Installation and connection
4.3 Connecting the HMI device
TP 270, OP 270, MP 270B (WinCC flexible)
4-8
Operating Instructions, Edition 03/2004, 6AV6691-1DD01-0AB0
4.3.3
Connecting the potential equalization
Ground connection
Connect the grounding connection of the HMI device to the rack ground. Use the grounding
screw provided and a conducting cable with a diameter of at least 2.5 mm
2
.
Potential difference
Potential differences can occur between separate system parts which leads to high
equalizing currents. An example would be when line shields are applied on both sides and
grounding occurs on different system parts.
The cause of potential differences could be different mains power feed.
General requirements of potential equalization
Potential equalization lines must be installed to reduce potential differences to an extent that
guarantee problem-free operation of the electronic components. Observe the following when
setting up potential equalizations:
•
The lower the impedance of the potential equalization lead or the larger the cross-section
of the potential equalization lead, the higher the efficiency of the potential equalization.
•
If two system parts are connected to each other by shielded data lines whose shielding
on both sides is connected to the grounding/protective conductor, the impedance of the
additionally laid potential equalization lead may be at the most 10% of the shield
impedance.
•
The cross-section of a potential equalization line must be sized for the maximum
equalizing current which flows. In practice, potential equalization lines between cabinets
with a minimum cross-section of 16 mm
2
have proven sufficient.
•
Use potential equalization lines made of copper or galvanized steel. Connect the potential
equalization lines to the grounding/protective conductor with large area contact and
protect them from corrosion.
•
Clamp the shielding of the data line on the HMI device flush and as close as possible to
the potential equalization rail using cable clips.
•
Lay the potential equalization and data lines parallel to each other and within the
minimum distance of each other (refer to following Figure, Point 6).
Notice
Potential equalization line
Cable shields are not suitable for potential equalization. Only use the intended potential
cable prescribed. Potential cable, for example, must have a minimum cross-section of 16
mm
²
. When setting up MPI and PROFIBUS DP networks, ensure cables with a sufficient
cross
section are used otherwise the interface modules may be damaged or even
destroyed.
