EMC implementation - Communication networks

It is highly recommended to follow the European Standards EN 50173 series to perform Information Technology cablings.

To ensure a reliable data transmission, the quality of the whole link shall be homogeneous. That means the category of the different cables shall be the same, the connecting interfaces shall be adapted to the cables.

Cables and connections of different categories may be mixed within a channel however the resultant performance will be determined by the category of the lowest performing component.

The shield continuity of the whole link (patch cords, Terminal Outlets, horizontal cable) shall be ensured and controlled by tests.

The Terminal Outlets (TO) could be used to earth the screen terminations in the cabinet. The choice of these TO is very important.

Communication networks are mostly extensive. They interconnect equipment located in different areas where the feeding power supplies could have different earthing systems.

Fig. R17 : How to reduce disturbing currents loop

If these different areas are not well equipotential, harsh transient currents could appear (lightning, main power fault, etc.) causing high voltage potential differences between interconnected equipment.

Communication interfaces (board, module, etc.) could be disturbed or damaged by this common mode over voltages.

The use of TN-S earthing system and well equipotential installation minimize this issue.

In any case, the use of Surge Protective Device (SPD) installed in Common Mode and/or Differential Mode is recommended.

If the different areas/zones are not equipotential, if the power supply earthing system is TN-C or IT, or if there is a doubt and the previous 2 points, optical fiber links are highly recommended.

To avoid electrical safety issue, the optical fiber link should not have any metallic parts.

Protection against coils disturbances

AC and mostly DC Coils (relay, contactor, actuator, etc.) are very disturbing sources.

Fig. R18 : TVS reduces the arcing voltage

To minimize these High Frequency disturbances the following solutions could be implemented. (In grey, the preferred choice).

Symbol	Transient Voltage Suppression type	For AC	For DC	Overvoltage limitation	Contact fall time
	R-C network	Y	Y	2 to 3 . Un	1 to 2 times the standard time
	Metal Oxide Varistor	Y	Y	< 3 . Un	1.1 to 1.5 times the standard time
	Transient Voltage Suppression Diode Bidirectional	Y	Y	< 2. Un	1.1 to 1.5 times the standard time
	Transient Voltage Suppression Diode Directional	N	Y	Un + 0.7 V	3 to 10 times the standard time
	Free wheeling diode	N	Y	Un + 0.7 V	3 to 10 times the standard time
	Resistor	Y	Y	< 4 . Un	1.5 to 2.5 times the standard time