PC Control / Profi Control 645-12M Instruction Manual pdf - Kuhnke

PROFIBUS hardware installation
B.1.3. Shielding
EN 50 170 leaves it up to the operator whether a shielded
or an unshielded cable is to be used. Unshielded cables
are allowed for non-interference environments.
However, for the following reasons, we recommend that
you always use shielded cables:
a)Non-interference environments may exist inside of
shielded switching cabinets if at all. However, as soon as
these cabinets house relays, they can no longer be classified
as non-interference environments.
b)The use of unshielded cables requires you to install additional
means to protect the bus signal inputs against
overvoltages.
This recommendation also applies to any supply cables between
external power supply units and PROFIBUS devices.
In highly EMI-contaminated environments use lines with a
double shielding. To provide optimal protection in this
case, use an earthing clip-ring to connect both the outer
(mesh shield) and the inner (foil shield) to protective earth
at both ends of the line.
When using a shielded bus cable we recommend that you
establish a low-inductance connection to protective earth at
both ends of the line. This ensures optimal EMC characteristics.
(One exception would have to be made for separated
potentials, e.g. in refineries. These applications usually allow
earthing at only one end of the line.)
of D-Sub plugs
Preferably, the connection between the cable shield and
protective earth is to be made via a metal device casing
and the locking screw of the D-Sub plug.
Other plug-type connectors
Directly connect the cable shield to function or protective
earth on the device. In many cases, this connector is located
directly on or at the device casing.
B - 4