Siemens S5-135U/155U appendix Shielding of Cables and Lines

Installation Guidelines

3.3.3Shielding of Cables and Lines

Shielding is a method of attenuating magnetic, electrical or electro-magnetic interference fields. Interference currents on cable shields are given a path to ground via the shield bar which is electrically connected to the housing. A low-impedance connection to the protective conductor is particularly important so that these interference currents themselves do not become an interference source.

Where possible, only use lines with a braided shield. The coverage density of the shield should be more than 80 %. Avoid lines with a foil shield because the foil can be very easily damaged by tensile strain and compression during fitting; this results in reduced effectiveness of the shield.

As a rule, line shields should always be connected at both ends. This is the only way to achieve a good degree of interference suppression in the higher frequency region.

Only in exceptional cases should the shield be connected at one end only, as this only achieves attenuation of the low frequencies. Single-ended shield connection may be more advantageous when:

San equipotential bonding conductor cannot be laid;

Sanalog signals (of a few mV or mA) are to be transmitted;

Sfoil (static) shields are used.

With data lines for serial communication, always use metal or metallized connectors. Secure the shield of the data line to the connector case. Do not connect the shield to Pin 1 of the connector.

For stationary operation, it is advisable to fully strip the insulation from the shielded cable and connect it to the shield/protective conductor bar.

Note

In the event of potential differences between ground points, a circulating current may flow through the shield connected at both ends. In this case, install an additional equipotential bonding conductor (see Section 3.3.2).

System Manual	3-15
C79000-G8576-C199-06