and
is therefore non-conductive!
The shields of all cables which are routed into a
cabinet from the outside must be clamped at the
point of entry inside the cabinet and connected to
the cabinet ground with a large contact surface
area.
When removing the cable jackets, it is important
to ensure that the braid shield of the cables is
not damaged. Tin-plated or galvanically stabilized
surfaces are ideal for optimum contacting bet-
ween grounding elements. With zinc-plated
surfaces, suitable threaded connections must
be provided for the required contacts. Painted
surfaces at the contact points are unsuitable.
Shield clamps/contact points should not be
used as strain relief devices. Contact with the
shield bus could otherwise deteriorate or break
completely.
4.4 Installation
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