Measures To Guard Against Electromagnetic Interference - Siemens S5-100U User Manual

S5-100U
Shielding Cables
Both ends of shielded cables should have a good electrical connection to the cabinet's chassis
ground. You can effectively suppress interference of all coupled frequencies only if the cables are
shielded at both ends. The shield should reach the module, but it should not be connected to the
module.
Note
There can be a compensating current flowing across cables shielded at both ends if
there are ground potential fluctuations. You should therefore join the connected
components with an additional equipotential bonding cable.
It would be unusual to shield only one cable end because shielding only one cable end dampens
only the low frequencies. Shield a single cable end in only the following instances.
• It is not possible to lay an equipotential bonding cable.
• Only very low analog signals (a few mV or µA) are transmitted.
With SIMATIC controllers, the interference current on cable shielding is discharged to ground both
via the shielding rail and the equipotential bonding cable. To prevent these discharged currents from
becoming a source of interference, ground them on a low-resistance path as follows:
• Tighten the hold-down screws on the cable connectors, modules, and equipotential bonding
cables.
• Protect the contact surfaces of equipotential bonding cables against corrosion.
3.4.3

Measures to Guard against Electromagnetic Interference

Measures to guard against electromagnetic interference are frequently not taken until after the
controller is operating and problems develop with the reception of a signal.
When you install your controller, you can significantly reduce the number of problems caused by
electromagnetic interference if you proceed as follows.
• Arrange devices and cables correctly.
• Ground all inactive metal parts to chassis.
• Filter power cables and signal cables.
• Shield devices and cables.
• Take specific interference suppression measures.
Physical Arrangement of Devices and Cables
It would cost a lot of money to significantly decrease the constant magnetic fields or alternating
fields that occur at low frequencies (e.g., 50 Hz). You can frequently avoid this problem by having
as much space as possible between the interference source and any potentially susceptible device.
3-32
Installation Guidelines
EWA 4NEB 812 6120-02