Remarks regarding shielding

The IEC61158-2 standard and in particular the IEC61784-5 EtherCAT installation profile demand a fully shielded cable for EtherCAT transmission links. This also corresponds to the general state of the art for communication cables. Twisting and differential transmission in the so-called twisted pair medium provide for fundamental interference immunity, while an overall shield around the transmission link supports the interference-free transmission of data. Over and above that, even single-pair shielded cables are used in higher connection classes (Cat6, Cat7).

The following notes correspond to the state of the art. VDI guidelines, EN/IEC standards or EMC guidebooks can be consulted as sources.

• The overall shield around the core pairs provides for protection against external electromagnetic interference fields for the enclosed communication cores.
  It is important for effectiveness that the shield coverage has a low impedance throughout and that it is also implemented without interruption or holes at transition points (EN50174-2:2009, chapter 4.7). Holes in the sense of this documentation are uncovered areas of the order of centimeters.
• The shield should be connected at each end of the cable to the machine earth via an electrically conducting connection with a large surface area and low impedance. A pigtail, i.e. the twisting together and point contact of the shield, must be avoided.
  



• If the shield is not earthed, there is no protection against the influence of magnetic fields.
• It must be ensured by means of sufficient parallel earthing in the plant that no equalizing current flows via the shields of the communication cables. This can destroy connected devices.
  Ethernet devices can therefore connect the shield to ground internally with an RC combination. Static equalizing currents are this prevented, while high frequency interference is eliminated.
  To support this, therefore, the shield of the communication cable should additionally be earthed with a low impedance at the control cabinet entry point and, if necessary, on the device itself by suitable means.
  Beckhoff offers appropriate installation material in the ZB8500 product group.



• Shield contacting should also be present at transition points (e.g. plug -> cable, couplings) and should encompass them 360°.
• The shield material is not to be used for strain relief.
• Suitable materials, preferably copper, are to be used as shield material. If aluminum is used, the special characteristics of aluminum must be considered.
  Note: in the case of foiled versions, the foils for Industrial Ethernet are always made of aluminum.
• The “coupling attenuation” shielding effect, i.e. the qualitative effectiveness or implementation, cannot generally be measured in the field or on installed cables at present (2011). The usual measuring instruments/certifiers are limited to a static continuity measurement. Beyond that, however, there are well-known and standardized laboratory-assisted measuring methods, such as the pipe-in-pipe or drain wire methods, which can also determine the high frequency characteristics and the attenuating behavior of the shield and in this way make it possible to check adherence to a coupling attenuation of >40dB (EN50174-2:2009, 30-100 MHz). However, these are not economical for series use in industry. Therefore, perfect workmanship must be ensured in the execution from the outset.

An inadequate shielding effect may become visible by transmission errors. The diagnostic means in the EtherCAT Master TwinCAT and the EtherCAT Slaves permit the continuous, in-depth location of such defects. Observe the notes in the EtherCAT system documentation regarding this.

Optical fiber cables/optocouplers The use of Ethernet optical fiber cables between the components may provide a solution in difficult EMC environments.