T 4.21 Transient currents on shielding

If IT devices supplied electrically by a TN-C network are interconnected by means of data lines with the shielding connected on both ends, transient currents may occur on the shield (an explanatory drawing can be found in S 1.39 Prevention of transient currents on shielding).

This is a consequence of a peculiarity of the TN-C network that protective (PE) and neutral (N) conductors are jointly routed as PEN conductors up to the individual distributors. They are only divided into N conductor and PE conductor in the distributor. According to VDE 0100 "Erection of power installations with rated voltages up to 1000V", this installation is admissible!

If the interface shielding connected via PE of devices connected to different distributors are interconnected by means of shielded data lines, the PEN conductor is connected in parallel between the distributors and the shielding between the interfaces. The transient current flowing over the shielding as a consequence may damage the interfaces and entail personal hazards when performing work on data lines.

No transient currents flow over the shielding of data lines between devices connected to the same distributor in a TN-C network or between devices connected to different distributors in a TN-S network.

In TN-CS networks, some partial areas are designed as TN-C network and others as TN-S network. As long as data lines with the shield connected on both ends are only routed within similar partial areas in each case, the same conditions are applicable as in the respective networks. However, if IT devices from different areas are connected by means of data lines with the shield connected on both ends, transient currents may also flow in the TN-S area.