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If IT appliances supplied by electricity via a TN-C network are connected with double-sided shielding, the result may be transient currents on the shielding (an explanatory diagram is to be found in S 1.39 Prevention of transient currents on shielding).
The reason for this is the nature of the TN-C network, whereby protective (PE) and neutral (N) conductors are led together to the various distribution points as a PEN conductor. The separation into N and PE conductors only takes place in the distribution. This installation is permissible according to VDE 0100!
If the interface shieldings of appliances (supported by different distribution points) that are connected with PE are connected together by shielded data lines, the result is a parallel connection of the PEN conductor between the distributors and the shielding between the interfaces. The transient current flowing over the shielding can lead to damage of the interfaces and to the risk of personal injury when working on the data lines.
No transient currents flow over the shielding of data lines between appliances which are connected to the same distribution in a TN-C network or between appliances which are connected to various distributions in a TN-S network.
With regard to TN-CS networks, some parts are designed as a TN-C network, others as a TN-S network. As long as data lines with double-sided shielding are only led within one section, the same will apply as in the relevant networks. However, if IT appliances in different areas are connected via data lines with double-sided shielding, transient currents can also flow in the TN-S area.
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by Bundesamt für Sicherheit in der Informationstechnik |