Potential Equalization - Siemens SIMATIC S5 Manual

Routing cables; connecting and installing bus connectors
5.1.4

Potential equalization

When do differ-
ences in potential
occur?
Avoiding potential
differences
Rules for potential
equalization
5-8
Differences in potential can occur, for example, when different mains
supplies are used. Under these circumstances, potential differences may
occur between the various parts of a system, if
programmable controllers and inputs/outputs are connected by
potential-bonded couplings,
or
cable shields are connected at both ends and grounded to different parts of
a system.
It is important to install potential equalization lines in order to minimize
potential differences and ensure the functionality of the electronic
components.
Note the following points:
The smaller the impedance of the potential equalization line, the higher
the efficiency of potential equalization.
If shielded signal cables are installed between certain parts of the system
and connected at both ends to ground/protective conductors, it is
important to ensure that the impedance of the additional potential
equalization line is not in excess of 10 % of the shield impedance.
Select potential equalization lines having a cross-section adequate for the
maximum equalization current.
The following are empirical values for the cable cross-sections:
2
– 16 mm Cu for potential equalization lines up to 200 m in length
2
– 25 mm Cu for potential equalization lines more than 200 m in length
Use potential equalization conductors made of copper or galvanized steel.
Use connectors with large contact areas to connect the potential
equalization conductors to the ground/protective conductor.
Protect the potential equalization conductors against corrosion.
Route the potential equalization conductors in such a way that the areas
bounded by the potential conductor and the signal lines are as small as
possible.
ET 200 Distributed I/O System
EWA 4NEB 780 6000-02a